A reference or formula for the largest pin which will fit in a hole

[bradhaz]

Hi everyone!

First many thanks to all who are part of the hundreds of threads I've already read in this jewel of a forum . My employer is implementing AS9100 (small aerospace machine shop) and this site is proving invaluable in our pursuit.

For many years I have "known" that a 1 inch pin will not (easily) fit into a 1 inch hole. That one must use a gage pin "a tenth or two smaller". For some reason it just makes sense. Although the quotes imply some authority is behind this reasoning I am at a total loss to quantify and document it. I've spoken with leading gage makers who suggest I just use the next smaller size pin. Does anyone know of a reference or formula that might be used to define the largest size pin a hole will accept at perfect form and nominal hand force?

Please don't ask me if it's nominal for Daisey or Bubba and why do I find myself questioning conventional wisdom?

[Jim Wynne]

In Reply to Parent Post by bradhaz

Hi everyone!

First many thanks to all who are part of the hundreds of threads I've already read in this jewel of a forum . My employer is implementing AS9100 (small aerospace machine shop) and this site is proving invaluable in our pursuit.

For many years I have "known" that a 1 inch pin will not (easily) fit into a 1 inch hole. That one must use a gage pin "a tenth or two smaller". For some reason it just makes sense. Although the quotes imply some authority is behind this reasoning I am at a total loss to quantify and document it. I've spoken with leading gage makers who suggest I just use the next smaller size pin. Does anyone know of a reference or formula that might be used to define the largest size pin a hole will accept at perfect form and nominal hand force?

Please don't ask me if it's nominal for Daisey or Bubba and why do I find myself questioning conventional wisdom?

If hole and pin are (theoretically) the same size, it will be impossible for the pin to fit into the hole, because it's physically impossible for the OD of the pin and the ID of the hole to occupy the same plane simultaneously. No formula necessary. How IDs get measured is usually a function of criticality, when size is critical, it's alway best to get the customer to concur with your measurement method.

[bradhaz]

Thanks Jim - I’m glad we share the same opinion. My question stems from the use of go–nogo attribute gaging for non-interference fit holes. Some of my customers are old school and want to see a 1” pin pass thru a 1” hole. The functional approach is hard for me to take issue with as I’m in agreement with it’s use whenever possible. How do I justify using a smaller size as a go member? How much smaller? If you would provide your source for the ‘impossible’ part I can do the math backwards. I want to give as much of the tolerance as possible to manufacturing and not use it in gaging (tolerance is a terrible thing to waste). I’ve searched GIDEP to no avail. Maybe JPL or the likes have already covered this ground. Has anyone come across a good site or search engine for this type of research? Perhaps I should be asking what constitutes an interference fit and go from there.

[andrewb91]

I use minus pins that are .0002 or .0003 smaller than the size. I always use the "snug fit" method for determining hole size. If diameter is that critical why not use a CMM, Smartscope or Optical Comparator?

[Jim Wynne]

In Reply to Parent Post by bradhaz

Thanks Jim - I’m glad we share the same opinion. My question stems from the use of go–nogo attribute gaging for non-interference fit holes. Some of my customers are old school and want to see a 1” pin pass thru a 1” hole. The functional approach is hard for me to take issue with as I’m in agreement with it’s use whenever possible. How do I justify using a smaller size as a go member? How much smaller? If you would provide your source for the ‘impossible’ part I can do the math backwards. I want to give as much of the tolerance as possible to manufacturing and not use it in gaging (tolerance is a terrible thing to waste). I’ve searched GIDEP to no avail. Maybe JPL or the likes have already covered this ground. Has anyone come across a good site or search engine for this type of research? Perhaps I should be asking what constitutes an interference fit and go from there.

It should be considered axiomatic that two masses cannot occupy the same physical plane at the same time. Your customers are asking you to defy the laws of physics, and there is no formula to explain the concept, because no formula is necessary. Well, I'll take that back. I'm sure that equations could be used to prove the point, just as equations may be used to prove that you can't flap your arms and take flight, but it would be unusual for someone to ask you to provide mathematical proof.

For clearance holes, standard practice is to use -.0002" pins. If the margin of error in the application falls within .0002", you will need to use pins closer to the nominal size, and perhaps even have pins custom-made, but that would be very unusual for a clearance hole. There are other issues with holes as well, such as cylindricity and roundness, which pins can't help you with. Sometimes bore gages are used when you need to know something about the shape of the hole along all or part of its length.

[Jim Wynne]

In Reply to Parent Post by andrewb91

I use minus pins that are .0002 or .0003 smaller than the size. I always use the "snug fit" method for determining hole size. If diameter is that critical why not use a CMM, Smartscope or Optical Comparator?

CMMs and optical comparators, especially the latter, are not very reliable for measuring the ID of a hole. There are almost always R&R problems with CMMs and hole IDs, and optical comparators are marginally useful only for measuring the opening at the surface.

[bradhaz]

I think ASME Y14.43 Dimensioning and Tolerancing Principles for Gages and Fixtures will address the issue if not answer my question. I was lucky in finding this link to the chairman’s website:

http://www.geotolmeadows.com/newslet...bruary2003.htm

Very good stuff and worth a look for those who are curious.

In a nutshell there is a 5-10% rule addressing size and wear allowances. Three policies govern how the allowance is applied: Absolute, Optimistic, and Tolerant (ranging from sometimes rejecting a good part to sometimes accepting a bad part). Implementation is up to the company. I’ll have to get the spec and find out more.

Thanks for the input. Just trying to justify things I've always done. (those pesky auditors!)