Machine Tool Design Lab Manual

EXPERIMENT NO.1 Object: - Study of functional requirements of machine tools . Introduction: - Any machine should satisfy the following requirements. 1. High productivity 2. Ability to provide the required accuracy of shape and size and also necessary surface finish 3. Simplicity of design 4. Safety and convenience of control 5. Good appearance 6. Low cost of manufacturing and operation

1. Productivity: - Productivity of a metal cutting machine tool is given by the expression Q= (1/tc+tn0).n tc = machine time tn0 = non-productivity time that include job handling time. a. Cutting down machining time: - This is possible if high cutting speeds and feed rates are available on the machine tool in accordance with the latest development in cutting tool material and design. b. Machining with more than one tool simultaneously: - This principle employed in multiple-spindle lathes, drilling machine etc. c. Improving the reliability of the machine tool to avoid break down and adopt proper maintenance policy to prevent unscheduled stoppages and delays.

2. Accuracy: - The accuracy of a machine tool depends upon its geometrical and kinematic accuracy and its ability to retain this accuracy during operation. Accordingly the ability of a machine tool to consistency machine parts with a specified accuracy with in permissible tolerance limits can be improved by the following method. a. Improving the geometrical accuracy of the machine tool: This is mainly determined by the accuracy of guideways, power screw etc. b. Improving the kinematic accuracy of the machine tool: This is determined the relationship between velocities of two or more forming motion and a nd it depends upon the length of kinematic accuracy of machine tool can be improved.

d ynamic stiffness of machine tool structure. The greater in the static c. Increasing the static and dynamic stiffness of the machine tool structure the smaller will be its deformation due to cutting forces and will be the accuracy of machining. d. Providing accurate devices for measuring distance of travel. e. Arranging the machine tools units in such a manner that the thermal deformation during the machining operation result in the least possible change in the relative position between the tool and the workpiece.

Department of Mechanical Engineering,RTU

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3. Simplicity of design: - Simplicity of design of machine tool determines the ease of its manufacture and operation. The design of machine tool can be simplified by using standard parts and sub assembly as far as possible. The complexity complexity of design of of a machine tool depends depends to a large extend upon the degree of its university. Thus a general purpose machine tool is a rule more complex than a special purpose machine tool design doing similar operation.

4. Safety and convenience of control: - A machine tool cannot be deemed fit for use unless it machine tools the requirement of safety and convenience of operation. a. Shielding the rotating and moving parts of the machine tool with hoods. b. Protecting the worker from chips, abrasive dust dust and coolant by means of screws shield etc. c. Providing reliable clamping for the tool and workpiece. d. Providing reliable earthing of the machine, providing device for safe handling of heavy workpiece.

5. Appearance:- Good appearance of the machine tool influence the mood of the worker favourably and thus facilities better operations it is generally conceded that a machine tool that is simple in design and safe in operation and also good in appearance although factors, such as external finish colour. Nowadays, painting of machines in different colours according to t o the production purpose is becoming popular.

6. Cost of manufacturing and operation: - The cost of manufacturing a machine tool is determined by the complexity of its design. Therefore factors that help in simplifying the machine tool design also contribute towards lowering its manufacturing cost. The cost can also be brought down by reducing the amount of metal require d in manufacturing the machine tool. This is achieved by using stronger materials and more precise design calculation pertaining to the strength and rigidly of parts to keep the safety margins as low as possible.

Department of Mechanical Engineering, RTU

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EXPERIMENT NO.2 Object: - Study of working and auxiliary motion of machine tool. Introduction: - Obtaining the required shape on the workpiece, it is necessary that the cutting edge of the cutting tool should move in a particular manner with respect to the workpiece the relative movement between the workpiece and cutting edge can be obtained either by the motion of the workpiece the cutting tool or by a combination of the motion of the workpiece and cutting tool. These motion which are essential are working to impart the required shape to the workpiece are known as working motion. Working motions are further classified into two categories: 1. Drive motion or primary cutting motion 2. Feed motion Working motion in machine tools generally of two types: 1. Rotary 2. Translatory

Fig: lathe

Fig: shaping

fig: drilling

fig: grinding

1 .For lathes and boring machines Drive motion: Rotary motion of workpiece Feed motion: Translatory motion of cutting tool in the axial or radial direction


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2 .For drilling machines Drive motion: Rotary motion of workpiece Feed motion: Translatory motion of drill 3 .For milling machines Drive motion: Rotary motion of the cutter Drive motion: Translatory motion of workpiece 4 .For shaping, planning and slotting machines Drive motion: Reciprocating motion of cutting tool Feed motion: Intermitted translatory motion of the workpiece 5 .For grinding machines Drive motion: Rotary motion of grinding wheel Feed motion: Rotary as well as translatory of the workpiece Besides the working motion a machine tool also has provision for auxiliary motions. In machine tool, the working motions are powered by sources of energy. The auxiliary motion may be carried out manually or may also be power operated depending upon the degree of automation of the machine tool. In general purpose machine tools, most of the auxiliary motions are executed manually.

Parameters defining working motions of a machine tool The working motions of the machine tool are numerically defined b y their velocity, the velocity of the primary cutting motion or drive motion is known as cutting speed while the velocity of feed motion is known feed. The cutting speed is denoted by ‘v’ and measured in the units m/min. Feed is denoted by‘s’ and measured in the following units. 1. mm/rev. in machine tool with rotary drive motion e.g. lathes, boring machine etc. 2. mm/tooth, in machine tool using multiple-tooth cutters e.g. milling machines. 3. mm/stroke, in machine tools with reciprocating drive motion e.g. shaping and planning machine. 4. mm/min, in machine tools which have a separate power source for feed machines. In machine tools with rotary primary cutting motion, the cutting speed is determined by the relationship v=

 

m/min

d= diameter of workpiece or cutter n= revolution per minute (rpm) of the workpiece or cutter


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In machine tools with reciprocating primary cutting motion, the cutting speed is determined by u=

 Tc

m/min

L= length of stroke mm Tc = time of cutting stroke min If the time of the idle stroke in minutes is denoted by T i, the number of strokes per minute can be determined as n=

 Tc +Ti

Generally, the time of idle stroke Ti, is less than the time of cutting stroke, if the ratio T c/Ti is denoted by K, the expression for number of strokes per minute may be written as n=

  (+)  (+) =

Now combining equations the relationship between cutting speed and number of strokes per minute may be written as follows v=

(+) 

The feed per revolution and feed per stroke are related to the feed per minute by the relationship Sm = s.n Where, Sm = feed per minute s = feed per revolution n = number of revolution The feed per tooth in multiple tooth cutter is related to the feed per revolutions as follows: S = Sz.z Where, S = feed per revolution Sz = feed per tooth of cutter z = number of tooth on the cutter The matching time of any operation can be determined from the following basic e xpression Tm = Where, Tm = matching time, min

Sm

min

l = length of machined surface, mm Sm = feed per minute


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Experiment No.-3 Objective: Design criterion for machine tool structure, Static & dynamic stiffness. Introduction:-

Machine tool consists of machine tool structure, bed, column, housings. These are the base of machine tool on which the guideways, spindle, carriage, etc. are mounted. These elements must able to withstand at higher permissible load. These elements are discussed in detail in the following section. Objectives is to understand    

Functions of machine tool structure and the design criteria for selection of material for sideways, The design of bed, The design of column, and The design of housing.

