GD&T question

[[To...]]

Top of part is Datum A. Part is 1.000 thick. Angled feature on part is dimensioned with a BASIC dimension of .625 from bottom of part. Do I treat the location of this feature using 1.625 as BASIC or do I need to establish location as .625 BASIC from bottom of part?

[[Cl...]]

I see this a lot. Whom ever drew this, might actually be wrong, but I usually give them the benefit of the doubt.
I try both methods, to see the difference between the two. That might shed some light on which one is correct?

[[Cl...]]

http://www.etinews.com/blog/tag/basic-dimension/

[[To...]]

Here's the drawing

TPos question.jpg

[[Cl...]]

Does Datum -A-, or the opposite surface mate with another part?
Looks like the opposing surface mounts to something.

[[Br...]]

Clarke is correct. There needs to be a basic dimension back to Datum Feature A for that Position requirement. The .6250 Basic is incorrect. You could add the 1.000 to it and assume that should be be basic dimension. But unless there is more to this drawing that I cannot see, it would appear that this is drawing incorrectly.

[[Ri...]]

It looks like the pivot point is 2.502 Basic from -B-. You could test this by creating a theoretical plane, parallel to -B- with that 2.502 Basic. Once you have that, create a couple of intersection points with the cylinders. They won't buy the part but they could be informative. If all that passes muster, then create another alignment with a theoretical point from the 2.502 and whatever other Basics might apply and rotate that alignment to the appropriate Basic angles.

[[Aa...]]

Going with my gut from what little info is there, I would say this is an issue we see all too frequently. The datum features are arbitrarily selected sides of the part that have absolutely no bearing on the function of the part. (Despite Y14.5 section 4.8.) This suspicion is mostly based on datum feature C, since the part is symmetrical in that direction and one side has been selected. Also, as Clarke points out, the counterbores on what appear to be bolt holes are positioned for the bolts to draw the part against the face opposing A.

I don't know what the general tolerance is for a 3-place decimal in this case, but I doubt it's even the same order of magnitude as the .0003" diametrical tolerance on the holes in question (especially since the bolt holes are 3-place). The difference in results when you switch from A as the primary to the opposite side as the primary will depend mostly on how close that thickness actually is to 1.000. And how many parts are being made? You could test the difference for a single part, but it may not be very reflective of all parts.

At the end of the day, Brett is correct that without enough basic dimensions to fully define the basic position of the controlled holes in the DRF, the drawing is incomplete. A toleranced dimension (even general tolerance) technically can't be substituted for a basic dimension. (If they say that a specific CAD model defines the basic geometry anywhere, this point is moot.)

I would try to go back to who designed the part and find out what they intend. Technically, you have to work from the feature they indicate as the datum. Realistically, if they've made a mistake, that won't ensure the part will function as intended. And really, the information you don't have should be really important to production/manufacturing.

[[Me...]]

I know this part Tom....Good Luck.