Crimping or twisting the blade is illustrated in Fig. 4.
Fig. 4 - Twisting the bit.
The cutting end of the hot blank is inserted in the
"buck A, the rear end of the blade being supported in
the fork B while the square on the shank is held by
tongs in the operator's left hand.
With his right hand, the operator turns a crank
connected to the spindle of the machine. Twisting is
continued until in the judgment of the operator the
number of turns and the lead are correct, no
hard-and-fast rule being followed. Operators on this
work, however, become so expert that the results are
Bits for boring treenail holes in wooden-ship
construction and for other deep-hole work are not
twisted but are crimped between dies in a machine of the
bull-dozer type. Such bits must have the bottoms of the
crimps accurately formed so that the chips will be
brought out of the hole and not clog the bit.
After twisting, the bit is reheated at the end and
headed. In this operation, both the cutting and scoring
lips are formed in dies, the action of the machine being
similar to that of a bolt-heading machine.
The next step is to even up the twist between dies in
the lever-operated fixture, Fig. 5, after which the bit
Fig. 5 - Evening the crimps.
The blank for the leader screw is hollow-milled to size
and shape in a hand lathe and the outside diameter is
ground to size.
In this latter operation, the bit is held in a chuck in
the live head of the machine, Fig. 6, and supported at
the other end by a female center into which the blank
for the leader screw fits.
Fig. 6 - Grinding to size by a friction wheel.
The action of the machine is similar to that of any
cylindrical grinding machine except that the actual
grinding is not done by an abrasive wheel, as would
naturally be supposed, but by a steel wheel running at a
very high peripheral speed. The removal of metal from
the bit is by friction. There seems to be little wear on
the wheel which is evidenced by the fact that the wheel
now on the machine has been in constant use for about 17
Fig. 7 - Grinding the clearance.