FUNCTIONS OF MACHINE TOOL STRUCTURE: Machine tool structure consists of bed, base, columns, box type housings, overarms, carriages, tables etc. The structures are divided into three categories according to their functions : Category 1 An element, upon which various subassemblies are mounted, falls under this category. Example: bed and base. Category 2 Elements consist of box type housings in which individual parts are assembled fall under this category. Example: Speed box housing, spindle head, etc. Category 3 Elements consist of parts that are used for supporting and moving the workpiece and cutting tool fall under this category. Example: Table, carriage, knee, tailstock etc.

Machine tool structure must satisfy the following requirement : (a) The initial geometrical accuracy of the structure should be maintained for the whole life of the machine tool. (b) All mating surfaces of the structure should be machined with a high degree of accuracy to provide the desired geometrical accuracy. (c) The shape and size of structure should not only provide safe operation and maintenance of the machine tool but also ensure that working stresses and deformation should not exceed specific limits. (d) The selection of material and high static and dynamic stiffness are the fundamental requirement to fulfill above-mentioned requirement.


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DESIGN CRITERIA FOR MACHINE TOOL STRUCTURE: The simple machine tool bed with two-side wall is represented as a simply supported beam. Figure 3.1 depicts a simply supported beam. Point load F acts at its center. The maximum normal stress acting on the beam is given by

Figure 3.1: Simply Supported Beam

Where Bmax = Maximum bending moment = FL/4 , In = Moment of inertia of the beam section about the neutral axis=bh 3/12 On substituting these values in Eq. σ nmax changes to

The permissible normal stress under tension for the beam material is given by

Or minimum volume of material (V min) required to make sure that beam has sufficient strength is given by

The maximum deflection of simply supported beam is given by the following expression : Department of Mechanical Engineering, RTU

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Where E is young modulus of beam material . If the deflection of the beam d per is not to exceed a permissible value, then

Where Vmin = minimum volume of metal required to make sure that deflection of the beam under load does not exceed the permissible value. The condition of optimum design is given by

Hence Eq. Indicates that for every structure, there exists an optimum ratio l/h and the ratio l/h depends upon: a) Operation constraint i.e. d per. b) The material of the structure i.e. E and σ per. Materials for Machine Tool Structure The commonly used material for machine tool structures are cast iron and steel. Earlier cast iron structures were widely used but due to advances in welding technology, welded steels are widely used now days. The selection of material for machine tool structure depends upon following factors: Material properties a) Cast iron has higher damping properties than steel. Welded steel also shows good damping properties. b) Cast iron has better sliding properties. c) Steel has higher strength under static and dynamic loading. d) The unit rigidity of steel under tensile, torsional and bending loads is higher than cast iron.


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Manufacturing Problems

Welded structures of steel have much thinner wall thickness as compared to cast structure. Walls of different thickness can be welded more easily than casting it. Machining allowances for cast structures are generally greater than for weld steel structures. Machining allowance is necessary in casting to remove defects such as inclusions, scales, etc. Welded structure can be easily repaired as compared to cast structure. Economy The selection of material for structure will also depend upon its cost. The weight of steel is lesser and but actual metal consumption is higher than that of cast iron. Hence in such cases the cost increases. Holes are obtained with the help of core in the casting structure but holes are made in welded steel structure by machining. These will not only increase the materi al cost but also increases labour cost. Cost of patterns, welding fixtures, and cost of machining are considered while selecting material for structure. On considering above factors, the cast iron and steel ma y be used for following application: a) Cast iron should be used for complex structure subjected to normal loading which are to be produced in large number. b) Steel should be used for simple and heavy loaded structures which are to be produced in small number. c) Combined welded steel and cast iron should be used where steel structure is economically suitable. Example: Cast bearing housings that are welded into the feed box.

Machine Dynamics: The machining and machine dynamics within the machine system should be well understood, optimized and controlled, because they have the following direct effects:     

They may degrade machining accuracy and the machined surface texture and integrity. They may lead to chatter and unstable cutting conditions. They may cause accelerated tool wear and breakage. They may result in accelerated machine tool wear and damage to the machine and part. They may create unpleasant noises and sounds on the shopfloor because of the chatter and vibrations.

Loop Stiffness within the Machine-tool-work piece System: Stiffness: stiffness normally can be defined as the capability of the structure to resist deformation or to hold a position under the applied loads. Static stiffness in machine tools refers to the performance of structures under the static or quasi-static loads. Static loads in machine tools normally come from gravity and cutting force etc. apart from the static loads, machine tools are subjected to constantly changing dynamic forces and the machine tool structure will deform according to the amplitude and frequency of the dynamic excitation loads, which is termed dynamic stiffness Machine-tool-workpiece Loop Concept

From the machining point of view, the main function of a machine tool is to accurately and repeatedly control the contact point between the cutting tool and the uncut material - the ‘machining interface’. Figure 3.2shows a typical machine tool-work piece loop. The machinetool-work piece loop is a sophisticated system which includes the cutting tool, the tool holder, the slideways and stages used to move the tool and/or the workpiece, the spindle holding the workpiece or the tool, the chuck/collet, and fixtures, etc. If the machine tool is being taken as Department of Mechanical Engineering, RTU

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a dynamic loop, the internal and external vibrations, and machining processes should be also integrated into this loop as shown in Figure 3.3. Stiffness can normally be defined as the capabilit y of the structure to resist deformation or hold position under the applied loads. Whilst the stiffness of individual components such as spindle and slideway is important, it is the loop stiffness in the machine-tool system that determines machining performance and dimensional and forming accuracy of the surface being machined, i.e., the relative position between the workpiece and the cutting tool directl y contributes to the precision of a machine tool and correspondingly leads to the machining errors.

Fig. 3.2 A typical machine tool loop

Fig.3.3 The machine-tool-workpiece loop taking account of machining processes and dynamic effects

Static Loop Stiffness Static loop stiffness in machine tools refers to the performance of the whole machine-tool loop under the static or quasi-static loads which normally come from gravity and cutting forces in machine tools. A simplified analogous approach to obtaining the static loop stiffness is to regard the machine tool individual elements as a number of springs connected to each other in series or in parallel, so that the static loop stiffness can be derived based on the stiffness of each individual element:


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Typically, a well-designed machine-tool-workpiece system may have a static loop stiffness of around 50N/µm; a figure of 500 N/µm is well desired for heavy cutting machine tools in particular. While a loop stiffness of about 10N/µm seems not rigid enough, it is quite common in precision machines. Static loop stiffness can be predicted at the early design stage by analytical or numerical methods and thus design optimization and improvement are essential; also, a continuous process because of the increasing demands from the various applications. Dynamic Loop Stiffness and Deformation

Apart from the static loads, machine tools are subjected to constantly changing dynamic forces and the machine tool structure will deform according to the amplitude and frequency of the dynamic excitation loads, which is termed dynamic stiffness. Dynamic stiffness of the system can be measured using an excitation load with a frequency equal to the damped natural frequency of the structure. Following Equations provide a rough approximation of dynamic stiffness k dyn and deformation xdyn:

Where F is the dynamic load applied to the machine tool, k static is the static stiffness of the machine tool, and Q is the amplification factor which can be calculated from:

Where M and c is the mass and damping:


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Therefore,

In order to accurately predict and calculate dynamic loop stiffness or the behaviour of a whole machine-tool system, a dynamic model including all elements in the machine-tool loop needs to be developed. The finite element method has be en widely used to establish the machine tool dynamics model and provide the solution with reasonable accuracy.


