Fits

[mklotz]

Fits of shafts and holes are frequently referred to by a term, e.g, press, shrink, etc., rather than an explicit numerical size difference. Professionals memorized all this in their apprenticeship but It can be very frustrating for the amateur machinist.

Machinery's Handbook devotes nearly fifty pages to the subject of various fits of a shaft to a hole. Tubal Cain's Model Engineers Handbook devotes less than half a page to the subject. I extracted the latter material and converted it into a program (FITS) that can be downloaded from my page. However, for the benefit of folks who don't wish to download the program, here is the information it contains in numerical form. The necessary math is trivial so the program, while convenient, isn't really necessary to obtain an answer.

Entries in the table below are: fit name, constant, allowance
constant is measured in thousandths of an inch
allowance is measured in thousandths of an inch per inch of shaft diameter

Examples:

For a push fit on a nominal 1" shaft, machine the hole to exactly 1.0000",
and machine the shaft to -0.35*(1.0) - 0.15 = -0.5 thou less than the nominal size (0.9995").

For a shrink fit on a nominal 1" shaft
1.5*(1.0) + 0.5 = 2.0, i.e. shaft size = 1.002"

Shrink,0.5,1.5
Force,0.5,0.75
Drive,0.3,0.45
Push,-0.15,-0.35
Slide,-0.3,-0.45
Precision Running,-0.5,-0.65
Close Running,-0.6,-0.8
Normal Running,-1.0,-1.5
Easy Running,-1.5,-2.25
Small Clearance,-2.0,-3.0
Large Clearance,-3.0,-5.0