Datum reference frame of three-bearing arrangement shaft

[PetkovStoyan]

Hi all,

just a general question how to proceed if we have to inspect long shaft, which is supported not on two, but three (or more) bearings. When the shaft is supported on two bearings the standard practice is to use the corresponding bearing journals as datums and measure runout with regards to common datum (A-B for example). What should we do if the shaft is supported on three bearings, should we perform measurements with regards to three datums, as we have three corresponding bearing journals? It doesn't make sense, since two datums are sufficient to provide unique reference frame, but on other hand, if the portion of the shaft that overhangs is too heavy this might influence the measurements. There are shafts which work with third bearing, which has the purpose of preloading the overhung part, so it doesn't wobble like dog tail during operation. How is it correct to proceed in such occasions? Another question is, if we decide to go with two datums, which two should we choose, and what is the criteria of using exactly those two, and not another two datums?

Thank you.

 

[3DDave]

greenimi,

What is valid is what is in the contract. End of story.

That you are not coordinating with the gauge suppliers to allocate their acceptable variation is a problem that needs to be dealt with in the contract.

No standard can possibly fix a problem that is outside the contract. If you have a contract with GM then they are the only party to carry concerns about "validity" to.

You might want a discussion with the legal department in your company about the interpretation of that contract.

 

[greenimi]

I think they are mis-applying part 2.b.

I agree with you.
And they did it (mis-apply) because it is most convenient to them.
I am in the CMM group / metrology and being asked to measure and qualify the gages. Sic!!
What tolerances shoud I use?
Whatever tolerances which protect my product and not their gage.

 

[Doug Hunter]

Greenimi,

I'm assuming you are within GM. Do you have a communication channel to the Product Development group that designs the parts that the gauge gauges, or the Purchasing group that buys the parts and gauges? The best solution (which you probably already know) is to get the gauge tolerances specified on the part drawing, or in the Purchase Order T&C. We know that that kind of communication and desired change can be like moving mountains within GM (or any OEM). Someone in the Quality department ought to have a voice with those other internal departments. I would think that management would want to respond positively to Quality-impacting issues. Purchasing may not want to help if it drives up the gauge cost. Keep fighting the good fight and protecting your product quality!


Best regards,
Doug Hunter
Altarium Technical Consulting

https://www.altarium.us/

 

[3DDave]

One thing that might make your legal department take notice is if you mention "void for vagueness." If the contract cannot be reasonably interpreted in a singular way, then the contract can be set aside. You don't have to meet it and they don't have to pay you. Lawyers will be attracted like vultures to a dead elephant.

 

[Doug Hunter]

3DDave, in Automotive the legal department doesn't get involved unless the cost number has 7+ digits. Greenimi is caught between a rock and a hard place. Production Operations will come after him if he approves a fixture that won't work, and Purchasing and Program Management will come after him if he doesn't approve the fixture and launch timing is held up. His lesser of two evils is to protect the product and hopefully he has a good boss that backs him up when Purchasing and Program Management confront him.


Best regards,
Doug Hunter
Altarium Technical Consulting

https://www.altarium.us/

 

[3DDave]

That may be true, but it also makes no difference. There isn't a place for him to stand and hold the position without the legal department. It's a contract failure and someone signed that contract for his company to make parts and someone signed a contract to buy inspection equipment.

If Production Operations and Purchasing and Program Management are signing stupid contracts, then it's time for putting out a resume and bailing.

 

[PetkovStoyan]

3DDave,
maybe you can share some thoughts on my problem? Interesting conversation though, I have to say.

 

[Burunduk]

Regarding the original question:
The idea of the Common Datum Features such as those referenced as A-B or A-B-C is establishment of a single datum from two or more datum features that arrest the same degrees of freedom together.
Whether a third coaxial datum feature cylinder should be part of the coaxial common datum feature, to establish that single datum axis, should depend on the answer to the questions:

-Is it non-redundant?
-Is it functionally appropriate?

I say yes to both.
I guess it's easy to see why it's functionally appropriate - the design uses 3 bearings to support the shaft, and for good reasons.
As for whether it's non-redundant: Remember that the datum axis is simulated by the datum feature simulators, which is the inspection equipment such as mutually aligned V-Blocks or simultaneously adjustable coaxial chucks that represent the bearings in the application. That datum simulating fixture should be of high accuracy and can be assumed near-perfect for the runout evaluation purpose. When you are placing the part in the datum simulating fixture, you are relating the actual part to the datum so that the features to be controlled relative to it can be measured. The actual part is imperfect and has form, size, location and orientation deviations in the datum features among other things. That means you are mating an imperfect part (each time a different one) to a presumably perfect datum simulator (each time the same one). You will therefore get different results for the same actual part depending on if that near perfect datum simulating fixture is constructed from 2 or 3 elements, because you would be relating the actual part to a datum axis differently. The preferred scenario is the one representing the functional assembly condition better.