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Experiment No: 4 Objective: - Function & important requirements of spindle unit. Functions: The spindle unit of machine tool performs the following functions:.

1.

Centering the workpiece, e.g., in lathes, turrets, boring machines, etc., or the cutting tool as in drilling and milling machines, 2. Clamping the workpiece or cutting tool, as the case may be, such that the workpiece or cutting tool is reliably held in position during the machining operations, and 3. Imparting rotary motion (in lathes) or rotary cum translatory motion (drilling machines) to the cutting tool or workpiece. The operational capabilities of a machine tool in terms of productivity as well as accuracy and finish of machined parts largely depends upon the extent to which these functions are qualitatively satisfied. Important design requirements of spindle units are list ed below: The spindle should rotate with a high degree of accuracy. The accuracy of rotation is determined by the radial and axial runout of the spindle nose, and these must not exceed certain permissible values which are specified depending upon the required machining accuracy. The rotational accuracy is influenced maximum by the stiffness and accur acy of spindle bearings, particularly the one located at the front end. The spindle unit must have a high static stiffness. The stiffness of the unit is made up of the stiffness of the spindle unit proper and the spindle bearings. Machining accuracy is influenced by bending axial as well as torsional stiffness. The spindle unit must have high dynamic stiffness and damping. Poor dynamic stability of the spindle unit adversely affects the dynamic behaviour of the machine tool as a whole, resulting in poor surface finish and loss of productivity due to restriction on the limiting unreformed width of cut. The mating surfaces that are liable to wear restrict the life of the spindle unit. These surfaces, such as journals, quills(in drilling machine) etc., must be hardened to improve their wear resistance. The spindle bearing must also be selected or designed to retain the initial accuracy during the service life of the machine tool. The deformation of the spindle due to heat transmitted to it by the bearings, cutting tool, workpiece, etc. should not be large as this has an advers effect on the machining accuracy. The spindle unit must have a fixture which provides quick and reliable cantering and clamping of the cutting tool or workpiece. The centering is achieved by means of an external or internal taper at the front end of the spindle. 












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Table: Spindle Ends

DESIGN OF SPINDLE Figure 4.1 shows schematic diagram of spindle. A spindle represents a shaft with (a) length ‘a’ which is acted upon by driving force F 2, and (b) Cantilever of length ‘m’ acted upon by external force F 1.

(c) The total deflection of spindle nose consists of deflection d 1 of the spindle axis due to bending forces F 1 and F 2 and deflection d 2 of the spindle axis due to compliance of the spindle supports. When the spindle has tapered hole in which a center or cutting tool is mounted, the total deflection of the center or cutting tool consists of deflections d 1, d 2 and d 3 of the center or cutting tool due to compliance of the tapered joint.


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d max

≤ d

per

F2

F1

a

m

Figure 4.1: Principle of Working of Spindle

Deflection of Spindle Axis due to Bending To calculate the deflection of the spindle nose due to bending, one must establish a proper design diagram. The following guidelines may be used in this regard. (a)

If the spindle is supported on a single anti-friction bearing at each end, it may be represented as a simply supported beam, and

(b)

If the spindle is supported in a sleeve bearing, the supported journal is analyzed as a beam on an elastic foundation; for the purpose of the design diagram the sleeve bearing is replaced by a simple hinged support and a reactive moment M r acting at the middle of the sleeve bearing.

The reactive moment is given as : M r = C . M

Where

M C

= bending moment at the support, and = constant = 0 for small loads and 0.3 to 0.35 for heavy load.

F2

k

F1

b

M

(a)

Mr

F2

F1

(b)

Mr

(c)

Figure 4.2 : Effect of Various Force on Spindle


d1

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Figure 4.2 (a) shows schematic diagram of spindle. Figure 4.2 (b) depicts the design diagram of the spindle and figure 4.2 (c) illustrates deflected axis of the spindle. Consider the spindle shown in Figure 4.2 (a). By replacing the rear ball bearing by a hinge and the front sleeve bearing by a hinge and reactive moment M r, the spindle can be reduced to the design diagram as shown in Figure 4.2 (b). The deflection at the free end of the beam (spindle nose) can be determined by Macaulay’s method and is found out to be

Where

E I a

is Young’s modulus of the spindle material. is average moment of inertia of the spindle section. The

deflection of the beam is shown in Figure 4.2 (c).

Materials of Spindles The blank for a machine tool spindle may be: 1. Rolled stock in the case of spindles having diameter < 150 mm, and 2. Casting in case of spindles having diameter >150 mm. In machine tool spindle design the critical design parameter is not strength but stiffness. If we compare the mechanical properties of various steels, we found that modulus of elasticity is more or less equal, although the strength of alloy steels can be considerably greater than that of mild steel. Since stiffness is primarily determined by the modulus of elasticity of the material, it may be concluded that no particular benefits accrues from using costly alloyed steels for making spindles. Recommendations for selecting spindle materials: 1. For normal accuracy spindles, plain carban steels C45 and C59, hardened and tempered to R C =30. 2. For above normal accuracy spindles- low alloy steel 40Cr1Mn60Si27Ni25 induction hardened to Rc= 50-56. 3. For spindles of precision machine tools, particularly those with


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Experiment No. 5 Objective: Importance of machine tool compliance with respect to machine tool accuracy.


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Consider a uniform shaft being machined between centres on a lathe (Fig. 5.1). Let KA be the stiffness of centre A and Kg that of centre B. Due to radial component Py of the cutting force, centre A will be displaced by a distance YA = PA /K A And centre B by YB= PB/K B

Fig. 5.1 Schematic diagram of a simple turning operation Here PA and P B are the forces of reaction at ends A and B, respectively. They can be determined from the following equations of static equilibrium: 1. Moment of Forces about Point B = 0, i.e., PAl = Py (l — x)


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Substituting the values of yA and yB from Eqs.

If it is assumed that K A/K B = α, Eq.


