proper hole gageing

[dodge78]

When gaging critical holes, using go/nogo gages, what is the correct method? We have a hole callout of .1250/.1248. we are using deltronic gage pins. My interpretation is that the .1248 pin must go, and that the .1250 cannot go. I have some people saying that the .1247 can go (meaning that the hole size is therfore at least .1248) and that a .1251 cannot go. Is there any mention of the correct practice in a publication? ie; ansi y14.5...etc...

Thank you

 

[Belanger]

Here, you are getting into the world of gage tolerances. Theoretically, the "go" gage would be exactly .1248 (and it should be a full-depth gage), and the "no-go" gage would be exactly .1248 (and it should be just a two-point check, not full-form).

Now, we might say that a .1248 pin won't fit into a .1248 hole, so for these on-the-fence parts, we do have to make a judgement. Plus, on top of all that, the gage itself will have some tolerance (often given as 10% of the part's tolerance). So should we make the gage pin a little smaller in order to allow the .1248 hole to pass? Or leave the gage pin right at .1248, knowing that a good part of .1248 may be rejected?

There is no ASME standard about this, but most people are of the philosophy that it's better to reject a good part once in a while rather than let a bad part slip out the door to your customer. In short, your interpretation is better (IMO) than the interpretation of your colleagues.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[dodge78]

Thanks John. Your no go should have been .1250 correct? (typo)

 

[Belanger]

oops -- yes, that's right!

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[dgallup]

This type of thing can get really hairy. It's best to spell it out exactly on a print if it's that important. I've got one part where we had to say the go gage pin had to pass completely through with a certain amount of weight on the pin. Still got into arguments with the supplier about how fast the gage pin could go. They were throwing the thing like a dart to get the momentum to carry it through!

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[fsincox]

I imagine this is somewhat naive on my part but if the gage can "broach/hone" the hole to size, I would say it is good.

 

[pmarc]

dodge78,
There is ASME Y14.43 standard on design of gaging and fixtures which mentions at least 4 different gaging policies. The one that J-P decribed is called Absolute (Pessimistic) Tolerancing Policy.

And be careful, a NO-GO gage pin is not a good idea for LMC size check of internal features of size, like a hole in your case. Regardless whether it is .1250 or .1251.

Think of a situation where the hole at its very top and at its very bottom is 0.1249 for instance, so absolutely within size limits. Your NO-GO pin will be useless in such case, since you will never be able to slide it from any side deeper inside the hole. The actual diameter in the middle may be .1249 or .1260 or even .1300, but you will never be able to verify it by your gage pin. Only two-point measurements along whole depth of the hole can deal with the problem.

 

[dodge78]

Thanks pmark,

I understand your concern with respect to the center of the hole having a larger condition. We also use our Zeiss cmm to verify that condition, however at the machine our method available is with the Deltronic pins. The method some of my collegues are suggesting, by using a .1247 pin leaves doubt and room for the hole to actually be slightly less than the required .1248 After some recent research into Go/Nogo design requirements, Go gages are designed 10% of the tolerance larger, and nogos are designed 10% of the tolerance smaller. This may reject a good part, but wuth this method you are sure to ship a compliant part.

Thank you.

 

[Belanger]

Good call, pmarc. Duh, I have a copy of Y14.43 on my shelf, not more than 16 inches from my head. But I forgot to mention that document to the OP!

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com