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Non-repeatability of straightness measurements

Non-repeatability of straightness measurements

Non-repeatability of straightness measurements

We have been using a Rank-Pneumo (now Precitech) to measure straightness of ceramic and metal shafts for some years.
Recently we received a batch of metal shafts from our vendor and performed the usual inspections including straightness, roundness diameter, surface finish.  All is as normal except the straightness traces are unrepeatable. (Normally within a few millionths)
Taking older metal stock and performing multi-traces gives satisfactory results.
Older ceramics are also fine.
Slowing the tracking rate appears to reduce the problem suggesting (almost unthinkable) that inertia is doing it's thing and tossing the probe tip around on the surface.  Surface finish is 2 microinch so that seem unlikely.
The guage head was replaced a few months ago but the new one has performed comparably.  Remember that we have retested older stock in an attempt to eliminate the head.
Any ideas folks?

RE: Non-repeatability of straightness measurements

Most Laser Alignment systems can perform Straightness measurements. The D510 includes 15 programs including: Horizontal, Vertical, Softfoot, Cardan Shaft, Offset and Angle, Spindle, Straightness, Center of circle, Flatness, Plumbline, Squareness, Parallelism and Flange. For additional info on the system, go to:

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