And

yields

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EXPERIMENT NO.6 Object – Study of different mechanism used for transforming rotary motion into translatory motion. (Application and sketching of Slider-crank mechanism, Cam mechanism, Rack & pinion mechanism, Nut & screw mechanism.) 1. Cam mechanism 2. Rack and pinion mechanism 3. Nut and screw mechanism These elementary transmissions are employed in feed mechanism of most of the Theory – These machine tools and also in the drives of machine tools have a reciprocating primary cutting motion. The Important elementary transmissions that are used in machine t ools for transforming rotary motion into translatory motion are:

1. Slider crank mechanism –

Fig: Slider crank mechanism The machine consists of a crank, connecting rod and slider. The forward and reverse stroke each take place during a revolution of crank therefore the need speed of forward and reverse speed in slider crank mechanism since metal removal occur during one stroke. It is desirable from the point of view of productivity to have a higher speed of the other stroke. Due to this property of slider crank mechanism is used only in an appreciable increase of productivity productivity e.g. in the driving of primary cutting motion of gear shaping machine the length of stroke may be change by adjusting the crank radius and is equal to L = 2R,

where R is the crank radius


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2. Crank and Rocker mechanism –

Figure. Crank and Rocker mechanism The crank and rocker mechanism consist of a rotating crank which makes the rocker arm oscillate by means of a block sliding along the groove in the rocker arm the clockwise rotation. The forward cutting stroke takes place during the clockwise rotation of the crank through angle ‘ ’ and the reverse stroke during rotation of the crank through angle ‘ ’ since “ ” and the crank rotation with uniform speed. The ideal stroke completes transfer than the cutting stroke. str oke. The length of stroke can be varied by b y adjusting the crank radius with a decrease in crank radius. The ratio of angle decrease and the speed of cutting and reverse stroke tend to become equal preferred in machine tool with large stroke (up to 1000 mm) where it can be effectively employed e.g. in drive of the primary cutting motion of shaping and slotting machine.



 >

⁄

Length of stoke can be calculated,



L = 2( ) R mm L = length of rocker e = offset distance R = radius of crank

3. CAM Mechanism –

Figure. Cam follower and link mechanism

The cam mechanism consists of a cam and a follower the cam mechanism provides the desired translatory motion is a suitable profile is selected. The profile may be provided. a. On the periphery of a disc-disc type t ype mechanism. b. On the face of a disc-face type cam mechanism. Department of Mechanical Engineering, RTU

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c. On a cylindrical surface-drum type cam mechanism. The main advantage of cam mechanical is that the velocity of the operative element is independent of the design of driving mechanism and is controlled by the cam profile.



In a disc type cam if the radius change from from R 1 to R 2 along an spiral while the cam rotate through angle , the velocity of the follower can be determined from the ex pression.

v= n = rpm of the cam

2− 

. 360.



m/min

R 1, 1,R 2 = radius mm In face or drum type cam mechanism the speed of the follower depends upon the steepness of the grove consider for instance. The profile development of drum cam segment a deplict the steep rise of follower corresponding to the rapid advanced segment deplict the slow rise corresponding to the steep full corresponding to the rapid withdraw of cutting tool.

ℎ ..

v = . h = rise during the working stroke

m/min

b = length of the working stroke D = diameter of the drum in mm s = rpm of drum

4 .Nut and Screw transmission –

Figure Schematic diagram of anti-friction nut and screw transmission A nut and screw mechanism is schematically depicted the screw and nut have a trapezoidal thread. The direction movement can reverse by reversing rotation of the screw. The nut and screw transmission is compact but has a high load carrying case capacity its other advantage are simplicity case of manufacturing the possibility of achieving slow and uniform movement of the operating member. The speed of operating member can be found from relationship Sm = t.k.n mm/min t = pitch of thread


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k = number of thread n = rpm of the screw

5. Rack and pinion transmission

Figure: rack and pinion transmission When the rotating gear meshes with a stationary rack, the centre of the gear moves in straight line on the other hand if the gear axis is stationary then the rack executes translatory motion. The direction of motion can be reversed by reversing the rotation of the pinion.



Sm = .z.n

mm/min

Sm = feed per minute of the operative member m = module of the pinion z = number of teeth of the pinion n = rpm of the pinion Rack and pinion transmission is the simplest and cheapest among all types of transmission used in reversible driven. It also has high efficiency and provides a large transmission ratio which makes it possible to use it in the feed as well as main drive mode.


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EXPERIMENT NO.7 Objective: Discuss various device for intermittent motion and draw the schematic diagram for various application. (Application and sketching of Ratchet gear mechanism, Geneva mechanism, Reversing mechanism, Differential mechanism, Norton mechanism, Mender’s mechanism.) 1. Ratchet gear mechanisms 2. Geneva mechanism 3. Reversing mechanism 4. Differential mechanism

Introduction – Devices for intermittent motion – In some machine tools, it is required that the relative position between the cutting tool and workpiece should change periodically. a .Machine tools with a reciprocating primary cutting motion e.g. shaping machine in which the workpiece must be intermittently upon completion of one full stroke of the cutting tool. b .machine tools with reciprocating feed motion.

1. Ratchet gear mechanism –

The Ratchet gear mechanism is generally consists of a pawl mounted on an oscillating pin. During each oscillation in the anticlockwise direction, the pawl turns the ratchet wheel through a particular angle. During the clockwise oscillating in the opposite direction, the pawl simply slides over the ratchet teeth and the latter remain stationary. The ratchet wheel is linked to the machine tool table through a nut and screw transmission. Therefore the periodic rotation of the ratchet wheel is transformed into the intermittent translator motion of the table for a particular nut and screw pair of some constant transmission ratio. The feed of the table during each oscillation depends upon the swing of the oscillating pawl. The rotation of the ratchet wheel in one stroke of the pawl should not exceed 45’. The ratchet gear mechanism is most suitable in case when the periodic displacement must be completed in a short time.


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2. Geneva mechanism -

Fig: Geneva mechanism Geneva mechanism consists of a driving disc which rotates continuously and a wheel a wheel with four radial slots. The arc on the driving disc and wheel provide a locking effect against rotation of the slotted wheel e.g. position of the wheel cannot rotate. As the disc continuous to rotate, point A of the disc comes out of contact with the arc and immediately thereafter pin ‘p’ mounted at the end of the driving arm enters the radial slot. The wheel now begins to rotate when it has turned an angle 90 the pin comes out of the radial slot and immediately thereafter point ‘B’ comes in contact with the next arc of the wheel preventing its further rotation. In the Geneva mechanism the angle of rotation of the wheel cannot varied. °

Application – (i) (ii) (iii)

Mainly used in torrents. Single spindle automatic machine for indexing cutting tools. Multi spindle automatic machines for indexing spindle through a constant angle.

3. Reversing mechanism –

These mechanisms are used for changing the direction of motion of the operative member. Reversing is accomplished generally through spur and helical gears. A few reversing arrangements using spur and helical gear. In this arrangement the gear on the driving shaft are mounted rigidly. While the idle gear and gears on the driven shaft are mounted freely. The jaw clutch is mounted on a key, rotation may be transmitted to the driven shaft ei ther through gear Department of Mechanical Engineering, RTU

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(A/B), (B/C) or through D/E depending upon whether the jaw clutch is shifted to the left to mesh with gear C or to the right to mesh with gear E. In the second arrangement, the gears on the driving shaft are again rigidly mounted and the idle gear is free. On the driven shaft a double cluster gear is mounted on a spline. By sliding the cluster gear transmission to the driven shaft may again be achieved either through gear (A/B), (B/C) or through gear pair D/E. In the third arrangement gear A on the driving shaft and gear D on the driven shaft are both rigidly mounted. A quadrant with constantly meshing gear B and C can be swivelling about the axis of the driven shaft. By swivelling the quadrant with the help of a lever transmission to the driven shaft may be achieved through (A/C), (C/D) or through (A/B) (B/C)(C/D).

