Placing a datum on bore for positional tolerance

[AJWoodburn]

Hello everyone,

I've looked through many threads, googled, and referenced the ultimate GD&T pocket guide, and I'm just dense enough to need my specific problem answered by someone experienced.

I have a clamp that consists of stacked leaves of a polypropylene. The important location is the partial bore at the very center, but seven of these need to stack on each other with thru-bolts pinning the stack together. I figure that is reason enough to try to control the position of the bolt holes tightly in relation to the bore. I want to use a positional tolerance to locate the pattern, but I don't have a third plane to apply a datum to, so I am using the bore surface as a datum and it doesn't feel right.

What I'm doing here is simple enough, but is it legal?

Thank you

AJ

 

[pmarc]

Use the pattern of 4 thru holes as secondary datum feature B (controlled with a position callout to A first), then define bore radius basic and apply a profile tolerance to bore surface with relation to |A|B(M)|. You will not need tertiary datum feature at all.

 

[dtmbiz]

AJWoodburn

Dont believe the pocket guide will show the answer(s) to your posted drawing, little more complex than that.

1. Consider your assembly interfaces and part function for the DRF.

2. Datum A sounds reasonable considering your description (OP)

3. Datum B; not so much per your posted print.
- You can do this; however should you use this feature as you have delineated it?
- You have identified a partial cylindrical surface as a feature of size. Can of worms!
- Some will say thats okay. I say no. How will you determine the axis of a partial cylindrical surface?
- I would say if your stuck on that partial cylinder, then use target points or target lines to establish it
4. From your description in (OP), I would consider the bolt pattern as Datum B
- This is an assy interface
- Controlls 5 DOF's
- Would only need Datum B & A to full connstrain part. I would use bolt pattern as Primary Datum, however you
know the part.
5. Why do you have a multi-segment vs. a composite positional control for the bolt pattern?

6. IMO
- Make bolt pattern Datum B and controll perpendicularity to Datum A
- Control Datum A with a flattness
- Control opposite surface (stacking surf) from Datum A with parallel control
- Profile center partial cylinder with Profile control to DRF B | A
- Use basic dims accordingly for partial cyl and definition

7. FYI: Sect A-A should be behind the sectionline cutting plane.

Others may correct me... wrote this in quickly...

 

[dtmbiz]

must of been typing while pmarc was posting
agreed

 

[AJWoodburn]

Guys,

See attached, let me know if I made an interpretive mistake.

Thank you. I think I got hung up on trying to do it all with a positional tolerance.

In response to your question about the missing composite frame. It's missing because I forgot to check the check box.

Thanks again, much appreciated

AJ

 

[AJWoodburn]

dtmbiz,

I did not include a perpendicularity, flatness, or parallelism tolerance because I think that's going overboard for the part. this part is stacked seven leaves tall, and is only clamping a cable in the central partial bore...I'm only concerned that the bore position is controlled in relation to the bolt clearance holes, so that when it stacks there is a reasonable alignment between bore centers. If this part were made from a more stable material than polypropylene, I would include these tolerances.

Again, thank you both for your replies. I know this was a simple problem, and I appreciate that time you spent to address my question.

AJ