True Position to Control symmetry

[Ksplice]

Hi,

See the attached example. I want to use true position to control symmetry of the 13.34 to Datum C (the 19.94 dimension), but I want the inspector to square up the part using datums B and C to measure that. would this be the correct way to call it out on the drawing? does this make sense? thanks,

 

[pmarc]

If you want to control perpendicularity of toleranced pad to datum features C and B plus position (symmetry) to A then this is absolutely legal callout.

However IMO constraining to C and B is not needed if symmetrical relationship between pad and datum feature A is you major concern. I would rather use A only in position callout and assign perpendicularity of A wrt C & B instead.

 

[Ksplice]

How would you measure the perpendicularity to C and B? would i need a gage to check that? thanks.

 

[MechNorth]

Ksplice, you have the right callout for your stated intent.

I disagree with PMarc that you could just do position wrt datum A, then just control datum feature A back to B&C. Once you establish your datum reference frame, you don't break it down to "simplify" (for want of a better term) the datum reference frame; you use that drf to ensure that you have the complete relationship established wrt all the datums.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[pmarc]

Jim,
I do not see a reason why symmetrical relationship could not be controlled using A as a datum only. This tells straight that only 'symmetry' of toleranced feature to datum feature A is the subject of concern. And this was actually my intention - not not to break DRF down but to grasp needed functionality described by OP.

If B & C are included in the DRF perpendicularity of this feature to B and C is controlled which in fact not necessarly might be the goal. If refering to |C|B|A| is fine for Ksplice that is OK. Like I said in my first post this callout is absolutely legal. However from geometrical point of view it has to be kept in mind that specifying |C|B|A| datum reference frame for positional callout gives different results than having |A| only. And if 'symmetry' is really a major concern single |A| is better choice in this case.

Side note: if we really want to debate on this I think first mutual relationships between datum features C, B & A have to be defined.

 

[MechNorth]

Pmarc, if you don't reference C and B in the drf, then datum A is established without any reference to the C & B. That means that you find the unrelated actual mating envelope (Y14'09), which is not controlled wrt C & B. In other words, you find the smallest set of parallel planes that encompass the datum A surfaces and find the center plane; if the surfaces skew the center plane off of the perpendicular wrt C and B, then that's what you get. To be clear, when you invoke a single datum in the drf, there is no relationship between that datum and any other datums, whether or not the datum features may be otherwise controlled wrt other datums. That changes the intent that the op gave. The op wants the inspector to square up (i.e. establish the perpendicularity) wrt to datums C and B. You can't get that without the full drf of /C/B/A.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[pmarc]

Jim,
You are right that without B and C the squarness cannot be guaranteed.

I was just trying to give an option, because I've had an impression that the function actually does not require squaring up to B & C. OP said he was applying |C|B|A| just because he wanted to help the inspector in measuring that relationship. If that is the case IMO the function of controlling symmetrical relationship is not properly grasped.

Maybe in my initial post I should have stopped after first statement so if I brought any confusion I really appologize for that.

 

[fsincox]

In essence with a complete framework the feature will only be symmetrical to some derived point of datum feature A established with respect to the primary & secondary datum features B & C, As opposed to symmetrical to a plane derived totally from the datum feature A.
Frank

 

[MechNorth]

Pmarc,
I read the op differently; I read in the op, "...I want the inspector to square up the part using datums B and C to measure that." Not that he wanted to make it easier for the inspector, but for that relationship to be intended. I think Ksplice should clarify the intent.
Frank, yup.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Ksplice]

Fsincox...that was my intent to control the symmetry of the feature to datum A relative to datums B and C.

so if i was to going to measure this part using an optical measuring machine for example:

I would lay the part flat on datum C (to establish that datum)
then I would push the shoulder (datum B) against a square surface. I would then find the center of datum A and that would be my zero, then I would find the center of the 13.34 feature to find the symmetry of the feature to datum A.

how would you measure that call out? what inspection equipment would I use to measure that?