4. Differential mechanism –

Differential mechanisms are used for summing two motions in machine tools in which operative member gets input from two separate kinematics trains. They are generally employed in thread and gear cutting machines where the machined surface is obtained as a result of the summation of two or more forming motions. A simple differential mechanism using spur or helical gears is shown. The mechanism is essentially a planetary gear mechanism consisting of sun gear A, planetary gear B and arm C. The planetary gear is mounted on the arm which can rotate about axis of gear A. suppose gear A makes nA and arm C, n C revolutions per minute in the clockwise direction. The relative motion between the elements of the mechanism will remain unaffected if the whole mechanism is rotated in the anticlockwise direction with n C revolution per minute. The transmission ratio of the mechanism may be written as nA – nC/nB – nC =- zB/zA Where zA and zB are the number of teeth of gear A and B, respectively. The above expression may be written as follows. nB = nC(1+zA/zB) – nA(zA/zB) Differential mechanisms are using a double cluster planetary gear. The mechanism consists of gear A, cluster gear block B-B’ mounted on arm C and gear D. If n A, n B, n C are the rpm’s of gear A, arm C and gear D, respectively then the transmission ratio of the kinematic train between gear A and D may be expressed as nD – nC/nA – nC = zA/zB . zB’/zD


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The mechanism consist of bevel gears A and D and planetary level gears B and C. Planetary gear can be rotated about the common axes of gear A and D. 1. By means of a ring gear – this differential is used in automobiles. 2. By means of a T- shaped shaft – this differential is used in machine tools. 3. if gear A,B and D make n A, nB, nD revolutions per minute, respectively, then the transmission ratio of the kinematic train between gear A and D can be written as nA – nB/nD – nB = - zA/zB . zB/zD Where zA, zB, zD are the number of teeth of gears A, B and D respect ively.


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EXPERIMENT NO.8 Objective: Aim of speed & feed rate regulation, stepped regulation of speed. Speed and Feed Rate Regulation:

Various Laws of Steeped Regulation:


31

Table: Diameter range for different rpm values in an A.P.


32

The rpm values constitutes a G.P.

Table: Diameter range fordifferent rpm values in G.P.


33

The rpm values constitutes a H.P.

Table shown below:


34

Table: Diameter range fordifferent rpm values in H.P.


35

EXPERIMENT NO.9 Object: Which Speed Series are used in machine tool gear box. (G.P. series is used in steeped regulation of speed.)

Gear boxes -Ap & Gp for steeping speeds of gears. -Structural Formula & Structural diagrams.

Gear boxes Machine tool characterized by their large number of spindle speeds and feeds of cape with the requirements of machine parts of different materials and dimension using different types of cutting tool materials and geometries. The cutting speed is determined on the bases of the cutting ability of the tool used. Surfaces finish required and economical consideration.

Speed Range for different Machine Tools

Machine

Range

Numerically Controlled lathes

250

Boring

100

Milling

50

Drilling

10

Surface Finish

4

Stepping of Speed According to Arithmetic Progression (AP) Let

 2   2 3 2  

Then

,

,…….., -

=

be arranged according to arithmetic progression.

-

= Constant

The saw tooth diagram in such a case is show in fig. Accordingly, for an economical cutting Speed , the lowest speed is not constant, it decrees with increasing dia. Therefore, the arithmetic progression does not permit economical machine at large diameter ranges. The main disadvantage of such an arrangement is that the percentage drop from step to step decrees as the speed increase. Thus the speed are not evenly distribution and more concentrated and closely stepped , in the small diameter range than in the large one. Stepping speeds according to arithmetic progression are used in Norton gear box with a sliding key when the number of shaft is only two.


36

Fig. 8.1 Speed stepping according to A.P.

Stepping of Speed According to Geometric Progression (GP) As show in Figure 8.2, the percentage drop from one step to the other is constant, and the absolute loss of economically expedient cutting speed ∆v is constant all over the whole diagram range. The relative loss of cutting speed ∆ / is also constant Geometric progression. Therefore, allow machining to take place between limits and independent of the WP diagram, where is the economical cutting speed and is the allowable minimum cutting speed. Now suppose that , ,…….., are the spindle speeds. According to the geometric progression

  2

       2 = 2 = ∅

Where Ø is the progression ratio. The spindle speed can be expressed in term of the minimal speed n1 and progression ratio Ø

n1

n2

n3

n4

nz

n1

n1 Ø

n1 Ø2

n1 Ø3

n1 Øz-1


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Fig.8.2 Speed stepping according to G.P.

Hence, the maximum spindle speed n z is given by

 = ∅− ∅ =  2 = √   = lloogg∅ 1

Where z is the number of spindle speed, therefore

ISO Standard values of progression ratio Ø (1.06, 1.12, 1.26, 1.4, 1.6, 1.78, 2.0) Justify ensuring with reason

1. Transmission ratio imax =2, imax =1/4, ig = imax/ imin=8 2. Minimum total shaft size

The torque transmitted by a shaft is given by

From the strength consideration:

 ∝ 1 2 = 2/3

3. For last radial dimensions of gear box imax* imin = 1 4. No of gears on last shaft should be minimum. 5. No of gears on any shaft should be limited to 3


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EXPERIMENT NO. 10 Object: Design Procedure of machine tool gear box design. (Design a four / six speed Gear Box.) Gear Box

Machine tools are characterized by their large number of spindle speeds and feeds to cope with the requirements of machining parts of different materials and dimensions using different types of cutting tool materials and geometries. The cutting speed is determined on the bases of the cutting ability of the tool used, surface finish required, and economical considerati ons. A wide variety of gearboxes utilize sliding gears or friction or jaw coupling. The selection of a particular mechanism depends on the purpose of the machine tool, the frequency of speed change, and the duration of the working movement. The advantage of a sliding gear transmission is that it is capable of transmitting higher torque and is small in radial dimensions. Among the disadvantages of these gearboxes is the impossibility of changing speeds during running. Clutch-type gearboxes require small axial displacement needed for speed changing, less engagement force compared with sliding gear mechanisms, and therefore can employ helical gears. The extreme spindle speeds of a machine tool main gearbox n max and nmin can be determined by

 = 1000   = 1000 

where Vmax = maximum cutting speed (m/min) used for machining the most soft and machinable material with a cutting tool of the best cutting property V min = minimum cutting speed (m/min) used for machining the hardest material using a cutting tool of the lowest cutting property or the necessary speed for thread cutting dmax, dmin = maximum and minimum diameters (mm) of WP to be machined The speed range R n becomes

 =  =  .  = 

R v = cutting speed range R d = diameter range In case of machine tools having rectilinear main motion (planers and shapers), the speed range R n is dependent only on R v. For other machine tools, R n is a function of R v and R d, large cutting speeds and diameter ranges are required. Generally, when selecting a machine tool, the speed range R n is increased by 25% for future developments in the cutting tool materials.

Design procedure for gear box

1. Determine the maximum and minimum speed of the output shaft. Then calculate the number of steps or speeds reduction stages for this range. This depends on the


39

application as well as space optimization. Higher reduction stages require more space because of more number of gears and shafts requirements. 2. Select types of speed reduction or gear box based on the power transmission requirements, gear ratio, and position of axis space available for speed reducer. Also make sure that for low gear ratio requires single speed reduction. Select worm gear for silent operation and level gear for interesting axis. 3. Determine the progression ratio which is ratio maximum speed and minimum speed of output shaft of the gear box the nearest progression ratio should be a standard one and it taken either from R20 or R40 series. 4. Draw the structural diagram and kinematic arrangement indicating various arrangement possibilities during speed reduction or increment.

