Washer madness 1

[Kedu]

New on the form. Glad I found this site.
Story: shown washer received from supplier, measured by QE department found non-conforming, disposition rejected.
Supplier yelling the parts meet the print.
Inspectors using X min and X max methods to determine the worst case.

Question: what would be the theoretical values (absolute minimum and absolute maximum)?



Supplier states:

x min.: .7285

X Max.: .7745



QE/Inspectors :

X min.: .7335

X Max.: .7665.



Parts measured on CMM are closer to the supplier values (and obviously they are defending their calculated numbers)

Algorithms used: maximum inscribed and minimum circumscribed circles (not LSQ)


Which theoretical values you agree with (if any)? Supplier or QE/Inspector?

 

[pylfrm]

Kedu,

Assume the outside diameter size tolerance is satisfied. If the minimum wall thickness is at least (3.005 - (1.508 + .020))/2 = 0.7385, the position tolerance is satisfied. If the minimum wall thickness is less than 0.7285, the position tolerance is not satisfied. If the minimum wall thickness is between 0.7285 and 0.7385, more information is required to determine whether the position tolerance is satisfied.

By the way, how thick are these washers? Do you know anything about their intended function?


pylfrm

 

[Kedu]

pylfrm,

Thickness: gage 0000 (13/32)
Support for a sealing surface O-Ring, washer centered on OD, but O-Ring is not allowed to be extruded from ID, hence need of minimum material between ID and OD.

 

[TheTick]

I would remove "x max" entirely. You can already reliably measure OD and ID. "x min" is a bit of a stab but doable. After that, there's no point, and additional measurement can only cause conflict.

 

[MikeHalloran]

WAIT!

Subject part is a spacer?

Or an anti-extrusion washer?

What material?

What do all of the mating parts look like?



Mike Halloran
Pembroke Pines, FL, USA

 

[Kedu]

pmarc,

Took me a couple of weeks to go over all the simple stacks recommended and found here on eng-tips.

May I ask, why do you think datum displacement has no influence over min-max calculations? Otherwise stated, if datum feature A is modified at MMB X min/ x max stays exactly the same. Why is that?
Since our washer is assembled with clearance, has been some meeting "brainstorming sessions" to change the print to show A(M) in order to allow for functional gaging.

 

[FACS]

The title of this thread is perfect

 

[greenimi]

I kind of brought all those stackup discussions together (I was involved in some) and here is what I have concluded. Please feel free to agree or disagree with my statements:

Regardless of which modifier is used (MMC, LMC, RFS) same min-max. values (same wall thickness) are obtained as a result of the stackups calculation if datum feature is modified at RMB or MMB.
(Simple OD-ID association)
Example 1: Feature at MMC: same min-max values regardless if datum feature is RMB or MMB.
Example 2: Feature at LMC: same min-max values regardless if datum feature is RMB or MMB
Example 3: Feature at RFS: same min-max values regardless if datum feature is RMB or MMB
Example 4: Datum feature at LMB: same min-max values regardless if feature is RFS or MMC.

Wall thickness calculations (min-max) a.k.a. mutual diameter relationship offers much similarity between RFS, MMC (and LMC to a certain extent).

Probably a good question for the experts (pmarc I am sure knows to answer it) is:

What would be ASME theoretical explanation why datum displacement/ datum shift does not have an effect in the above stackup calculations?
Is the default Rule#1 the main culprit?