A new method of roundness measurement of machine elements based on the three-point method is studied. In this method a workpiece is surrounded with three detectors
M1,
M2 and
M3, each of which is different in its detective magnification, and is able to rotate freely in them. The outputs of detectors are summed together and are indicated on a indicator.
The rotational error of a workpiece (eccentric motion or play of rotating centre) does not appear on the indicator if the relative angular positions of the detectors r (between
M1 and
M2) and φ (between
M1 and
M3) are adequately arranged. Let the ratios of the detective magnifications of the detectors
M1,
M2 and
M3 be 1 :
a :
b, then the combinations of (τ, φ) or (
a,
b) can be selected as follows so as to cancel the rotational error of a workpiece.
1 +
a cos τ
b cos φ = 0
asinτ-sin
bφ=0
These equations correspond to the magnifying power of the Fourier coefficient of first order of workpiece profile Under these conditions the combinations of (τ, φ) and (
a,
b) are calculated numerically in which the magnifying powers of the Fourier coefficients of the higher order of workpiece profile are as large as possible.
It is proved experimentally, that this method can be used practically for measuring roundness without any high precision turn device.
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