5. Select materials for gears so that gear should sustain the operating condition and operating load. Normally cast iron is chosen for housing and cast steel or other all can be selected as per the load requirements. 6. Note down the maximum power output in horse power (H.P) or transmission power and revolution per minute of shaft i.e. rpm of each shaft.

7. Determine the centre distance between the driven and driver shaft based on the surface compressive stress. 8. Determine the module of gear by beam strength as well as fix the number of teeth required. 9. Calculate the diameter of the shaft by torque requirements and bending moment consideration. 10. Calculate the key size, shape or t ype of transmission key for each gears. 11. Select appropriate fit and tolerance for matting parts like shaft and gear. 12. Select bearing types or the loading and operating conditions. Also make sure to include consideration of maximum speed and expected life of gear and gear box.

13. Make the shaft stepped or provide collar to prevent axial displacement of bearing and gear. 14. Provide suitable clearance between gear and walls of the housing of gear box and based on this considerations design the casing/housing of gear box.


40

15. Complete the design of casing in drawing by providing fires if necessary to have increased heat transfer by convection and conduction. Put inspection hole/man hole as well as drain hole to drain lubricating oil. Also provide oil level indicator to have proper amount of oil during operation, if not out, this will lead to failure of gear a nd shaft due to over heating or due to friction failure. 16. Draw neat a clean working drawing in suitable software like auto cad, pro engineer etc. indicating required details during manufacturing or assembly. 17. One can also perform finite element analysis of the complete gear box after it completely designed.


41

EXPERIMENTS NO. 11 Objective: Study of Lathe Bed. (Design of Lathe bed. (Including Torque analysis of lathe bed, bending of lathe bed, designing for torsional rigidity, use of reinforcing stiffener in Lathe bed))

Figure. Lathe

Explanation of the standard components of most lathes: Bed: Usually made of cast iron. Provides a heavy rigid frame on which all the main components are mounted. Ways: Inner and outer guide rails that are precision machined parallel to assure accuracy of movement. Headstock: mounted in a fixed position on the inner ways, usually at the left end. Using a chuck, it rotates the work. Gearbox: inside the headstock, providing multiple speeds with a geometric ratio by moving levers.


42

Spindle: Hole through the headstock to which bar stock can be fed, which allows shafts that are up to 2 times the length between lathe centers to be worked on one end at a time. Chuck: 3-jaw (self centering) or 4-jaw (independent) to clamp part being machined. Chuck allows the mounting of difficult workpieces that are not round, square or triangular. Tailstock: Fits on the inner ways of the bed and can slide towards any position the headstock to fit the length of the work piece. An optional taper turning attachment would be mounted to it. Tailstock Quill: Has a Morse taper to hold a lathe center, drill bit or other tool. Carriage: Moves on the outer ways. Used for mounting and moving most the cutting tools. Cross Slide: Mounted on the traverse slide of the carriage, and uses a handwheel to feed tools into the work piece. Tool Post: To mount tool holders in which the cutting bits are clamped. Compound Rest: Mounted to the cross slide, it pivots around the tool post. Apron: Attached to the front of the carriage, it has the mechanism and controls for moving the carriage and cross slide. Feed Rod: Has a keyway, with two reversing pinion gears, either of which can be meshed with the mating bevel gear to forward or reverse the carriage using a clutch. Lead Screw: For cutting threads. Split Nut: When closed around the lead screw, the carriage is driven along by direct drive without using a clutch. Quick Change Gearbox: Controls the movement of the carriage using levers. Steady Rest: Clamped to the lathe ways, it uses adjustable fingers to contact the workpiece and align it. Can be used in place of tailstock or in the middle to support long or unstable parts being machined. Follow Rest: Bolted to the lathe carriage, it uses adjustable fingers to bear against the work piece opposite the cutting tool to prevent deflection.

Table below showing commonly used bed section and wall arrangement and their applications.


43

Wall Arrangement

Applications 1.) Beds on logs or sheas a.) without stiffening, diagonal wall, used in lathe, turrets, etc. b.) without stiffening diagonal wall has 3040% height stiffness than arrangement (a); used in multiple tool and height production lathes.

(a)

(b) c.) with stiffening wall and provision of chip disposal through opening in rear wall, used in large sized lathes & turret with stiffing wall, also used in large-sized laths. (d) With stiffening wall also used in large size lathe and turret

(c)

(d)

2.) Covered top closed profile bed, used in plan milling, clothing & boring machines.

3.) Open top closed profile bed, used when the bed is also require, commonly employed in grinding machine.


44

EXPERIMENT NO. 12 Object: Free body diagram of following machines. (Analysis of force under headstock, tail stock and saddle.) (1) Lathe (2) Drilling (3) Shaping (4) Milling

Introduction Lathe The lathe is a machine tool used principally for shaping articles of metal (and sometimes wood or other materials) by causing the workpiece to be held and rotated by the lathe while a tool bit is advanced into the work causing the cutting action. The basic lathe that was designed to cut cylindrical metal stock has been developed further to produce screw threads, tapered work, Drilled holes, knurled surfaces, and crankshafts. The typical lathe provides a variety of rotating speeds and a means to manually and automatically move the cutting tool into the workpiece. Machinists and maintenance shop personnel must be thoroughly familiar with the lathe and its operations to accomplish the repair and fabrication of needed parts.

Types of lathe Lathes can be divided into three types for easy identification: engine lathes, turret lathes, and special purpose lathes. Small lathes can be bench mounted, are lightweight, and can be transported in wheeled vehicles easily. The larger lathes are floor mounted and may require special transportation if they must be moved. Field and maintenance shops generally use a lathe that can be adapted to many operations and that is not too large to be moved from one work site to another. The engine lathe is ideally suited for this purpose. A trained operator can accomplish more machining jobs with the engine lathe than with any other machine tool. Turret lathes and special purpose lathes are usually used in production or job shops for mass production or specialized parts. While basic engine lathes are usually used for any type of lathe work. Further reference to lathes in this chapter will be about the various engine lathes.


45

Figure. FBD of Lathe

Drilling Machine A drilling machine comes in many shapes and sizes, from small hand-held power drills to bench mounted and finally floor-mounted models. They can perform operations other than drilling, such as counter sinking; counter boring, reaming, and tapping large or small holes. Because the drilling machines can perform all of these operations, this chapter will also cover the types of drill bits, took, and shop formulas for setting up each operation. Safety plays a critical part in any operation involving power equipment. This chapter will cover procedures for servicing, maintaining, and setting up the work, proper methods of selecting tools, and work holding devices to get the job done safely without causing damage to the equipment, yourself, or someone nearby. A drilling machine, called a drill press, is used to cut holes into or through metal, wood, or other materials. Drilling machines use a drilling tool that has cutting edges at its point. This cutting tool is held in the drill press by a chuck or Morse taper and is rotated and fed into the work at variable speeds. Drilling machines may be used to perform other operations. They can perform countersinking, boring, counter-boring, spot facing, reaming, and tapping. Drill press operators must know how to set up the work, set speed and feed, and provide for coolant to get an acceptable finished product. The size or capacity of the drilling machine is usually determined by the largest piece of stock that can be center-drilled. For instance, a 15inch drilling machine cans center-drill a 30-inch-diameter piece of stock. Other ways to determine the size of the drill press are by the largest hole that can be drilled, the distance between the spindle and column, and the vertical distance between the worktable and spindle. All drilling machines have the following construction characteristics: a spindle, sle eve or quill, column, head, worktable, and base. 1. The spindle holds the drill or cutting tools and revolves in a fixed position in a sleeve. In most drilling machines, the spindle is vertical and the work is supported on a horizontal table. 2. The sleeve or quill assembly does not revolve but may slide in its bearing in a direction parallel to its axis. When the sleeve carrying the spindle with a cutting tool is lowered, the cutting tool is fed into the work: and when it is moved upward, the cutting tool is withdrawn from the work. Feed pressure applied to the sleeve by hand or power causes the revolving drill to cut its way into the work a few thousandths of an inch per revolution. 3. The column of most drill presses is circular and built rugged and solid. The column supports the head and the sleeve or quill assembly.


