GDT True Position Secondary Callout

J

jdtrixler

I have searched this site for a similar answer to see what other have found for this question but I did not find anything that was quite the same... Moderators, if this is not in the right area please move!!

I am working in a fabrication shop. We make large steel fabricated parts (non-automotive). We have multiple customers who use GD&T and call out true position of a hole as follows:

POS DIA1.5MMC A B MMC C MMC
POS DIA0.5MMC A

My question is how does everyone interpret the secondary box? The way that I have worked with this in the past is that this is the holes to themselves as a group. To me that would mean to measure these I would create a secondary datum structure that is only referring to these holes. Then you would call the group of holes to this new datum structure. However, the way that others in this organization believe this to be read is taking the first hole back to the original datum structure and then relating the rest of the holes in this group back to the first hole.:argue::frust:

How would everyone here setup for this? I understand (sort of) the way that they have done it here for a long time, but I have not done it this way in any of my previous lives.

:thanx:Thank you all in advance for your help!!!
 
D

David DeLong

I have searched this site for a similar answer to see what other have found for this question but I did not find anything that was quite the same... Moderators, if this is not in the right area please move!!

I am working in a fabrication shop. We make large steel fabricated parts (non-automotive). We have multiple customers who use GD&T and call out true position of a hole as follows:

POS DIA1.5MMC A B MMC C MMC
POS DIA0.5MMC A

My question is how does everyone interpret the secondary box?

The bottom section of the feature control frame controls the holes to themselves and perpendicular to datum A within the pattern. This is called a "Feature Relating Tolerance Zone Framework". The top section of the feature control frame also controls that location of the pattern but you are only interested in the bottom section here.

This is best confirmed with a checking fixture since it can check all the holes simultaneously. If one was using a CMM, it becomes more difficult and the Operator would end up using one hole as the origin and confirming the others to that particular hole. It certainly isn't correct but that is what happens. It is quite possible that the CMM could reject a hole as being out of position while the attribute gauge (made correctly) would accept it. The checking fixture supersedes the CMM approach.

Hope this helps.
 
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jdtrixler

We are using the CMM (actually a Romer arm, but same difference) approach to obtain variable data for PPAP dimensional.
The way that I have done it in the past is to mimic the check template/gage. By creating an alignment datum structure in the middle of the holes themselves, independent of any other feature. Would this not get the same point across as the fixture, but give me variables data?
 
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True Position

The bottom section of the feature control frame controls the holes to themselves and perpendicular to datum A within the pattern. This is called a "Feature Relating Tolerance Zone Framework". The top section of the feature control frame also controls that location of the pattern but you are only interested in the bottom section here.

This is best confirmed with a checking fixture since it can check all the holes simultaneously. If one was using a CMM, it becomes more difficult and the Operator would end up using one hole as the origin and confirming the others to that particular hole. It certainly isn't correct but that is what happens. It is quite possible that the CMM could reject a hole as being out of position while the attribute gauge (made correctly) would accept it. The checking fixture supersedes the CMM approach.

Hope this helps.

I know for a fact Calypso does not do these calculations incorrectly. If even uses the MMC bonus tolerancing on the secondary and tertiary datums even to provide the movement or 'slop' you would gain with the hard fixture and does not just add extra tolerance to the true position.

Generally on an automotive print the first datum will be the large mating surface of the part, true position to this surface is usually considered to be perpendicularity (of a bore). If you have say a 4 circle bolt circle with each hole being A, B, C, and D all machined into a plane Datum E you were to ask for true position of B with respect to E then A the effective measurement would be the 2d distance between the centers. If it were B to -E- -A- -C- the method would be to set E as the primary plane, A as the origin, and rotate C to one of it's locations then check the hole position to that alignment.
 
D

David DeLong

I know for a fact Calypso does not do these calculations incorrectly. If even uses the MMC bonus tolerancing on the secondary and tertiary datums even to provide the movement or 'slop' you would gain with the hard fixture and does not just add extra tolerance to the true position.

Generally on an automotive print the first datum will be the large mating surface of the part, true position to this surface is usually considered to be perpendicularity (of a bore). If you have say a 4 circle bolt circle with each hole being A, B, C, and D all machined into a plane Datum E you were to ask for true position of B with respect to E then A the effective measurement would be the 2d distance between the centers. If it were B to -E- -A- -C- the method would be to set E as the primary plane, A as the origin, and rotate C to one of it's locations then check the hole position to that alignment.

It is great that there is finally software out there that has the ability to confirm the FRTZF (feature relating tolerance zone framework) that corresponds to a checking fixture but I wonder how many companies have this software. If they don't have it, they would end up using on the the holes as an origin which is not correct.

Positional tolerances at MMC are difficult to confirm to its true position using CMM equipment. One could have a CMM reject the part while the checking fixture accepts it.
 
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True Position

It is great that there is finally software out there that has the ability to confirm the FRTZF (feature relating tolerance zone framework) that corresponds to a checking fixture but I wonder how many companies have this software. If they don't have it, they would end up using on the the holes as an origin which is not correct.
That is true, a lot of CMM software out there does not work to the standard. Also a lot of CMM programmers out there don't understand the differences anyway.

Positional tolerances at MMC are difficult to confirm to its true position using CMM equipment. One could have a CMM reject the part while the checking fixture accepts it.

That's usually when I put the checking fixture on the CMM.
 
J

James Lusk

I'm looking for software that will measure and report the true positions. (FRTZF). Any recommendations? Thank you in advance. James
 
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