Lathes Page: 84
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49-51 AIR CORPS
wheels, pulleys, sleeves, etc., as well as for the assembly of permanent
parts where load and service requirements are not severe and other
holding methods are impractical.
(6) Force.-This fit is comparable to the driving fit but has a
greater difference in sizes. This difference makes it necessary to use
some mechanical means to force the members together. Forcing
presses of various types are used to accomplish assemblies with this
class of fit. In both the drive and force fit it is possible that the
members may gall when put together. To prevent this a heavy
lubricant of some kind should be applied before assembly.
(7) Shrink.-In this fit the outer member is made slightly smaller
than the inner member, then expanded by heat, slipped over the inner
member, and allowed to cool. Shrink fits are used where maximum
grip is required between the parts and are found valuable in work
where an intensity of stress will be applied to the united parts.
(8) Expanding.-There are times when a shrink fit is desired
between certain members but the nature of the work will not allow
the outer member to be heated. In this case the inner member is
cooled to a low temperature by use of solidified carbon dioxide
(-1100 F.) or liquid air (-2200 F.). This causes the member to
shrink in size. While in this contracted condition it is assembled
and allowed to expand.
(9) Thread.-The National Screw Thread Commission has estab-
lished five distinct classes of screw thread fits known as: class I
(loose fit), class 2 (free fit), class 3 (medium fit), class 4 (close fit),
and class 5 (wrench fit).
50. Allowance.-It will be noted throughout the different fits that
certain intentional differences in the sizes of mating members must
be made to obtain the result desired. This intentional difference is
termed "allowance." Before an allowance can be specified certain
information must be available, such as the kind of fit, class of fit,
kind of metal, mass of metal, length of bearing, and finish of surface.
51. Tolerance. The term "tolerance" is used to denote the varia-
tion permitted in the dimensions of a machined part and may be
defined as the difference between the maximum and minimum limits
of a dimension. It is generally specified as being plus or minus a
certain number of thousandths or ten-thousandths and is written
immediately following the dimension. The greater the tolerance the
more rapidly the work can be produced, although the constant
demand is for smaller limits in all machine work. Charts giving
allowance and tolerance for the various fits may be found in various
handbooks on machine work.
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/86/: accessed July 27, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.