PRECISION MEASURING AND GAGING - OD1642 - LESSON 1/TASK 1
for the assembly and disassembly of stationary parts. Stationary means that
the part will not rotate against another, although they may rotate together
as part of a larger assembly.
The allowances used as examples in the
following descriptions of the various fits represent the sum of the
tolerances of the external and internal parts.
To achieve maximum
standardization and to permit common size reamers and other fixed sized
boring tools to be used as much as possible, use unilateral tolerance
method.
(1) Locational Clearance Fits. These are broken down into 11 classes
of fits.
The same basic diameter with a class 1 fit ranges from a zero
allowance to a clearance allowance of 0.0012 inch. A class 11 fit ranges
from a clearance allowance of +0.014 to +0.050 inch. The nearer a part is
to a class 1 fit, the more accurately it can be located without the use of
force.
(2) Locational Transitional Fits.
These type of fits have six
different classes which provide either a small amount of clearance or an
interference allowance, depending on the class of fit selected. The 2.500
inch basic diameter in a class 1 fit ranges from an interference allowance
of -0.0003 inch to a clearance allowance of +0.0015 inch.
A class 6 fit
ranges from an interference allowance of -0.002 inch to a clearance
allowance of +0.0004 inch.
The interference allowance fits may require a
very light pressure to assemble or disassemble the parts.
(3) Locational Interference Fits.
These fits are divided into five
different classes, providing an interference allowance of varying amounts.
A class 1 fit for a 2.500 inch basic diameter ranges from an interference
allowance of -0.0001 to -0.0013 inch. In comparison, a class 5 fit ranges
from an interference allowance of from -0.0004 to -0.0023 inch.
These
classes of fits are used when parts must be located very accurately while
maintaining alignment and rigidity. They are not suitable for applications
where one part is subjected to a force that causes it to turn on the other
part.
d.
Interference Fits.
There are five classes of fits within the
interference type.
They are all fits that require force to assemble or
disassemble parts. These fits are often called force fits. In
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