Positional callouts on this print - GD&T true position question

CycleMike

Registered Visitor
Okay all you GD&T gurus, can you help me interpret the positional callouts on this print?

It's a sheetmetal stamping with PEM studs in two of the holes. It seems as though it's not complete, but I'm not sure.


Any help is appreciated.
 

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David DeLong

Re: GD&T true position question

Okay all you GD&T gurus, can you help me interpret the positional callouts on this print?

It's a sheetmetal stamping with PEM studs in two of the holes. It seems as though it's not complete, but I'm not sure.


Any help is appreciated.

It appears that the 2 studs must be in relationship to each other and also perpendicular to datum A to become datum B. The only problem that I have with it is the fact that both studs become datum B rather than the single stud as shown. Usually, 1 hole or stud becomes a secondary datum rather than 2 studs. Other than that, it is OK.

If I were measuring the stamping, then I would use the same feature control frame and data although the print notes that assembled stud. The stud can only follow the hole location.l

If this doesn't answer your question, please let me know.

PS - I have been in GD&T training for over 20 years and am a ASME certified GD&T professional - Technologist & Senior so I have been around the block a bit on GD&T.
 

CycleMike

Registered Visitor
Re: GD&T true position question

It appears that the 2 studs must be in relationship to each other and also perpendicular to datum A to become datum B. The only problem that I have with it is the fact that both studs become datum B rather than the single stud as shown. Usually, 1 hole or stud becomes a secondary datum rather than 2 studs. Other than that, it is OK.

If I were measuring the stamping, then I would use the same feature control frame and data although the print notes that assembled stud. The stud can only follow the hole location.l

If this doesn't answer your question, please let me know.

PS - I have been in GD&T training for over 20 years and am a ASME certified GD&T professional - Technologist & Senior so I have been around the block a bit on GD&T.

Dave,

Thanks for the quick response. I've read many of your posts and have learned a lot from them. I'm using a Zeiss CMM with Calypso and it isn't giving me any results using that FCF as shown. I will have to make sure I haven't made any mistakes in the measurement plan. Calypso, I understand, follows the standard quite well so either the GD&T is wrong or I'm doing something wrong (quite likely the latter).
 
T

True Position

Re: GD&T true position question

Calypso should probably spit that out. My best guess on how to inspect it would be to use datum A as your spatial alignment, pick one of the studs or the hole before it's welded as one of the points(likely making it the origin), and do a special alignment to rotate the other stud to position in X, Y, or Z. Then you could use the FCF to check the second stud to the first one. (obvious one of the axis will be perfect since you rotated it to be so, but it could vary in the other direction)

I see similar alignments for parts that are assembled using pins, but generally each pin would be it's own datum and you end up with plane / center that you're rotating around / hole / hole as the four features for the alignment.
 

Paul F. Jackson

Quite Involved in Discussions
Re: GD&T true position question

The 2X [position|dia 0.5|A] on the M9 studs makes them a pattern. That position note controls each studs orientation to A and the spread between the studs simultaneously.

The only problem that I see is that no subsequent tolerance callouts are refer to [A|B]! Surface [C], the 2X hole pattern [D] and furthermore all of the remaining untoleranced dimensions are autonomous to the Datum features [A|B]!

I do not have a problem with the 2X pattern being used as a secondary datum feature pattern if that is the way it functions in the mating assembly (where both studs limit or constrain the remaining three degrees-of-freedom simultaneously). From your specification however you cannot assume that serves as a secondary datum feature pattern because it is not specified as such!

This looks to me like an "in-process" assembly print because the sheet metal edges or holes [D] are not specified for their location relative to the 2X studs! If they were and there was some tolerance to [A|B] then with your CMM you would establish the primary (median plane) using points probed in the two target areas (A1) and (A2) then you would establish the secondary by moving reference to the mid-point of the spread between the axes of the 2 studs and rotate the system by the arc tan of the 128.75/196.76.

If there were callouts referencing [A|B] and their basics were shown relative to on stud or the other you could move reference one half the basic spread and measure from that reference or just leave the reference at the center of the pattern and adjust the measured value by the rectangular X-Y displacements of half the basic spread at the specified angle of the spread. Making one stud secondary and the other tertiary makes sense only if they function that way (like if the mating part has a hole and a slot).

Paul
 

CycleMike

Registered Visitor
Re: GD&T true position question

Calypso should probably spit that out. My best guess on how to inspect it would be to use datum A as your spatial alignment, pick one of the studs or the hole before it's welded as one of the points(likely making it the origin), and do a special alignment to rotate the other stud to position in X, Y, or Z.

Do I use "Rotate by distance" in "special"? I tried that but end up with the alignment rotated by some strange angle.

<edit>

Wait....................had to rotate by only one axis. In this case X 126.75.
 
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CycleMike

Registered Visitor
Re: GD&T true position question

If there were callouts referencing [A|B] and their basics were shown relative to on stud or the other you could move reference one half the basic spread and measure from that reference or just leave the reference at the center of the pattern and adjust the measured value by the rectangular X-Y displacements of half the basic spread at the specified angle of the spread. Making one stud secondary and the other tertiary makes sense only if they function that way (like if the mating part has a hole and a slot).

Paul

I tried getting it changed to make it simpler to inspect but the customer's engineer dug in his heels. The part bolts to the frame of a pickup truck using the two holes, and some kind of electronic module attaches to the two studs. For some reason, he just didn't want to connect the dots. I suggested that it would make inspection much easier and thus help us to produce a better part. I might as well send suggestions like that into outer space!:lmao:

Mike
 
T

True Position

Re: GD&T true position question

Do I use "Rotate by distance" in "special"? I tried that but end up with the alignment rotated by some strange angle.

<edit>

Wait....................had to rotate by only one axis. In this case X 126.75.

No, Rotate Distance to Axis. If you were to pick the lower left as your origin, rotate the other stud to either distance from one axis.

I just made up a quick sample with two circles and a plane. The two circles being the studs (or holes). Then I set the spatial rotation to the plane. The X and Y origins to one hole (in this case I used the lower left as the origin), set the Z origin as the plane. Then set hole B as the planar rotation and under special went 'Rotate Distance to Axis' as 126.75. It was aligned pretty closely to how the print seems drawn.

By rotating distance to axis you can then just ask for the true position of the second hole and it will be perfect in one direction (X if you rotate it to 126.75 from +Y) and then can see the error in Y.

Good luck.
 
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Paul F. Jackson

Quite Involved in Discussions
Cyclemike,

If you are trying to inspect the 2X [position|dia 0.5|A] then first check each stud for its perpendicularity to the surface created by (A1 and A2) if that is OK then check the direct spread distance between the tops and bottoms of the pins it should measure (128.75^2+196.76^2)^0.5 +/- 0.5. The same procedure for 2X [D] relative to [C]. There is nothing else left to inspect on that print except the profile of [A] and flatness of [C]... nothing else has tolerance.

Paul
 
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