46

4. The head of the drill press is composed of the sleeve, spindle, electric motor, and feed mechanism. The head is bolted to the column. 5. The worktable is supported on an arm mounted to the column. The worktable can be adjusted vertically to accommodate different heights of work. or it may be swung completely out of the way. It may be tilted up to 90° in either direction, to allow for long pieces to be end or angled drilled. 6. The base of the drilling machine supports the entire machine and when bolted to the floor, provides for vibration-free operation and best machining accuracy. 7. The top of the base is similar to a worktable and maybe equipped with T-slots for mounting work too large for the table.


47

FBD of Drilling Machine

Shaping Machine The main functions of shaping machines are to produce flat surfaces in different planes. The cutting motion provided by the linear forward motion of the reciprocating tool and the intermittent feed motion provided by the slow transverse motion of the job along with the bed result in producing a flat surface by gradual removal of excess material layer by layer in the form of chips. The vertical infeed is given either by descending the tool holder or raising the Department of Mechanical Engineering, RTU

48

bed or both. Straight grooves of various curved sections are also made in shaping machines by using specific form tools. The single point straight or form tool is clamped in the vertical slide which is mounted at the front face of the reciprocating ram whereas the workpiece is directly or indirectly through a vice is mounted on the bed.

Fig. Shaping Machine

Milling Machine Milling is the process of machining flat, curved, or i rregular surfaces by feeding the workpiece against a rotating cutter containing a number of cutting edges. The milling machine consists basically of a motor driven spindle, which mounts and revolves the milling cutter, and a reciprocating adjustable worktable, which mounts and feeds the workpiece. Milling machines are basically classified as vertical or horizontal. These machines are also classified as kneetype, ram-type, manufacturing or bed type, and planer-t ype. Most milling machines have selfcontained electric drive motors, coolant systems, variable spindle speeds, and power-operated table feeds. Free body diagram of the milling machine shown in figure below


49

Fig. Milling Machine


50

EXPERIMENT NO.13 Object: Application of slide ways profiles and their combinations. (Design of Guide ways / Slide ways.)

FUNCTIONS OF GUIDEWAYS The Guideway is one of the important elements of machine tool. The main function of the guideway is to make sure that the cutting tool or machine tool operative element moves along predetermined path. The machine tool operative element carries workpiece along with it. The motion is generally circular for boring mills, vertical lathe, etc. while it is straight line for lathe, drilling, boring machines, etc. Requirements of guideways are: (a) Guideway should have high rigidity. (b) The surface of guideways must have greater accuracy and surface finish. (c) Guideways should have high accuracy of travel. It is possible only when the deviation of the actual path of travel of the operative element from the predetermined normal path is minimum. (d) Guideways should be durable. The durability depends upon the ability of guideways to retain the initial accuracy of manufacturing and travel. (e) The frictional forces acting on the guideway surface must be low to avoid wear. (f) There should be minimum possible variation of coefficient of friction. (g) Guideways should have good damping properties.

Guideways can be classified as: (a) Guideways with sliding friction (b) Guideways with rolling friction

Guideways with Sliding Friction

The friction between the sliding surfaces is called as guideways with sliding friction. These guideways are also called as slideways. The slideways are further classified according to the lubrication at the interface of contacting surfaces. The friction between the sliding surfaces may be dry, semi-liquid, and liquid. When the lubrication is absent in between contacting surfaces, it is called as dry friction. Dry friction is rarel y occurred in machine tools. When two bodies slide with respect to each other having lubrication between them, the sliding body tends to rise or float due to hydrodynamic action of the lubricant film. The principle of slider is shown in Figure 12.1.


51

Figure12.1 Principle of a Slider

The hydrodynamic force,

ℎ =  ∗

(1)

Where C is constant and depends upon wedge angle θ, the geometry of sliding surfaces, viscosity of the lubricant and parameter of lubricant film.

v

is sliding velocity.

W

is weight of the sliding body.

s

The resultant normal force acting on sliding body, R

= F h – W

From Eq. (1), it is clear that the hydrodynamic force increases with increase in sliding velocity. The sliding body rests on the stationary body when hydrodynamic force is less than the weight of the sliding body. Here, there are semi-liquid type friction conditions and under these conditions the two bodies are partially separated by the lubricant film and partiall y have metal to metal contact. The resultant normal force on sliding body starts to act upwards and the body floats as hydrodynamic force is greater than the sliding weight of the body. The sliding surfaces are completely separated by the lubricant film and liquid friction occurs at their interface. The slideways in which the sliding surfaces are separated by the permanent lubricant layer are known as hydrodynamic slideways. This permanent lubrication layer is due to hydrodynamic action. A permanent lubricant layer between the sliding surfaces can be obtained by pumping the liquid into the interface under pressure at low sliding speed. The sliding body is lifted by this permanent lubricant layer. Such slideways are call ed as hydrostatic slideways.


52

Guideways with Rolling Friction

These are also called as anti friction ways. The anti friction slideways may be classified according to the shape of the rolling element as: (a) Roller type anti friction ways using cylindrical rollers. (b) Ball type anti friction ways using spherical balls.

DESIGN OF SLIDEWAYS

Slideways are designed for wear resistance and stiffness. 1. Design of Slideways for Wear Resistance

The wear resistance of slideways is mainly dependent upon maximum pressure acting on the mating surfaces. This condition may be given as

pmax ≤ pmp

(2)

Where

pm = maximum pressure acting on the mating surface, and pmp = permissible value of the maximum pressure. It is seen during the subsequent analysis that slideway designed for maximum pressure is quite complicated. Sometimes, this design is replaced by a si mple procedure based upon the average pressure acting on the mating surfaces. The condition is that:

Pa ≤ Pap

(3)

Where Pa = average pressure acting on the mating surface, and Pap = permissible value of the average pressure. Hence from Eqs. (2) and (3), the design of slideways for wear resistance requires that. (a) pm and pa to be known, (b) pmp and pap to be known, and (c) The values of pmp and p ap are given for different operating conditions of slideways on the basis of experience. For determining pm and pa, the first and foremost task is to determine the forces acting on the mating surfaces. Forces acting on the mating surfaces in combination of V and flat slideways.

