Envelope Principle Interpretation

[DaddyBluto]

I have a problem with one of our customers and a difference of opinion over ASME Y14.5M - 1994 section 2.7.1.2 Variations of Form (Evelope Principle). We have a bushing that is supposed to be ø.218 - .229". It does not have a circularity/cylindricity call out or anything of that nature on it. Measuring with calipers they found a two point diameter that measures .232", but the same part measured with pin gages comes up as .228". I say the parts are good as their MMC creates the envelope that they need. They say they are bad since the LMC violates the tolerance. I need a few more opinions to help me determine if I am right or they are right. Please chime in.

[Cumofo]

All points must lie within that envelope, the problem is that you have a device that measures any TWO points. Not necessarily about the centriod of that feature. So try to get someone to buy into the principle that measuring a diameter with a caliper is not a best practice. (Good luck.) Maybe a full scan of the bushing, graphed could help.
Hmmm.
How much is that bushing?


There's an old saying:

"You can't save your face and your *** at the same time."





G

[bw_bob]

Last time I checked a diameter was defined by at least 3 points..IE gage pin picks up the 3 highest points....as Gabe said good luck

[Paul Sarrach]

All points must lie within that envelope, the problem is that you have a device that measures any TWO points. Not necessarily about the centriod of that feature. So try to get someone to buy into the principle that measuring a diameter with a caliper is not a best practice. (Good luck.) Maybe a full scan of the bushing, graphed could help.
G

I agree with Cumofo. calipers are not the correct tool to use. Can you get your hand on some I.D. Mics? If not, do a build up using gage blocks and at the min and max size of the Diameter and check it with the calipers. You may want to do this by yourself first to see what results you will get. But I totally disagree with calipers as a ID checking tool.

[Wingman]

Technically you are probably going to lose this argument.
Calipers are the wrong tool. However, if you have dial bore gages you will probably find it over spec with those and you will have a hard time arguing against them. Intramic (ID Mic) is the best way but there are those who will never concede the dial bore argument.
Calipers are no good for accurate ID measurements.
Try to get them to buy into the ID Mics is your best bet.

[inspector212]

IMO, measuring a diameter with a caliper is bad practice when a CMM is available.

Rule #1 states: Where only a tolerance of size is specified, the limits of size of an individual feature prescribe the extent to which variations in its geometric form, as well as size, are allowed.

IOW, if you want to use the caliper reading, the parts are bad. If you want to use a hard gage, the parts are good. But you are not checking form with either measurment.

It sounds like you may have a form issue with the bushing. I would run a multi-level, multi-point, max inscribed cylinder in the bushing and use cylindricity to check the form and compare that number to the diameter. Subtract the diameter from 0.229. This number is the tolerance for the form control. Size may pass but form may fail. 0.02

[Cumofo]

http://www.pcdmisforum.com/image.php...ine=1181154602
I'll be at the Copacabana, BYOV

Bring Your Own Vole?
http://sp1.mm-a6.yimg.com/image/3502312135
"Hmmm, not tonight, I have a headache."




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[Paul Sarrach]

This Tapered ID Gage is nice too. I have never had any complaints using this, as long as there was not a very visible out of round condition. These are also excelent for Plastic holes where a caliper would cut into the material.
http://i178.photobucket.com/albums/w...petedidmic.jpg

[Wingman]

Bring Your Own Vole?
http://sp1.mm-a6.yimg.com/image/3502312135
"Hmmm, not tonight, I have a headache."




G

I think that is 'Bring Your Own Version (of a Barry Manilow Song)'

[#2#]

You don't mention what this bushing is made of or how thick the wall is. Is it possible that a two point measurement is deforming the part .004?

[Underspec]

Why do you argue with the customer? If he wants you to dance on your CMM using the bridge as the pole, do it. It's their part, throw the ball back in their court and ask them how they want you to check it.

[Sean Harris]

Section 1.3.17 kinda leaves the door open to interpretation.... Sadly it proves both your and the customers arguement...looks like it may be one to chalk up to the old cliché "The customer is always right".... but then again maybe not.

[DaddyBluto]

You don't mention what this bushing is made of or how thick the wall is. Is it possible that a two point measurement is deforming the part .004?

It is solution treated stainless steel. Wall thickness is .062". I doubt they are deforming the part. I have not done any of the measurements on these. Their source inspector used calipers and our engineering assistant (source babysitter) did the pin measurements. The engineering manager came to me for documentation to determine letter of the law measurement. It sounds like they have us by the short hairs here.

[bw_bob]

Screw the customer is always right. In this case he is full of sh!te. I may not win the battle but i'm gonna argue like heck that his way is crap..

DB posted before me......nevermind

[Cumofo]

I think that is 'Bring Your Own Version (of a Barry Manilow Song)'

http://sp1.mm-a4.yimg.com/image/2892798040
Everything you always wanted to know about homosexuality but were afraid to ask




G