Roundness Testing Week 9: November 15, 2011 Metrology & - - PowerPoint PPT Presentation

Intrinsic and Extrinsic Roundness Testing

Week 9: November 15, 2011 Metrology & Instrumentations Fall 2011

• Dr. Belal Gharaibeh

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The difference between intrinsic and extrinsic properties

• An intrinsic property is an essential or inherent

property of a system or of a material itself or

• within. It is independent of how much of the

material is present and is independent of the form the material

• A property that is not essential or inherent is

called an extrinsic property.

• For example, mass is a physical intrinsic property
• f any physical object, whereas weight is an

extrinsic property that varies depending on the strength of the gravitational field in which the respective object is placed, http://en.wikipedia.org/wiki/Intrinsic_and_extrin sic_properties_(philosophy)

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• 1. 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 compared to other readings.

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• 2. 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

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• 3. Between Centers
• A dial indicator is positioned over the surface to a reference height.
• The part is then rotated on centers.
• The variations in the readings are then used to evaluate the part.
• Location of the center may lead to problems

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Intrinsic methods limitations

• All three of the intrinsic methods are inexpensive
• 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.

If two readings are taken for the case shown, both readings will indicate roundness, while in reality this is not true

Reading 1 Reading 2 Method one limitation:

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Method 2 limitations

This test will give large roundness difference reading due to fact that the part in non-cylindrical

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Diameter measurements

• When using The Flat Plane, or the Center to intrinsically measure

roundness, the diameters can be directly obtained,

• but when using the Vee block, some additional calculations are required

                 

    csc 1 csc ) sin( ) sin( radii in change height center in change

1

                 B A IR B A B A IR B A B A B A h h IR IR

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Other disadvantages (Vee block and center support methods)

• 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 center support method also has

disadvantages:

• The part may be bowed, or warped
• off center or degraded center holes

will decrease reading quality

• the centers themselves can also

affect readings

Even lobed Warped shape

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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

• 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

• This method of testing is used for large cylinders and assembled

cylinders

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Eccentricity

• Eccentricity is the deviation of multiple circles

from a common center

• Concentricity is the opposite word for

eccentricity

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Output reading of eccentricity test

C 15/11/2011 12

Measuring eccentricity

• Y: largest deviation between circumferences
• X: smallest deviation between circumferences
• C: distance between the two circles centers
• Magn: the magnification factor

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