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THE  WORK  OF  THE  MACHINES               229
oid which the rack-teeth in this system are slightly rounded, and the
:ms of the cutters for pinions below thirty teeth have two curves instead
one. Undercut can also be prevented by increasing the length of the
dendum of small pinions. But other views now obtain, chiefly in conse-
ence of the growth of generating methods, of the increasing employment
the short " stub " teeth, and the desire for the closest approximation to
ithematical accuracy. Pressure angles are now increased to 18°, 20°, and
sn 25°. Gears can thus be produced without undercut down to twelve
Machines using Form Cutters.—The type of these using rotary
tters is the Brown & Sharpe. One group is used for spurs only, another
:ludes the cutting of bevel gears. Later machines include provisions for
iltiple cutting. Form planing of spur and bevel gears is represented by
5 Gleason machines. These are made to be pitched by hand or automati-
Machines for Shaping  Gear Teeth.—The " Bilgram " was the                           j
ginal machine.    It is now made for shaping spur as well as bevel teeth.                           !
is, and the Robey-Smith, employs planing tools, the movements of which                           I
: controlled by links.    The Fellows machine cuts spurs, internal gears,                           j
i helical teeth.    It employs a pinion-like cutter.    The Sykes machine                           j
ploys two cutters, which operate simultaneously.    They produce spur                           \
I helical teeth.    The Gleason planer shapes the teeth by means of a yoke,                            j
the inside of which a segment is bolted which has the same angle as that                           I
the gear to be cut.    The Sunderland machine cuts spurs and spirals,                            j
ng a reciprocating cutter containing six rack-teeth.    The machines that                           j
pe by means of hobs, cut spur, spiral, and worm-teeth.                                                       i
DIVISION VIII                                                                     ;
Grinding Machines.—Grinding has invaded the  old territory of                            |
ning, boring, and facing.    The lathe is now often a mere satellite—a                            !
ghing, a first-operation machine, playing second fiddle to the grinder.                            {
:ut is taken with a coarse feed that leaves marked spiral ridges on the                            f
face of the work.    Then the grinder performs the second operation,                            I
nely, that of fine-finishing to precise limits.    The lathe reduces with                            I
ater economy than the grinding wheel, but the latter imparts a finish in
tere fraction of the time that would be occupied by the turner in producing
precise results.   When machining allowances are slight, the grinder takes                            >
rge of the entire work.    It is not necessary to pickle, as it is when milling                            I
ters have to remove small amounts.    The grinding wheel can operate                            !
b. allowances of iV in. or less, which would give trouble to the lathe man                            |
) has to get under the skin.                                                                                               ,
Cylindrical Grinding.—This represents by far the largest volume of
t done.   The common method, to which there are exceptions, is to                            \
ite the wheel and the work, and to traverse the wheel.   The object of
traverse is to get the maximum amount of duty from the wheel, and to