How is "fit" quantified

[kjk]

I have been trying to understand the various types of "fit" referred to in plans and in the various build threads. I've searched around the net but not been able to find any definitions which indicate how close in size one part is to another when a particular type of "fit" is indicated.

For example, if a 0.375 inch shaft is to run in a bushing, what should the size of the hole be? Is this what is described as a "close fit"?

Similarly, what would the size of the hole be for a "press fit"?

I've also seen mentioned "interference fit" which I gather is constructed by heating one part and chilling the other to the point where a "press fit" can be made. I may be completely wrong here.

There is also the technique used where a bearing for example is inserted into a bored hole and loctite used to secure it. What are the relative sizes of the parts in a case like that?

I realize that in practice a machinist will from experience, depending upon the materials being used and possibly the phase of the moon, know how close to machine the parts and will then (guessing here) sneak up on the final fit.

What I'm looking for as a complete rookie is a target to shoot at when undertaking these various types of "fit".

Is there a book that covers this sort of thing?

 

[steamer]

A Machinery's Handbook has all that and more....I would recommend getting one as fits are something that has been quantified.

It's OK to buy a used one...they are cheaper than the new ones and most of the machining information is still good even back as far as the 40's.

Dave

 

[kjk]

Thanks for that. I will see if I can find one.

 

[steamer]

Let wiki and google be your guide too.

http://en.wikipedia.org/wiki/Interference_fit

Dave

 

[Deanofid]

I think there is one on Google Books, Woody. An old edition, but perfectly usable in most respects, and
you can view it at no charge. You'll still want a copy for the shop someday.

Lots of people use completely inappropriate terms trying to describe one fit or another. It's easy to
make those mistakes, (myself included). You'll find that the dimensions for various fits change with
the size of the work piece, and what might be a running fit for one size shaft will be a push fit for another.

Dean

 

[kjk]

Free is good - especially since the new price is north of $100. I'll have a look in google books and check abebooks as well.

 

[gbritnell]

What Dave said is about the best information you can find. The only problem is knowing which one to use. In the case of two parts running together, sliding or rotating, you always need room for a layer of oil so in general .001 would be minimum but here again that depends on the overall sizes of the parts you're working with. Now a press fit. There are several of these. Here again how tight does the press need to be? Are the two pieces that are pressed together going to have a lot of radial or axial pressure applied to them or do they just need to be 'pressed' together? I'm not quite sure where 'interference fit' comes into play because this could fall into the 'press fit' category. Some pieces can be heated and chilled to fit together, a bearing in a bore for example, but for this application you have to know what the normal state dimensions would be. You can only get so much expansion or contraction of parts through heating and chilling.
gbritnell

 

[kjk]

Thanks everyone for your replies. I've got a digital copy of the book and it is proving very interesting reading. I had no idea that so many different variations of fit had been quantified.

 

[shred]

Over in this thread is a discussion of fits and a little table I made up with the common ones that might be useful.

http://www.homemodelenginemachinist.com/index.php?topic=6474.msg77445#msg77445

 

[mklotz]

My FITS program will tell you the clearance needed for most types of fits as a function of the input shaft/hole size. Download it for free from my page - URL in sig below.

 

[kjk]

Thanks Shred and Marv - Those will both help me reduce the number of opportunities in the scrap box. Thanks again to all who responded - it's a great forum.

 

[kvom]

While fits can be quantified as shown above, for us home machinists it's often difficult to actually obtain the "quantities" desired. I happen to own some over-under reamers for imperial sizes, so I will ream .001" over for a sliding fit and .001" under for a press fit. Otherwise reaming to size and using loctite is a good alternative. YMMV.