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11.4.0.1 - Roundness Testing

•Roundness is of particular importance when designing components for fit and function.

•Most of the methods considered so far are suited to measuring with single points, but a round shape is a collection of points, with each point having significant influence if out of tolerance.

•Precise roundness measurement equipment is expensive

•Two fundamental methods for measuring roundness are,

-Intrinsic - uses points on the round surface to measure from

-Extrinsic - uses a separate round surface for a reference (e.g. a precision bearing)

11.4.0.1.1 - Intrinsic Roundness Testing

• Three methods for Intrinsic roundness testing are shown below,

	90	0
		10

80	Dial Indicator
20

70

30

60

40 50

Diametrical Intrinsic Method

A dial indicator is positioned over the surface to a reference height. The part is then rolled

underneath. The peak height can then be Rolled this way

compared to other readings.

dia.

Datum Point

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	90	0
		10

80 20

70

30

60

40 50

Rolled this way

0

90

80

70

60

50

Dial Indicator

Vee Support Intrinsic Method

A dial indicator is positioned over the surface to a reference height. The part is then rolled underneath. The peak height can then be compared to other readings. The Vee support reduces the effect of a single datum point.

Datum Point

10 Between Centres

A dial indicator is positioned over the surface

20to a reference height. The part is then rotated on centres. The variations in the readings are

30	then used to evaluate the part. Location of
	the centre may lead to problems.

40

• All three of the intrinsic methods are inexpensive

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• The Intrinsic methods all have an important limitation. In particular, if the deformation of the round is small, the methods will deal with it reasonably, but if the deformation is large enough to make the shape non-cylindrical, then the results will err significantly.

With this test the two readings shown would indicate roundness, when in fact this is not true

This test would exaggerate the roundness error such that it would be greater than the actual error

• When using The Flat Plane, or the Centre to intrinsically measure roundness, the diameters can be directly obtained, but when using the Vee block, some additional calculations are required.

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indicator reading (IR)

A

B

h1

h0

θ θ

IR =	change in centre height + change in radii
=	( h1 – h0) + ( A – B)	=	A	B
			---------- – ---------- + ( A – B)
			sin θ	sin θ

= A – B( csc θ ) + ( A – B)

IR = A – B( 1 + csc θ )

where,

θ= 1/2 vee block angle

•The vee block method has particular disadvantages,

-a number of angles are required (the standard angle is 90°)

-only suitable for regular odd lobed figures

•The centre support method also has disadvantages,

-The part may be bowed, or warped

-off centre or degraded centre holes will decrease reading quality

-the centres themselves can also affect readings

11.4.0.1.2 - Extrinsic Roundness Testing

•The features of this methods are,

1.the reference datum is not points on the object, but a separate precision bearing

2.The axis of the part being measured is aligned with the machine bearing axis

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3.A stylus is moved in to contact the part, and then it moves about in a circular path

4.The deflection of the stylus is amplified onto a polar plot to be used in evaluation of the part

•We can measure the out of roundness value as the minimum distance between two concentric circles that enclose/envelope the trace profile. This distance must obviously be divided by the magnification.

•Only roundness deviations are amplified. This creates distortions in the trace.

•The Talyrond machine also uses a low pass electronic filter to reduce the roughness that is shown on the plot. But this still shows the lobing.

•Eccentricity - the talyrond can also be used to detect concentricity. A simple example is a bearing race shown below.

the stylus measures the profile for both the inside and outside, and then these can be compared to determine concentricityXXXXXXX

stylus

inside dia.

outside dia.

• An example of the part discussed above, is now shown in a trace from the Talyrond