Close fitting dowel restricting pos tol of clearance holes?

[randy64]

Situation: Two blocks held together with 4 screws thru clearance holes (.031 clearance on the diameter). Fixed fastener situation. In addition, there is a pin press fit into the block with the threaded holes. There is a corresponding hole for the pin in the other block that has a slip fit clearance on the diameter.

By using the fixed fastener pos tol formula we get .014 for the screw clearance holes, and .001 on the slip fit holes (equally splitting the tolerance between the holes in each block). The screw clearance holes were calculated in isolation, without any regard to the pin holes.

Question: Due to the tighter restriction put in place by the tighter fit of the pin to its clearance hole, what effect, if any, does that have on the pos tol of the clearance holes?

All holes are coming from the same datum structure.

I'm thinking I may have to tighten the pos tol of the screw holes, or open up their size. If this is the case, is there an easy formulaic way to get those numbers? If this is not the case, explain to me why.

Thanks!

 

[3DDave]

If there is a single DRF used by all the features and each tolerance is set to allow the features in the two parts to fit without regard to any other other feature pair, then nothing needs to be done.

One way to look at this is that the DRF for one part is assembled to the DRF of the other part with whatever basic offsets are needed to get the true positions of the holes to line up. When that has happened the contribution of the pin doesn't matter because each hole can be evaluated on its own. This appears to be the case you have.

If the pin is expected to be an interference fit in both parts then that will force the two parts to not be lined up according to the original DRF and they will be forced to follow the pin location, so the screw holes would have that additional offset from the original DRF to account for. If it's one pin, then that will be easy and just add the location tolerance the pin has to the clearance required for the screw holes. Most other cases are significantly more complicated and changing to use the pinholes as a DRF component makes for a far easier analysis.

 

[drawoh]

randy64,

Why not use the dowel pin as a datum?

--
JHG

 

[mkcski]

Or the opposite what drawoh said : the pattern of (4) threaded holes a datum.

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