Lathes Page: 83
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49. Types of fits.-a. General.-In the fitting of the various parts
of any mechanism, certain differences in dimensions of the mating
members must be made so that they may operate properly.
b. Description.-(1) Rlnning or revolving.--In this fit the shaft
or revolving member is slightly smaller than its bearing to provide
space for lubrication. This clearance is also necessary to avoid
any danger of seizing or galling due to expansion. Running fits vary
from 0.0002 inch to 0.015 inch, depending on the size, and class of
work being fitted. The lathe headstock spindle with its bearing is
a good example of this type of fit.
(2) Sliding.--In this class of fit the line of motion is at right
angles to that of the running fit, and the clearance can be made
somewhat smaller. It is considered a good practice to machine memn-
bers which require this class of fit so that the tool marks will be in
the direction of travel. If this is not possible, the work should be
draw-filed and polished so that the lines are parallel to the motion.
Good examples of the sliding fit are the lathe tailstock spindle in its
bearing and the lathe carriage on the ways.
(3) WVrirnging.-In this fit the clearance between the male and
female members is so slight that the parts have practically a metal-
to-metal contact. A fit of this kind is difficult to produce in the
lathe due to the close tolerances required. A good example of the
wringing fit is to be found in the standard ring and plug gages.
These gages, instead of being sized in the lathe, are hardened,
ground, and lapped. The fit derives its name from the action nec-
essary to bring the parts together. For example, it is not easy to
insert the plug into the ring with a straight push, but if this push is
accompanied by a slight rotary or wringing motion the gages will
go together readily.
(4) Tight.-In lathe work this is understood to mean a fit so close
as to make hand assembly of the members difficult yet not so close
as to require driving or other forcing by mechanical means. This
type of fit is used for gears, pulleys, etc., that are secured to shafts
by keys, set screws, or taper pins, so that they may be easily mounted
and dismounted when the retaining members are removed.
(5) Driving.-The internal member in this case must be distinctly
larger than the external member, although the difference must not be
so great that the parts cannot be put together by hammer blows. It
is a good policy to taper the receiving ends of such members slightly
so that they may start and line up easily. Assemblies with this class
of fit are generally considered permanent although they may be dis-
sembled. They are generally used for the placement of bushings in
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/85/: accessed March 28, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.