The combination of V and flat slideways is commonly used in lathe machines. The schematic diagram of slideways and the forces acting on the system for the case of orthogonal cutting are illustrated in Figure 12.2.


53

Figure 2 Forces Acting on Combination of V and Flat Sideways

The forces acting on V and flat slideways are : (a) Cutting force component Fz (in the direction of the velocity vector) and F y (radial), (b) Weight of carriage W, and (c) Unknown forces F1, F2 and F3 acting on the mating surfaces. The unknown forces are calculated from fol lowing equilibrium conditions: Sum of components of forces acting along Y-axis = 0

Substituting value of F3 in Eq., we get


54

If the apex angle of the V is 90 o, and assume that present angle γ may change to γ = 90 – λ, the solution of simultaneous algebraic Eqs. gives :

Substituting the values of F1 and F2 in Eq., we get,

Above eq. represents the forces acting on the mating surfaces in combination of two flat sideways. The schematic diagram of the slideways and the forces acting on the system under orthogonal cutting conditions are shown in Figure 12.3.

Fig.12.3 Forces Acting on Combination of Two Flat Slideways The forces acting on combination of two flat slideways are : (a) Cutting force components, i.e. axial Fx, radial Fy, and Fz in the direction of velocity vector. (b) Weight of carriage, W. (c) Unknown forces F1, F2 and F3 acting on the mating surfaces. (d) Frictional forces μF1, μF2, μF3, where μ is the coefficient of friction between the sliding surfaces.


55 The unknown forces F1, F2 and F3 are calculated from following equilibrium conditions :

From above eq.

On substituting the value of F3 in Eq.


56

Slideway Profile and Combinatio n for Bads

Open V Open V

Sketch

+

Application

Planning Machines

Closed V + Closed V

Precision lathes and turret lathes

Open flat + Open V

Surface-grinding machines

Closed flat + Closed V

Genral-purpose lathes & heavy duty boring machine

For vertical columns

Closed flat + Closed flat

Most Commonly used for all types of vertical columne


Knee types milling machine small vertical drilling machine and traverses of radial drilling machine


57

Closed flat + heavy-closed dovetail

Same us above

For cross slides and compound rests

Closed devotail

Cross slides & compound rests


Cross slides of heavy duty machine tools

For Rotary Blades

Flat

Surface-grinding machine and small hobbing machine

W

Precission gearhobby machine


58

Shapes of slideways

(a)

(b)

(c)

(d)

Slideways profiles: (a) Flat; (b) Symmetrical V; (c) Asymmetrical V; (d) dovetail (e) Cylindrical


(e)

59

EXPERIMENT NO. 14 Object: Draw a neat schematic diagram of herring bone gear and explain Introduction A herringbone gear, a specific type of double helical gear, is a special t ype of gear that is a side to side (not face to face) combination of two helical gears of opposite hands. From the top, each helical groove of this gear looks like the letter V, and many together form a herring bone pattern (resembling the bones of a fish such as a herring). Unlike helical gears, herringbone gears do not produce an additional axial load. Like helical gears, they have the advantage of transferring power smoothly because more than two teeth will be in mesh at any moment in time. Their advantage over the helical gears is that the side-thrust of one half is balanced by that of the other half. This means that herringbone gears can be used in torque gearboxes without requiring a substantial thrust bearing. Because of this herringbone gears were an important step in the introduction of the steam turbine to marine propulsion. Precision herringbone gears are more difficult to manufacture than equivalent spur or helical gears and consequently are more expensive. They are used in heavy machinery. Where the oppositely angled teeth meet in the middle of a herringbone gear, the alignment may be such that tooth tip meets tooth tip, or the alignment may be staggered, so that tooth tip meets tooth trough. The latter alignment is the unique defining characteristic of a Wuest type herringbone gear, named after its inventor.


60

Benefits Since a herringbone gear is non-linear in the teeth the gears won't slip out from grabbing one another if the axle or another force moves the gears up and down. This is also a benefit with machinery that needs very straight movement, because a herringbone gear is designed to 'self center' and is much less likely to skip a tooth or fall out of place. With some gears sets that use herringbone gears; an axle can be lost and the gear will stay in place, a herringbone planetary gear system.

Manufacture A disadvantage of the herringbone gear is that it cannot be cut by simple gear hobbing machines, as the cutter would run into the other half of the gear. Solutions to this have included assembling small gears by stacking two helical gears together, cutting the gears with a central groove to provide clearance, and (particularly in the early days) by casting the gears to an accurate pattern and without further machining. With the older method of fabrication, herringbone gears had a central channel separating the two oppositely-angled courses of teeth. This was necessary to permit the shaving tool to run out of the groove. The development of the Sykes gear shaper made it possible to have continuous teeth with no central gap. Sunderland, also in England, also produced a herringbone cutting machine. The Sykes uses cylindrical guides and round cutters; the Sunderland uses st raight guides and rack-type cutters. The W. E. Sykes Co. dissolved in 1983 – 84. Since then it has been common practice to obtain an older machine and rebuild it if necessary to create this unique type of gear. Recently, the Bourn and Koch Company has developed a CNC-controlled derivation of the W. E. Sykes design called the HDS1600-300. This machine, like the Sykes gear shaper, has the ability to generate a true apex without the need for a clearance groove cut around the gear. This allows the gears to be used in positive displacement pumping applications, as well as power transmission. Helical gears with low weight, accuracy and strength may be 3D printed . The herring bone gear is essentially a pair of helical gear in which the helix angel is oppositely direct. In a gear transmission, the rpm of the drives shapes is determined as

2 =  . 2  2  2  2 Where

=rpm of the driven shaft =rpm of the driving shaft

=no. of teeth of the drawing gear =no. of teeth of the driven gear

The ratio

/

is known as the transmission ratio of the gear driven and is constant for a

particular gear pair.


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EXPERIMENT NO.15 Object: Description of stick-slip and sliding friction in machine tool design. Stick-slip Friction

Stick-slip can be described as surface alternate between sticking to each other and sliding over each other with a corresponding change in the force of f riction coefficient between two surfaces is larger than the reduction of the friction to the kinetic friction can cause a sudden jump in the velocity of the movements. The attached picture shows symbolically an example of stick-slip. V is the drive system, R is the elasticity in the system and M is the load i.e. lying on floor and is being pushed horizontally. When the drive is started, the spring R is loaded and its pushing force against load M increases until the static friction coefficient between load M and floor is not able to hold the load anymore. The load start sliding and the friction coefficient decreases from its static value to its dynamic value. At this moment, the spring can give more power and accelerate M. During M’s movements, the force of the spring decreases, until it is insufficient to the overcome the dynamic friction. From this point M de-accelerate to a stop. The drive system however, continues and the spring is loaded again etc.

Fig.- Stick-Slip Phenomenon

Sliding (motion) Friction

Sliding is a type of friction motion between two surfaces in contact. This can be constructed to rolling friction. Both types of motion may occur in bearing. Friction may damage or wear the surface in contact. However, it can be reduced by lubricat ion. The science and technology of friction, lubrication, and wear is known as tribology.

Sliding may occur between two objects of arbitrary shape, whereas rolling friction is the friction force associated with the rotational movement of a somewhat dislike or other circular object along the surface.


Machine Tool Design Lab Manual

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