True position question

[[Ja...]]

We had a gauge made with 6x 10.015 (+0.003mm) pins on a 96mm bcd and each pin has a true position callout of 0.010mm. When I spoke with our CMM operator after it failed our annual verification (and has failed every year), he said that 0.010mm is really tight for true position. The mating part has a 0.1mm true position with max material condition. I want to know if anyone else has ever seen this before? When I spoke to the guy who worked with the guy that made the drawing, he said all the pins have to be checked individually and because the pins can be bigger or smaller, this was taken into account when the drawing was designed to DIN 1938-1. Has anyone else seen true positions this tight? Do you think its possible that its an error?

[[Ma...]]

Hi,

without knowing anything about drawings, parts and intended purpose we can not say for sure if it is a fraud.
Tolerance can by based on how it is machined and how it is important in assembly.

I would ask engineer if change is possible - TP tells not just position - you can have center on exact spot, but cylinder is tilted, ...

[[Ja...]]

Here is some of the gauge print and part print.You can see the AB datums that relate to the true position, and the perpendicularity callout is to the ID of that fixture. The diameter of the holes on the part being measured is 10.1 +0.25 and the true position callout is 0.1 MMC to AB.

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[[Ma...]]

Thanks for sharing some info.

I would agree that 0,01mm tolerance is too tight. Half of tolerance is used by runout on inserted pins ( 6x ).
ID of 2nd image have to be grinded, same for pins - but there is some tolerance for hole and cylinder of "C" on pin.

I would guess - 0,02mm tolerance for TP is the closest you can have on drawings, but i think you are measuring about 0,028-0,035 TP values?

[[Ja...]]

Yes. How were you able to determine that? These are the results. We ordered 2 gauges for this and neither have ever passed under 0.010mm for all 6 pins. We have to order 2 more for a bigger size but before I do, I want to get this situation worked out.

[[Ma...]]

If you think i got close to numbers - just pure guess with a little help of drawing and machinist practice.

I don't know if you are evaluating TP as a pattern - you should.

Problem here is that TP 0,01 is for assembly - if it would be for holes, then it should be fine - those pins are long.

[[Ma...]]

I agree with Martin, the pins/holes should be evaluated with pattern using Chebyshev fit allowing rotation only.