Acceptance Tests and Maintenance of Machine Tools Shour Shourav av Ahmed Ahmed Lecturer, Department of IPE
Introduction •
The basic function of a machine tool is to • • •
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produce a workpiece of the required geometric form with an acceptable surface finish at high rate of production
To evaluate the relative merits of machine tools it is essential to perform the acceptance tests Machine tools, which satisfy the specified accuracies in the standards of machine tool acceptance tests, will produce components that will meet the requirements of modern production in accordance with standard limits and fits Acceptance Testing of a Machine Tool can be defined as checking alignment of a component relative to the other component. It includes: 1. 2.
Geometrical tests or alignment tests, Dynamic performance tests or practical tests.
What is checked during Geometrical Tests or alignment Tests? 1.
Straightness
2.
Flatness
3.
Parallelism, equidistance and coincidence
4.
Squareness of the straight lines and planes
5.
Rotation: Eccentricity, out of true running, out of round etc.
6.
Movement of all working components.
Equipment for Geometrical Tests: •
Dial Gauges
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Test Mandrels
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Straight edges and Squares
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Spirit Levels
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Autocollimator
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Optical Alignment telescope
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Waviness-Meter
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Water level
Geometrical testing of a Lathe Machine 1. Leveling of Machine bed 2.
Parallelism of spindle axis to bed
3.
Straightness of carriage movement in horizontal plane
4.
Parallelism of tailstock quill movement to the carriage movement.
5.
Parallelism of spindle axis to the carriage movement.
6.
Difference in the height between headstock and tailstock.
7.
Parallelism of longitudinal movements of tool slide to the spindle axis.
8.
Run –out of spindle nose - centering sieve or cone.
9.
True running of the taper bore of the spindle.
10. Squareness of the transverse movement of the cross-slide to the spindle axis. 11. Axial slip of lead screw. 12. Parallelism of tailstock guideways with the movement of carriage. 13. Axial slip of main spindle and true running of shoulder face of spindle nose. 14. True running of headstock center. 15. Accuracy of pitch of lead screw
Geometrical testing of a Lathe Machine •
Leveling of Machine bed: a) Longitudinal direction b) Transverse direction.
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Measuring equipment: Spirit level
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Permissible error: 0.01 to 0.02 mm
Geometrical testing of a Lathe Machine •
Parallelism of spindle axis to bed.
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Measuring equipment: Test mandrel and dial gauge.
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Permissible error: 0.08 mm per meter
Geometrical testing of a Lathe Machine •
Straightness of carriage movement in horizontal plane
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Measuring equipment: Dial gauge and test mandrel.
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Permissible error: 0.015 to 0.02 mm
Geometrical testing of a Lathe Machine •
Parallelism of tailstock quill movement to the carriage movement
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Measuring equipment: Dial gauge
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Permissible error: 0.02 to 0.04 mm
Geometrical testing of a Lathe Machine •
Run-out/ true running of spindle nose
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Measuring equipment: Dial gauge
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Permissible error: 0.01mm
Geometrical testing of a Lathe Machine •
Parallelism of tailstock guideways with the movement of carriage.
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Measuring equipment: Dial gauge and a block
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Permissible error: 0.015 to 0.02 mm
Dynamic Performance tests or Practical tests According to approved general specifications, performance tests or practical acceptance tests of machine tools should include: 1. Idle run tests, operation checks of mechanisms 2. Load tests and productive output tests, 3. Checks of the geometrical accuracy, surface roughness, and accuracy of the work-pieces being machined 4. Rigidity tests of machine tools 5. Tests for vibration proof properties of machine tools in cutting
Dynamic performance test for lathe The dynamic performance of a machine tool can be compared by the results of cutting tests. These cutting tests have procedure must be followed accurately as follows:
1.
Condition of the Lathe: i. Mounting: The lathe to be tested must be either mounted as specified by the manufacture, or as envisaged when in normal use. ii. Locking of joints and sliding elements: All bolted joints, and sliding elements not required to be in motion during the test must be locked or clamped. Also, the tailstock, if not in use, must be either locked at the far end of the slideway, or removed. iii. Warm-up time: Before any test the spindle must be run at two-thirds maximum speed for one hour. The temperature of the bearing housing nearest the workpiece should then be recorded and this temperature maintained through all the tests to within ±2°C. iv. Ambient temperature: The ambient temperature must have been kept within ±5°C of a mean ambient temperature, which is recommended as 20°C, for at least 12 hours before the tests are commenced.
Dynamic performance test for lathe 2.
Workpiece: i.
ii. iii.
Material: If the lathe is intended for machining a particular material then this should be used for the workpiece; otherwise a normalized carbon steel should be used. All workpieces for test must be manufactured from the same bar. Initial preparation: The outer 5 mm of the stock as supplied must be removed before the workpiece is manufactured. Shape: If the lathe is intended for machining a particular shape of workpiece then this should be used for the tests. For else, the following shapes and sizes of workpiece are recommended for “turning” . The diameter ‘ Dm’ refers to the maximum diameter of workpiece that may be machined.
Dynamic performance test for lathe 3.
Cutting Tool:
Geometry: Considerable care should be taken to maintain cutting angles, radii and tool material characteristics constant ii. Tool Material : The tip grade must be kept constant for the same series of tests. iii. Fixing of tips: The position of the tip in the toolholder must be carefully maintained for each series of tests. iv. Tool height: The tool center height at the start of a test must be kept within +0.005 R of the true workpiece center i.
Dynamic performance test for lathe 4. Cutting condition: Approach angle: For all parallel turning and boring an approach angle of 60° or 75° should be used and for facing operations an approach angle of 0°, as shown in figure ii. Position of toolpost: For parallel turning and initial position when facing, the leading edge of the top slide should be made level with the cross slide iii. Speed: The recommended speeds according to the used workpiece material should be used. iv. Feed: Whenever possible a feed in the range 0.1 - 0.25 mm/rev should be used. However, if, in facing operations, the use of such a feed would lead to an excessive diameter reduction a smaller feed should be used. i.
The maintenance of the machine tool includes: •
Checking the accuracy of the finished workpiece.
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Lubrication of the moving components.
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Checking clearances between slideways.
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Preparation of materials and component parts necessary for repair work. Any repair work must be carefully planned beforehand. Instructions to machine shop foremen and operators in order to ensure correct use of the machines.
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Provision for emergency repairs.
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Estimates of maintenance and repair cost.
