I have a positional callout that is extremely tight.
fairly large part (for us)
total order will be for around 400 parts
metric
overall size is 515 x 515 x 60
two features with a TP of 0.05 to A,B & C
one is a hole
the other is a slot
no MMC so size tolerance won't help
now, A is the main face, B & C are two side faces
B is called out Perp to A within 0.30
C is called out Perp to A & B within 0.30
It is my understanding that this TP is not valid because the Datums themselves have more possible variance than the TP itself allows. The controlling features have to have as tight or tighter tolerance than the called out feature tolerance.
We have already tried to ask the customer to relax their TP callout and were turned down. They said these were critical features.
We feel that if they are that critical, these features should be used as Datums themselves so everything else would relate to them, but they are not.
I need some better verbage to send to their engineers so that they understand our problem.
If the order was for 10 or 20 parts this would not be an issue, but doing a large quantity this is a problem.
Any help from those GDT gurus out there?
Thanks
bob mappes
fairly large part (for us)
total order will be for around 400 parts
metric
overall size is 515 x 515 x 60
two features with a TP of 0.05 to A,B & C
one is a hole
the other is a slot
no MMC so size tolerance won't help
now, A is the main face, B & C are two side faces
B is called out Perp to A within 0.30
C is called out Perp to A & B within 0.30
It is my understanding that this TP is not valid because the Datums themselves have more possible variance than the TP itself allows. The controlling features have to have as tight or tighter tolerance than the called out feature tolerance.
We have already tried to ask the customer to relax their TP callout and were turned down. They said these were critical features.
We feel that if they are that critical, these features should be used as Datums themselves so everything else would relate to them, but they are not.
I need some better verbage to send to their engineers so that they understand our problem.
If the order was for 10 or 20 parts this would not be an issue, but doing a large quantity this is a problem.
Any help from those GDT gurus out there?

Thanks
bob mappes
Comment