ROUNDNESS
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Roundness
Roundness refers to a condition of a circular line or the surface of a circular feature wherein all points on the line or on the periphery of a plane cross section of the feature, are equidistant from a common center point . Examples : Disc, Sphere , Cylinder Cylinder,, Cone
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Errors of Roundness
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Errors of Roundness
Ovality : Difference appear in major and minor axes.
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Errors of Roundness
Lobing: Small Variation in diameter as shown in figure.
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Errors of Roundness
Irregularity : Random irregularities from a true Circles
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ROUNDNESS SYMBOL
The
geometrical characteristic symbol for roundness is simply a circle, having a diameter 75% of the feature control symbol frame height.
ROUNDNESS SYMBOL 7
ROUNDNESS TOLERANCE
The
variation should lie within the width of the Annular space between two concentric circles
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ROUNDNESS TOLERANCE
Incorrect
Incorrect
Correct
Roundness error is min. radial separation between two concentric circles within which all points on measured surface to lie. 9
ROUNDNESS ERROR MAY EXCEEDS SIZE BOUNDARY
Measurements between any two opposing points along the circumference shall be within the specified diameter tolerance limits.
The outer diameter of the roundness tolerance zone exceeds the actual measured diameter of the part by the amount of roundness tolerance.
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ROUNDNESS OF CYLINDERS AND SPHERES
ROUNDNESS FOR A CYLINDRICAL FEATURE
INTERPRETATION OF ROUNDNESS TOLERANCE
It is preferable to direct a roundness tolerance for a cylindrical feature to the view in which the feature appears as a circle.
The tolerance applies to all planes perpendicular to the axis.
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ROUNDNESS TOLERANCE APPLIED TO A SPHERE
The tolerance is shown in the same manner and applies to any or all planes, which pass through a section of maximum diameter.
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ROUNDNESS OF NON-CYLINDRICAL PARTS
Non-cylindrical
parts refer to conical parts and other features which are circular in cross-section but which have variable diameters. Since
many sizes of circles may be involved it is usually best to direct the roundness tolerance to the longitudinal surface as shown.
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ROUNDNESS MEASURING PRINCIPLE
The
measurement of roundness presents some problems, as it does not lend itself to direct measurement. Indirect
measurement involves establishing the relationship of the periphery of a feature with the geometry of a perfectly round form, regardless of its size or the exact position of its center. It
is immaterial whether the part is revolved in contact with a fixed indicator or whether the indicator is revolved around the part. 14
POLAR CHART & TRANSPARENT OVERLAY CHART
POLAR CHART
PROFILE OF PART
TRANSPARENT OVERLAY CHART
The indicator readings are entered directly on polar chart during roundness measurement of cylinder.
The profile is evaluated by means of transparent overlay chart on which concentric circles are scribed to the same scale as the polar chart.
Note : There are a number of commercial instruments available, based on optical, mechanical, or electronic principles, some of which produce a polar chart automatically as the part is revolved 15
ALTERNATIVE MEASURING PROCEDURES
(NOT) RECOMMENDED
It
is sometimes suggested that parts be checked for roundness by revolving them in suitable V-block, while measuring the upper surface with an indicator gage This
method does not measure in accordance with the definition of roundness, and is therefore not recommended for precise results. 16
ALTERNATIVE MEASURING PROCEDURES
An estimate of out of roundness errors can sometimes be obtained by making separate measurements on a part in V-blocks having different included angles, for example 180 , 120 , 90 and 60 .
If all measurements show little or no indicator movement it might be assumed that the part is satisfactory.
Full indicator reading is approximately equal to measurement over a diameter, instead of a radius. The roundness error will therefore be roughly half the indicator movement.
(NOT) RECOMMENDED
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ALTERNATIVE MEASURING PROCEDURES
USE OF TWO – BLOCKS To
avoid errors of readings due to bending of the parts it may be
necessary to employ two narrow vee-blocks. In this method one of the vee-blocks must always be directly under the point of measurement. 18
ALTERNATIVE MEASURING PROCEDURES
HOW LOBING CAUSES ERRORS IN MEASURING
V-blocks (not)recommend -Reasons Five Lobed part in 60 V-block-- indictor reading practically zero even though the component is out of round
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ALTERNATIVE MEASURING PROCEDURES
V-blocks (not)recommend -Reasons Three Lobed part in 60 V-block-- indictor reading greatly exaggerated for small variation of roundness.
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ROUNDNESS OF INTERNAL DIAMETERS
(NOT RECOMMENDED) ROUNDNESS OF INTERNAL DIAMETERS
Round holes can be toleranced for roundness in the same manner as external cylindrical features, If
the holes are large enough for insertion of a gaging probe, similar methods used for external measurement can be used. Indicator gauges used for measurement of internal roundness for bigger hole diameters. Drawback of this method is same as that of V-blocks For small holes, where suitable gaging probes or indicators cannot be used, it is recommended that roundness tolerances be replaced by cylindricity tolerances of zero MMC 21
ROUNDNESS ON MMC BASIS
It
is often desirable to ensure that any errors of roundness do not cause the periphery of the feature to cross the maximum material boundary.
This
shall ensure that the part will assemble satisfactorily with its mating part.
This
is accomplished by specifying a roundness tolerance on an MMC basis.
A
tolerance on this basis is generally directed to the diametric dimension, such as the zero MMC tolerance.
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ROUNDNESS ON AN MMC BASIS
ROUNDNESS TOLERANCE ON MMC BASIS
Max. Dia. of Tolerance Zone Min. Dia. of Part Minor Dia. of Tolerance Zone Mean Tolerance Zone
= 50.00mm = 49.88mm = 49.76mm = 0.12mm
PART MAY CROSS THE LEAST MATERIAL BOUNDARY 23
