Parallelism call out of .005" to Datum A - A GD&T question

P

ProProcess

#1
I'm not sure if this is the place for this, I appologize if it is not.

My question:

I have a Parallelism call out of .005" to Datum A.
Is there any obligation to the standard to place the datum on a reference surface and measure the feature?
Or are you free to do it the other way around, that is place the feauture on the reference surface and measure the datum?

Thanks in advance.
 
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Stijloor

Staff member
Super Moderator
#2
I'm not sure if this is the place for this, I appologize if it is not.

My question:

I have a Parallelism call out of .005" to Datum A.
Is there any obligation to the standard to place the datum on a reference surface and measure the feature?
Or are you free to do it the other way around, that is place the feauture on the reference surface and measure the datum?

Thanks in advance.
There is an (engineering) reason why that particular surface is specified as a datum feature. Simply follow the GD&T specifications and inspect accordingly.

BTW, why shouldn't you?

Stijloor.
 
P

ProProcess

#3
I make a part the has a slight deformity on the end of the Datum surface.
This deformity does not exopse itself when the datum is place on the reference surface and the parallelism checks out just fine.
My customer is doing the opposite i.e. clamping the feature and checking the datum.
This in turn exposes the deformity and could be construde as out of spec.

To me, and I think this is what you are saying, the design intent says to place the datum on the reference surface because this is how the part will function, which is in fact, the case for this particular part.
I just have to politely convince my customer that their interpritation of there own drawing is not correct, which is always a slipery slope!
 

Stijloor

Staff member
Super Moderator
#4
I make a part the has a slight deformity on the end of the Datum surface.
This deformity does not expose itself when the datum is placed on the reference surface and the parallelism checks out just fine.
My customer is doing the opposite; i.e. clamping the feature and checking the datum.
This in turn exposes the deformity and could be construed as out of spec.

To me, and I think this is what you are saying, the design intent says to place the datum on the reference surface because this is how the part will function, which is in fact, the case for this particular part.
I just have to politely convince my customer that their interpretation of their own drawing is not correct, which is always a slippery slope!
A classic example of an interpretation/inspection issue that should have been discussed and agreed upon during the contract review stage.

Stijloor.
 
P

ProProcess

#5
A classic example of an interpretation/inspection issue that should have been discussed and agreed upon during the contract review stage.

Stijloor.
True.
The thing is, we've been making this part for more than 20 years.
We/I worked hand in hand with the engineer on both the drawing and machining as the part is very small and barley "makable".
This condition has always been present and has never posed any issue during assembly or in the field.
The company has grown quite rapidly and has new personnel in place that do not work in concert with their own engineering department.
It's not so much that the contract was not reviewed, more like the rules have changed.
 
Q

qualitytrec

#6
Do you have the original first pieces with sign off from the company?
Also, you say there is a slight deformity on the part. Is there no way to correct the deformity?
The customer should measure to the drawing it is your standard that you build to it should also be the standard they check to.
I have never found the type of discussion you are talking about to be a slippery slope as long as it is done professionally and with integrity. I would pursue discussion of it with some people from the company that know some of the history and try to get some support going in.

mark
 
P

ProProcess

#7
Do you have the original first pieces with sign off from the company?...
No.

...Also, you say there is a slight deformity on the part. Is there no way to correct the deformity?
...
Actually, I mispoke in the original post as the tolerance is .0005" not .005". I did not edit it because I did not figure it was relevent for the discussion, but to answer your question, it is.
The area that has the deformity is only .009" thick.
We perform a milling operation that seperates the part from the stock.
At the moment that the remaining material is too weak to withstand the cutting forces, it defletct around .001".
We've tried to eliminate this, but there is not practical way.
Too the deflection does not interfere with the datum or FF&F so it's never been viewed as an issue, that is untill now.

...I have never found the type of discussion you are talking about to be a slippery slope as long as it is done professionally and with integrity. I would pursue discussion of it with some people from the company that know some of the history and try to get some support going in.

mark
I agree with this too.
Luckily the original engineer is still at the company and is scheduled to meet with us and their QAM some time next week.
Thanks for to all for the input, I do appreciate it.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#8
Placing the datum against the reference surface is the correct interpretation. If the customer measures it the opposite way and finds a problem, technically it is irrelevant. But, practically, they are the customer and write the checks. So, until the print can be fixed it would be appropriate to get some kind of written agreement as to how the part is to be measured so that if the proverbial bus runs over someone there is a record as to how you got to where you are now. As it stands, you are doing them a favor to deal with the problem.
 
G

Geoff Withnell

#9
A classic example of an interpretation/inspection issue that should have been discussed and agreed upon during the contract review stage.

Stijloor.
Well, to my mind it already was. If Y14.5 is called out, then inspect to Y14. That's what the OP is doing. That's the whole point of having standards, so that every fiddling detail of every drawing doesn't need to be discussed. The design engineer, the manufacturing engineer, the production operator, the supplier inspector and the customer inspection/quality folks should all be using the same rules - the ones in Y14.5!
If the customer insists on measuring and judging conformance some other way, that's an engineering change, which should drive a PO mod!

Geoff Withnell
 

Stijloor

Staff member
Super Moderator
#10
Well, to my mind it already was. If Y14.5 is called out, then inspect to Y14. That's what the OP is doing. That's the whole point of having standards, so that every fiddling detail of every drawing doesn't need to be discussed. The design engineer, the manufacturing engineer, the production operator, the supplier inspector and the customer inspection/quality folks should all be using the same rules - the ones in Y14.5!
If the customer insists on measuring and judging conformance some other way, that's an engineering change, which should drive a PO mod!

Geoff Withnell
Geoff, the majority of the engineering drawings that I see when I teach/consult GD&T do not even meet the ASME Y14.5M-1994 (or now the 2009) Standard. I will not even get into GD&T (application/verification)competency!

Stijloor.
 
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