Interpretation of datum -C-
Interpretation of datum -C-
(OP)
Greetings All,
My customer is rejecting this part claiming that the lower pattern of M3 holes are out of tolerance. When I check them the worst TP I get is about .028mm they are getting TP around .380mm. When I setup and measure I'm implying the bottom surface is parallel to datum A-B not shown in this view but, is an axis of 2 concentric diameters from which all the basic dimensions to said holes originate from. I also imply that Datum C is perpendicular to this surface since it's the surface from which I fixture from. I can then zero out on the large center hole and measure the 25,42, 55 and 60mm basic dims. Then I rotate the part 90 degrees making sure A-B is parallel to my surface plate, zero off the bore and proceed to check the hole that falls on datum -C-, 2, 28 and 30.5 mm dimensions. The are aligning datum -C- off the 2 small surfaces of the 30 degree notches which based on the angular tolerance can vary far more than the locational tolerance of the hole pattern so, I would consider this unusable as a datum feature for this reason. When I had the part setup at 90 degrees (rotated cw to view in drawing) I checked the location of these surfaces and the surface that would be to the right of the bore measures about +.1mm and the surface to the left measures around -.1mm which would skew this pattern. Am I correct in how I'm setting up to measure? Let me know if you need more info. Thanks for any feed back.
Doug
My customer is rejecting this part claiming that the lower pattern of M3 holes are out of tolerance. When I check them the worst TP I get is about .028mm they are getting TP around .380mm. When I setup and measure I'm implying the bottom surface is parallel to datum A-B not shown in this view but, is an axis of 2 concentric diameters from which all the basic dimensions to said holes originate from. I also imply that Datum C is perpendicular to this surface since it's the surface from which I fixture from. I can then zero out on the large center hole and measure the 25,42, 55 and 60mm basic dims. Then I rotate the part 90 degrees making sure A-B is parallel to my surface plate, zero off the bore and proceed to check the hole that falls on datum -C-, 2, 28 and 30.5 mm dimensions. The are aligning datum -C- off the 2 small surfaces of the 30 degree notches which based on the angular tolerance can vary far more than the locational tolerance of the hole pattern so, I would consider this unusable as a datum feature for this reason. When I had the part setup at 90 degrees (rotated cw to view in drawing) I checked the location of these surfaces and the surface that would be to the right of the bore measures about +.1mm and the surface to the left measures around -.1mm which would skew this pattern. Am I correct in how I'm setting up to measure? Let me know if you need more info. Thanks for any feed back.
Doug





RE: Interpretation of datum -C-
RE: Interpretation of datum -C-
Doug
RE: Interpretation of datum -C-
It would be easier to understand if it included the original callouts for the datum references. A datum should never be applied to a centerline.
It also leaves me unsettled to 'imply' a datum reference. It's either explicit or it doesn't count.
RE: Interpretation of datum -C-
Being placed on the Centerline extension I'd guess it's meant to be a plan on the axis of one of the diameters - I think the A-B you mention supports this.
However, it's not entirely clear what the 'clocking' feature is.
Could be the small hole at the bottom though that looks just off the center-line.
I don't think it would be the 30° notches as you can't tell which it is and being both conflicts with it being a plane on the axis.
What does the customer claim datum C is?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Interpretation of datum -C-
1.What is the datum compound A-B?
2.Datum feature C shall not be placed on the centerline, but on an actual feature. So, datum C is the axis of what feature?
3. Same question as above for datum feature D
Just make sure you abide to the rule of higher precedence datums. A-B probably will arrest 3 or 4 degrees of freedom. (3 if they are planes and 4 if they are axes). Then C will arrest the remaining ones--if capable. Then, finally D the ones left DF (s) if impose by the functionality of the product.
RE: Interpretation of datum -C-
RE: Interpretation of datum -C-
the customer is using the edge of one of the 30 degree opening and I agree with you on its usefulness as a datum feature.
RE: Interpretation of datum -C-
1. A-B is an axis through 2 concentric bores which is at the intersection of datum C and the horizontal centerline of the large bore
2. I interpret datum C to exist on the axis of A-B that passes through one of the M3 holes
3. Datum D exists on a face on the opposite side of the view I supplied that is perpendicular to A-B
RE: Interpretation of datum -C-
RE: Interpretation of datum -C-
You should have determined what they thought was the clocking feature before machining the part instead of guessing. Live and learn. Nothing on the drawings so far clarify the orientation or location of datum C.
RE: Interpretation of datum -C-
RE: Interpretation of datum -C-
RE: Interpretation of datum -C-
One problem with this approach is you assume the person on the other end understands GDT. But given the "bad" drawing, they most likely do not. So a learning session on their part may be necessary to get your response understood. Telling the customer they don't know GDT is dangerous. Tread lightly. Of note: this is one reason why I participate in this forum; my responses help with my written GDT communication skills
Certified Sr. GD&T Professional
RE: Interpretation of datum -C-
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
RE: Interpretation of datum -C-
RE: Interpretation of datum -C-