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Bidirectional True Position Tolerance Call-Out for Holes for Process Capability

A

Aitch

#1
A customer of ours has proposed going to a bidirectional true position call-out for some hole locations. I know this is a valid based on ASME Y14.5
The customer is wanting process capability numbers based on these proposed call-outs.
I have no idea how to calculate true position based on this bidirectional call-out. As usual I am probably over complicating things.
Any help or enlightenment is appreciated.
 

Miner

Forum Moderator
Staff member
Admin
#2
You will need to break the position out into the X and Y coordinates, and assess each separately.
 
T

True Position

#3
My appologies, the terminology is throwing me. Are you currently using limit tolerancing(Hole 1 is to be at 1.000 +/- .005 in X and 1.250 +/- .005 in Y) and your customer wants to go to 'True position of hole 1 is to be .005'?

Perhaps a photo would clarify things, if it's the above case and you're just not familiar with true position tolerancing, I'm sure someone would help you out(I would at any rate).

If the previous posted is correct, your customer wants to go away from true position more towards the old style limit tolerancing?
 

Miner

Forum Moderator
Staff member
Admin
#4
The position tolerance that the OP is referring to is most likely the type that would allow a position tolerance of say .05 in the X-axis and .07 in the Y-axis.
 

Jim Wynne

Super Moderator
#5
The position tolerance that the OP is referring to is most likely the type that would allow a position tolerance of say .05 in the X-axis and .07 in the Y-axis.
See the attachment. Because in my experience it's unlikely that the customer actually understands what he's asking for (insofar as capability reporting is concerned), I strongly recommend that whatever inspection method and capability calculating is done be reviewed and approved by the customer in advance.
 

Attachments

A

Aitch

#6
The rectangular bidirectional true position call-out is what I'm dealing with.

The call-out was originally: true position to be 0.9mm dia. MMC to A-B-C
with a 1.33 Ppk requirement.

After a 100% inspection of a 102 piece lot, we requested a 1.35mm dia. true position which would give us around a 1.39 Ppk.

The customer was unwilling to give us the 1.35 zone and opted to go with the bidirectional call-outs: true position to be 0.9 in the Y axis and 1.35 in the X axis. This call-out is just as the rectangular bidirectional example in the above attachment.

As indicated I have a ton of inspection data to the original TP call-out and need to understand how to convert this data to the new call-outs.
 
T

True Position

#7
Well, if you have the raw X,Y positional data stored, you could show PPK in each axis independantly, if your customer would accept that.

The big question is why are they going away from true position to rectangular coordinates?
 

Kingsld1

Involved In Discussions
#8
A bi-directional true position tolerance can be thought of as a plus or minus tolerance with datums attached. There are situations where functionally the feature can be allowed to vary in one direction more than another. The bi-directional true position allows this to occur while still providing a datum structure to make and inspect the feature to.

One potential problem with the capability study would be with the use maximum material or least material condition modifiers anywhere in the feature control frame. As far as I know there is no formally recognized way to deal with a varying tolerance in a capability study.

Dennis
 


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