GD&T Rule 1: Gage size for form check at MMC (maximum material condition)?

[Salman]

According to GD&T Rule 1, "size controls form".

If we have a cylindrical pin with dimension range 17.490 mm to 17.495 mm then according to tec-ease.com, following is the way we should inspect it.

1. We will need the pin to pass through a full form gage of size 17.495 mm (that is, at MMC),
2. and a mic to inspect the 17.490 mm (LMC) dimension.

(ww.tec-ease.com/tips/june-02.htm)

We use a mic to inspect size of such pin at both MMC and LMC, which is wrong as per Rule 1.
When I asked our Receiving Inspection guy to start using ring gage of size 17.495 mm to inspect for size at MMC, he told me that it is unlikely that a ring gage at 17.495 mm will allow a pin of the same size to pass through. In other words, the ring gage should be a bit oversize.

Now tec-ease.com did not say anything about using somewhat oversize ring gage, and yet what our Receiving guy is saying also makes sense to me.

Can anyone please explain if the gage should be oversize? If yes, then how much?

Thanks.

[David DeLong]

The ring gage should be made to a nomimal ID dimension of 17.495 which is the MMC size. The tolerance is usually 10% of the part tolerance and it is up to you how you would apply it. Do you want to use the tolerance as a range of a bi-lateral tolerance or unilateral? I would always (in automotive for sure) apply tolerances 1 way to be safe and the gage would reject a part made at MMC. It is better to be safe than sorry.

[harry]

I stumbled upon this thread which I thought is relevant for you: A reference or formula for the largest pin which will fit in a hole. Pay special attention to post #7.

[Stijloor]

In Reply to Parent Post by Salman

According to GD&T Rule 1, "size controls form".

If we have a cylindrical pin with dimension range 17.490 mm to 17.495 mm then according to tec-ease.com, following is the way we should inspect it.

1. We will need the pin to pass through a full form gage of size 17.495 mm (that is, at MMC),
2. and a mic to inspect the 17.490 mm (LMC) dimension.

(ww.tec-ease.com/tips/june-02.htm)

We use a mic to inspect size of such pin at both MMC and LMC, which is wrong as per Rule 1.
When I asked our Receiving Inspection guy to start using ring gage of size 17.495 mm to inspect for size at MMC, he told me that it is unlikely that a ring gage at 17.495 mm will allow a pin of the same size to pass through. In other words, the ring gage should be a bit oversize.

Now tec-ease.com did not say anything about using somewhat oversize ring gage, and yet what our Receiving guy is saying also makes sense to me.

Can anyone please explain if the gage should be oversize? If yes, then how much?

Thanks.

Hello Salman,

David provides excellent points. Allow me to add a few.

Your Receiving guy is correct because "metal to metal" does not fit.
So in order for that ring gage to verify that the boundary at MMC has not been violated, the actual gage size must be a "little larger."
This is called "gage clearance."

The actual values depend on the desired "precision" of the gage.

There are a few resources you can look at:

• Y14.43-2003 Dimensioning and Tolerancing Principles for Gages and Fixtures.
• Machinery's Handbook.

Stijloor.

[David DeLong]

In Reply to Parent Post by Stijloor

Your Receiving guy is correct because "metal to metal" does not fit.
So in order for that ring gage to verify that the boundary at MMC has not been violated, the actual gage size must be a "little larger."
This is called "gage clearance."

Stijloor:

I think that the ring gauge should have the tolerance applied to the minus size from the nominal gauge size to make sure that the boundary at MMC has not been violated rather than the plus size.

This gauge will reject a good part made at MMC but it is safe.

[Stijloor]

In Reply to Parent Post by David DeLong

Stijloor:

I think that the ring gauge should have the tolerance applied to the minus size from the nominal gauge size to make sure that the boundary at MMC has not been violated rather than the plus size.

This gauge will reject a good part made at MMC but it is safe.

David,

This may be "safe", but does it make economic sense?
But then again, what percentage of parts will be at their MMC size?
A smart process engineer will likely target the process so that these "extremes" will be avoided.

My take? Use Y14.43. That's what it's for....

Stijloor.

[David DeLong]

In Reply to Parent Post by Stijloor

David,

This may be "safe", but does it make economic sense?
But then again, what percentage of parts will be at their MMC size?
A smart process engineer will likely target the process so that these "extremes" will be avoided.

My take? Use Y14.43. That's what it's for....

Stijloor.

I do agree with you on that one. I just purchased the particular standard and they do have 3 levels of risk. I would still suggest that automotive suppliers still use what the standard calls "Absolute Tolerancing Gage" which would never accept a nonconforming product.

Just a note - A lot of gauge maker companies do not know GD&T that well. Make sure that you agree with the nominals and tolerances on the gage drawing. It is a weak area in today's quality system.

[Stijloor]

In Reply to Parent Post by David DeLong

I do agree with you on that one. I just purchased the particular standard and they do have 3 levels of risk. I would still suggest that automotive suppliers still use what the standard calls "Absolute Tolerancing Gage" which would never accept a nonconforming product.

Just a note - A lot of gauge maker companies do not know GD&T that well. Make sure that you agree with the nominals and tolerances on the gage drawing. It is a weak area in today's quality system.

Absolutely! Great points David. Thank you very much.

A lot of gauge maker companies do not know GD&T that well.

I guess that's where you and I come in to help them understand.....

Stijloor.