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]

Re: GD&T Rule 1: Gage size for form check at MMC?

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.

 

[Stijloor]

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.
• (broken link removed).

Stijloor.

 

[harry]

Re: GD&T Rule 1: Gage size for form check at MMC?

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.

 

[David DeLong]

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]

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]

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]

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.

 

[Salman]

Thank you very much for your valuable posts everyone.
I have a lot better understanding now.

 

[Wayne]

I deal almost exclusively with cylindrical and threaded gages and this is a topic that I deal with on a daily basis. One of Taylor's Principals is used in measuring both cylindrical and threaded products: The GO gage needs measure all features simultaneously; the NOGO gages need to measure individual features. Putting this to practice on cylindrical parts is simpler than more complex parts, like threads, because there is only one surface to be evaluated: the diameter. Measuring all features of the diameter is easily checked with one tool: a GO Ring Gage. The GO ring gage is made to the maximum material condition of the product to be tested. The NOGO condition may be evaluated with any number of tools, but a NOGO ring gage is still the simplest.

The standard and accepted practice in the USA is to use the products limits as the gage sizes.

The method of choice in the USA for determining the gage tolerance is the 10% Tolerance Rule. There are several methodologies for figuring gage tolerance, but the 10% Product Tolerance Rule, discussed in ASME B89.1.5:1998 p 4.1, is one of the simplest. Once the amount of tolerance is determined, it is divided between the GO and NOGO gages, converted into standard Gagemaker’s Tolerance, and applied using "Absolute Tolerancing" so that all tolerance falls with in the product limits and thus would never accept a nonconforming product.

There will be parts rejected that technically are good parts, but the intension here is that no bad part ever passes inspection. For a primer on gage design: (broken link removed) . This methodology is so well imbedded into the USA gage industry that if you wish to deviate from the normal method you will be questioned to be sure that you are aware that you are not using the common methodology. Even if you are allowed to purchase with tolerances that deviate from the norm, the gage maker must be carefully watched so that they make what you desire.

The Absolute Tolerancing works this way:
GO Ring Gage is made to maximum product size with a negative tolerance.
The NOGO Ring Gage is made to the minimum product size with a positive tolerance.
The GO Plug Gage is made to the minimum product size with a positive tolerance.
NOGO Plug Gage is made to maximum product size with a negative tolerance.

Summary:
Given the product size as an example: 17.495/17.490mm external diameter
Using the Tolerance Calculator to figure the 10% tolerance: XXX (0.00025mm)
This makes the GO Ring Gage: 17.495mm+0.0/-0.00025mm
This makes the NOGO Ring Gage: 17.490mm+0.00025/-0.0mm

One final note: When the gage maker’s tolerance gets to the point of XXX tolerance for GO and NOGO gages it is a good time to begin exploring air gauging to make the measurements.

I hope that this helps you achieve your quality goal.