Effective Training Inc. Message Board

I recently had a disagreement with a customer over a datum pin size. Let me explain. Datum -A- was a flat plane. Datum -B- is a hole, square to -A- with a size of 6.4/6.6 MM. Moving up on the page 39mm and to the right 101.4mm we come to the feature in question. A slot 6.9/7.1 on the width vertically and 9.9/10.1 horizontally. the FCF is a position of 0.75 MMC to A(RFS) and B(MMC) and this feature is called datum -C-. I have a hole on this part that is 7.0/7.2mm in dia with a FCF pos of 0.75 MMC to A and B(MMC) and C(MMC). My customer is telling me that to check the 7/7.2mm hole on the end, the datum -c- pin should be 6.15 vertically and 9.15 horizontally. Is he correct?

Hi SOC,

You ask a very good questions. I can not give you an absolute answer with out seeing the drawing. I will need to see how the feature control frame on the slot is placed. I need to know if it applies to just the width of the slot only or if it applies to the length of the slot also. The rule is when you have a geometric tolerance at MMC that references feature of size datums at MMC, you can use a functional gage to check the geometric tolerance. The gage pin size for the datum B hole will be its MMC (smallest) size minus its geometric tolerance. The gage pin size for the datum C slot will be its MMC size (smallest) size minus its geometric tolerance. Therefore, if the position tolerance that you stated is correctly applied so it applies to the slot width that is labeled datum C then its gage pin will be 6.15. The gage pin size for the 7 mm hole will be 7.0 (MMC) - 0.75 (Geometric tolerance) = 6.25 mm virtual condition size. The next question is, what is the perpendicularity tolerance on the hole labeled datum B. Its gage pin size will be its MMC size 6.4 minus its perpendicularity tolerance.

Please feel free to send a picture of the drawing showing the area in concern to my e-mail address below and I will be glad to review the drawing to verify the correctness of the GD&T and confirm the gage pin sizes.

Good luck. Keep sending us your questions.

_________________
Best Regards,
Roy Cross
ASME GDTP Senior Level S-0488
Dimensional Engineering Mentor
Effective Training, Inc.
2118 S. Wayne Rd.
Westland, MI. 48186
royc@etinews.com
http://www.etinews.com