Bonus Tolerance at RFS

[JLang17]

Threaded hole = position .005 RFS
Clearance hole = position .005 MMC

As the clearance hole departs from MMC it gains bonus tolerance. Let's say the size increases, but the fabricator manages to maintain a position within .005.

Is it then assumed that the remaining tolerance is applied to the threaded hole (overriding the .005 RFS)? Or is there a way of stating this on the drawing?

Btw, I know there has been some debate over RFS on a threaded hole. I don't want this to be another thread on that...no pun intended.

Thanks in advance!

 

[KENAT]

JLang17, the short answer is that as far as I know or can think of NO.

I assume that the threaded hole & clearance holes are in mating parts.

Unless these parts are somehow treated as a matched pair I don't see how you could hope to make this idea work, or am I missing something?

You might be able to achieve something similar by match drilling at the assy level but for separately detailed, interchangeable parts I don't see how your idea could work.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[JLang17]

I will try to clarify:

Picture the two holes concentric and increase the clearance hole size to its LMC, whatever it may be. Now the clearance hole has bonus positional tolerance. To simplify things, let's say the fabricator drilled the clearance hole spot on; exactly to the basic locating dimensions. Now we have this bonus tolerance that's unused, so can't we give it to the threaded hole?

 

[KENAT]

How?

How do you give it to the 'other part' without making them a matched set where the threaded hole is created to match the clearance hole in the other part?

If doing it at the assemly level and effectively creating matched parts you could probably do something like make the clearance hole a datum. You could then locate the threaded hole to it with the datum at MMC. However, from a manufacturing point of view I'm not sure you wouldn't just be better off just match drilling, will depend on factors you don't list.

Given the relative difficulty of creating typical clearance holes compared to threaded holes, you may be best off just reducing the position tol on the clearance hole, relying more on the 'bonus tol' from MMC assuming you have reasonable size variation, and give more of it to the threaded hole. Instead of your .005 each in your example maybe .008 to the threaded hole and .002 to the clearance hole, assuming a typical trill tol of something like -.0001+.005 or there abouts depending on drill size.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[drawoh]

JLang17,

The whole assumption behind clearance holes and proper tolerancing is that any part made to drawing[ ]A can be assembled with any part made to drawing[ ]B. The answer to your question is "No". There is no bonus tolerance when the mating part is oversize.

If you send the assembly drawing out for fabrication instead of the two fabrication drawing, I would interpret that the tolerance on your screw locates the assembled screws. The fabricator has all sorts of options, one of which is to pilot drill everything. Don't do this if you want interchangeable parts.

JHG

 

[JLang17]

This is just another case of overthinking myself into an irrelevant train of thought that ultimately leads to a slap on the forehead. You're both clearly right, interchangability is what I didn't consider.