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igo                              THE  MACHINE-SHOP
3.  The blades are fitted in recesses, and are expanded with wedges driven
beneath them.
4.  The blades fit in inclined slots, and are expanded by driving them
inwards towards the higher ends, with or without using locking nuts for
their retention.   The blades are ground in place while the body is mounted
on centres.
5.  In this group the blades are fitted in inclined slots, and are moved
up with nuts coned on the inside to retain the blades, with or without lock
nuts.   This design is much to be preferred to the last, because the movements
imparted to the blades are simultaneous and more precise, and regrinding
is not necessary after the setting.   Many of the best reamers are made in
this way.
6.  Blades in slots rest upon a central tapered plug or " cone bolt ",
which, being forced inwards, expands all the blades equally.   The locking
is effected with nuts.   In the " Vickers " design the expansion is imparted
without longitudinal movement of the blades.    In a sub-group the blades
are expanded with two cones, reversed, which are drawn towards each other.
A large range of diameters can be obtained with these.
7.  In some designs an eccentric or cam bolt has a series of cams like very
shallow ratchet teeth, which by their partial rotation cause the blades to move
Milling-cutters.—These have gone through a larger evolutionary
growth than any other single group of cutting-tools. They range from J in.
diameter to several feet; include true cutting as well as scraping teeth; can
be used to rough and finish work; and produce not only plane surfaces but
combinations of horizontal and vertical faces, and curved and irregular
Teeth, Speeds, Feeds.—Milling-cutters have very little in common
with the single-edged cutting-tools, since their teeth operate in quick suc-
cession over broad surfaces. In the edge mills taking deep cuts the angles
of presentation will change, and the teeth will rub on the leaving edge. Also
the chips will become entangled between the teeth and cause friction. The
teeth of all the early cutters were pitched too finely to permit of their use as
roughing-tools. Coarser pitches are imparted now, and roughing-cutters
may be had, but generally the same cutter is employed for both functions.
For roughing, the teeth are often notched to break up the chips, and all
except the narrowest cutters have spiral teeth which effect a gradual cut.
A cutter must not be run at a high speed, since its teeth would become choked
with chips, but the feed should be coarse. Feeds have been increased
amazingly. They are stated in terms of advance in inches per minute, or
in fractions of an inch per revolution of the cutter. A more practical test is
the number of cubic inches of material removed per minute. The end
mills are more efficient as roughing-tools than the edge cutters are, since
there is no change in cutting angles and the chips get away freely. As a