different ratios for screw-cutting. These again are doubled or trebled by altering the ratios of other gears connected therewith, so
Fig. 2.
A, Simple train which rotates lead screw in opposite direction to mandrel, and makes slide-rest feed away from the headstock.
B, Simple train with intermediate wheel on stud, which rotates lead screw in same direction as mandrel, making slide-rest feed towards the headstock. Intermediate on "stud" does not alter ratio.
C, Typical compound train arranged for cutting a screw finer than that of the lead screw.
D, Ditto for screw coarser than that of the lead screw.
that for each position of engagement of the stud wheel, two, or in some cases three, pitches can be cut. This avoids the waste of time involved in setting up fresh wheels on the swing-plate as often as a screw of different pitch has to be cut.
Another step in the direction of economy depends on the removal of all screw-cutting, except those screws which are of several feet in length, from the ordinary lathe to the special chasing and screwing machines. The screw-cutting arrangement of an engineer's lathe is a cumbrous apparatus to fit up and set in motion for the cutting of screws of small dimensions. When there was no other method available except that of common dies operated by hand or carried in a screwing machine, there was good reason why a true cutting tool should be operated in the lathe through change wheels. But the reason no longer exists, since for the single cutting tool of the lathe the two or three cutters of the chasing and screwing machines (figs. 3 and 4) are substituted, and the hollow mandrel embodied in the latter permits of screws being cut and parted from the solid bars of several feet in length. Except for the cutting of long screws and screws of odd pitches, the ordinary lathe is now a wasteful machine.
Fig. 3.—Bolt-Screwing Machine (John Stirk & Sons, Ltd., Halifax).
A, bed.
B, Spindle.
C, Four-step belt pulley, driving through triple spur gears D, to B.
E, Opening die head.
F, Bolt carriage racked to or fro along the bed by rotation of hand-wheel G.
H, Handle for opening and closing vee-jaws at a for gripping and releasing bolts by means of a right- and left-hand screw.
J, Handle for opening the dies.
K, Lever for automatically opening the dies, operating through J.
L, Rod having adjustable dog b, struck by carriage at a definite position of its travel, thus throwing the dies off the work.
M, Pump drawing lubricant from reservoir in bed.
Fig. 4.—Opening Die-head for Screwing Machine.
A, Spindle end.
B, Sliding collar.
C, Ring bolted to B, and enclosing ring having three coned grooves a, a, a, set eccentrically to close in or let out the chasers D.
E, Curled spring keeping chasers outwards in contact with a.
F, Piece screwed to end of A, and provided with three grooves to carry the chasers.
G, Cover plate confining the chasers, and unscrewed from F when changing chasers for other sizes.
The second method of cutting screws is that by means of hobs or leaders, and either comb or single-edged tools. That is, a short
