Multiple tolerances on a hole

[mhorbacz]

I have a part that is mounting an object to it via two holes. The hole on the left is my "primary" hole in the pattern. Don't REALLY care where it lands. The hole on the right, I want tied to the first hole. I need its vertical tolerance controlled very tightly (.01 positional), but I am able to loosen the horizontal tolerance (+/-.10). My original thought was to use datums A, D, and B in the feature control frame, but then I realized that I don't have a basic dimension going between the right hole and datums D. So would the correct feature control frame only reference datums A and B? Is there anything to be gained out of including D as the secondary datums? Or would I be better off using a two single segment feature control frame?

 

[Burunduk]

You described clearly enough which relationships you want more loosely controlled and which more tightly, but you didn’t provide much information about the function. There’s more than one way a plate can be used to locate a mating part via two holes, and some common methods don’t justify designating one as "main" and using it as a datum feature for controlling the other. So, without addressing whether this is the best way to tolerance the part for its exact function, and purely based on your description, I would suggest bi-directional position tolerances. Each of the red feature control frames creates a two-parallel-planes tolerance zone to control the axis of the right hole.



One scenario I could think of where this could be useful is if the holes are for dowel pins that are intended to mate with a hole and a horizontal slot on another part. The left ("primary") pin will accurately mate with a hole and locate that mating part, and the right pin will mate with a slot to clock it. Thus, the 2.00 distance can be less accurate, but the vertical alignment (normal to datum B) is important.

 

[mhorbacz]

You described clearly enough which relationships you want more loosely controlled and which more tightly, but you didn’t provide much information about the function. There’s more than one way a plate can be used to locate a mating part via two holes, and some common methods don’t justify designating one as "main" and using it as a datum feature for controlling the other. So, without addressing whether this is the best way to tolerance the part for its exact function, and purely based on your description, I would suggest bi-directional position tolerances. Each of the red feature control frames creates a two-parallel-planes tolerance zone to control the axis of the right hole.

View attachment 3645

One scenario I could think of where this could be useful is if the holes are for dowel pins that are intended to mate with a hole and a horizontal slot on another part. The left ("primary") pin will accurately mate with a hole and locate that mating part, and the right pin will mate with a slot to clock it. Thus, the 2.00 distance can be less accurate, but the vertical alignment (normal to datum B) is important.

Thanks for the reply! Can you explain how the the result is different between what I wrote and your suggestion? To me it seems that they would provide the same exact control