Stab pin size on B datum hole

Hello Everyone,

If I have a B Datum hole at 10.1mm Nominal. Size tolerance on the hole is +0.1mm / -0 and the positional tolerance is 0.1mm total. (MMC)

What size should the stab pin be for this hole? From my understanding it should be Nominal at 10.1mm. Am I wrong?

We were sent Stab pins at 10.0mm and I feel this is a mistake considering if we are at our maximum size of 10.2mm and the pin is 10mm we have 0.2mm of play!

If it is correct can you please explain why?

Thanks!

Belly Up over 1 year ago

Can you just use a larger pin?
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel
Belly Up over 1 year ago

Best fit.
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel
Matthew D. Hoedeman over 1 year ago

stab pin should be the SMALLEST diameter the hole is allowed to be MINUS the position tolerance for the hole. Always.
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel
Smasherofprobes over 1 year ago

Matthew D. Hoedeman Even on Datum holes? So I guess our process of keeping Datum holes at maximum size isn't correct, should we be closer to nominal ?
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel
Matthew D. Hoedeman over 1 year ago in reply to Smasherofprobes

IF it is an MMC pin (straight pin) and not an RFS pin (taper pin) then YES, smallest hole size minus position tolerance. This is so you can 'datum shift' the part around on the pins to try to balance it all into tolerance, which is the reason behind the size and position tolerance. The 'hard gage' method, instead of the software method (the idea behind Xact and the new replacement GD&T tolerance method). IF you have MMC stab pin to 'locate' the part on the fixture that allow the part to 'datum shift', then you 'should' be measuring them FIRST, then use them to align the part (or use them as the datums when you dimension the part). I've been doing this for decades. ANY time you CAN measure a datum hole in a part on a fixture, you measure it, then use that to align to before measuring the rest of the part. The joys of sheet metal!
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel
Smasherofprobes over 1 year ago

Thank you for your detailed explanation Matthew, that was my understanding as well, always do a part alignment if possible. I do not use the software methods (Xact). Thanks again!
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel