Looking for some help on a tolerance question

[UGMENTALCASE]

Morning all,

I have been asked a question which got me thinking. I have attached a little sketch. So the bottom image is a plate with 2 pins, the top image is a plate with 2 holes, they fit on top of each other.

Normally we would tolerance the holes and pins with GD & T and once assembled, finish machine the dimensioned edge. But it got me thinking is there a way to figure out where the edge would end up by finish machining each part separately. So we finish machine the pins on the plate. We finish machine the holes and edge on the top plate. Once assembled the edge needs to be within the tolerance shown for example.

So to get the holes fitting, you can use the formulas for fixed/floating fasteners, depending on the finished article.
So what happens to the edge? The size difference and position tolerance would affect where the face ends up. So is there another formula or some way of working out where the edge could end up?

Thanks in advance

 

[CheckerHater]

There is no formula per se, but the entire discipline called "TOLERANCE STACK-UP ANALYSIS"
Unfortunately, it's another "larger than life question" - thick books are written about it.

From GD&T point of view it will make sense to use your hole(s) as datum and apply profile to surface in question
Since you provided very limited information about fit form and function, it's hard to come up with simple instructions (if they are ever simple)

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[chez311]

Ditto what CH said - simply googling "tolerance stack up" should come up with a plethora of resources. I'm not sure how everyone else on this forum feels about it but I've found "Mechanical Tolerance Stackup and Analysis" by Bryan Fischer to be a good resource. Like most GDnT resources there are some errors but overall a pretty thorough treatment of the subject as well as relatively accessible/readable with plenty of examples.

As far as analyzing your described situation, perhaps an example of a drawing you have could be provided - or you could mark up an example with more complete dimensions/tolerances and geometric controls - and give what feature/condition is to be analyzed and we could walk through it.

 

[drawoh]

UGMENTALCASE,

Making pins insert into clearance holes is straight forward mathematics. How accurately do you need to locate stuff?

--
JHG

 

[UGMENTALCASE]

Tolerance stack up is the one book I've not got to yet. It brushes on it in the books I have by James D Meadows but I don't have the specific tolerance stack up. Maybe another investment is in order!

I don't have a specific example which I can share, but I had a feeling it would be something to do with stack up.

 

[UGMENTALCASE]

Checker hater, true about the datum as holes. This would be my normal practice, but in doing that and adding the assembly tolerance I got thinking there must be some sort of calcs which work it all out. So you would gd and t the holes position and fit. Which would allow an amount of float, however big or small. So that would have a knock on affect on the edge.

I think I'll check out some stack up examples and probably buy James D Meadows book on stack up as well