Section 13. qxd

PLANING AND SHAPING
Planers
are used to rough and finish large flat surfaces, although arcs
and special forms can be made with proper tools and attachments.
Surfaces to be finished by scraping, such as ways and long dovetails
and, particularly, parts of machine tools, are, with few exceptions,
planed. With fixtures to arrange parts in parallel and series, quantities
of small parts can be produced economically on planers. 
Shapers
are
used for miscellaneous planing, surfacing, notching, key seating, and
production of flat surfaces on small parts. The tool is held in a holder
supported on a clapper on the end of a ram which is reciprocated
hydraulically or by crank and rocker arm, in a straight line.
BROACHING
Broaching
is a production process whereby a cutter, called a 
broach,
is
used to finish internal or external surfaces such as holes of circular,
square, or irregular section, keyways, the teeth of internal gears, multi-
ple spline holes, and flat surfaces. Broaching round holes gives greater
accuracy and better finish than reaming, but since the broach may be
guided only by the workpiece it is cutting, the hole may not be accurate
with respect to previously machined surfaces. Where such accuracy is
required, it is better practice to broach first and then turn other surfaces
with the workpiece mounted on a mandrel. The broach is usually long
and is provided with many teeth so graded in size that each takes a small
chip when the tool is pulled or pushed through the previously prepared
leader hole or past the surface.
The main features of the broach are the pitch, degree of taper or
increase in height of each successive tooth, relief, tooth depth, and rake.
The 
pitch
of the teeth, i.e., the distance from one tooth to the next,
depends upon tooth strength, length of cut, shape and size of chips, etc.
The pitch should be as coarse as possible to provide ample chip clear-
ance, but at least two teeth should be in contact with the workpiece at
all times. The formula 
p 
0.35 
may be used, where 
p
is pitch of
the roughing teeth and 
l
the length of hole or surface, in. An average
pitch for small broaches is 
to 
in (3.175 to 6.35 mm) and for large
ones 
to 1 in (12.7 to 25.4 mm). Where the hole or other surface to be
broached is short, the teeth are often cut on an angle or helix, so as to
give more continuous cutting action by having at least two teeth cutting
simultaneously.
The degree of 
taper,
or increase in size per tooth, depends largely on
the hardness or toughness of the material to be broached and the finish
desired. The degree of taper or feed for broaching cast iron is approxi-
mately double that for steel. Usually the first few teeth coming in con-
tact with the workpiece are undersize but of uniform taper to take the
greatest feeds per tooth, but as the finished size is approached, the teeth
take smaller and smaller feeds with several teeth at the finishing end of
nearly zero taper. In some cases, for soft metals and even cast iron, the
large end is left plain or with rounded lands a trifle larger than the last
cutting tooth so as to burnish the surface. For medium-sized broaches,
the taper per tooth is 0.001 to 0.003 in (0.025 to 0.076 mm). Large
broaches remove 0.005 to 0.010 in (0.127 to 0.254 mm) per tooth or
even more. The teeth are given a