CHASERS, ETC.
It is not always an easy task to chase a true thread on a piece of
work, and even “the boldest holds his breath for a time,” if he has a
nice piece of work all done but the thread, and that in a critical
part. It is so easy to make a drunken thread, or one in which the
spirals are not true, but diverge or waver in their path around the
shaft, that many are made. That they are more common than true threads,
is well known to mechanics. To start a thread true is quite easy with
an inside chaser; for, strange as it may seem, it is seldom that a
drunken thread is made on inside work; only have the bore itself true,
and the chaser will run in properly. The case is different when a bolt
or shaft is to be cut. With fine threads, the slightest obstruction on
the rest will cause the chaser to catch and stop slightly. No matter
how slight the stoppage, it is certain to damage the thread. The injury
is more perceptible on fine threads than on coarse, for, in the former,
if the threads do not fit (as they will not if they are drunken, one
crossing the other, when both parts are put together), the drunken
thread will not come fair with the other. In coarse threads, however,
it will not be so apparent, for, by making the drunken thread smaller,
it will have play and accommodate itself to its place. This is not
workmanship, it is “make-shift.”
To chase a true thread the rest must be smooth and free from burrs or
depressions. Nice workmen keep a special rest, with a hard, polished
steel edge, expressly for this purpose.
If the chasers themselves are smoothly finished at the bottom, on an
emery wheel, they are all the better. With these precautions, and
others noted below, success is certain. When a thread is to be started,
take a fine diamond-pointed tool, and hold it on the end of the shaft
to be chased. Set the lathe going, and give the tool a quick twist with
the wrist, so that a spiral will be traced on the work, like Fig. 17.
[Illustration: Fig. 17.]
Some part of this will correspond with the pitch of the thread to be
cut, and there is less liability of making it drunken. By a little
practice, one is able to hit the pitch of the chaser exactly in making
a start.
“There is no trouble, after you once know how.” We have chased
quantities of small screws, with forty-eight threads to the inch, and
not a sixteenth of one inch in diameter. If the chaser once hesitates
on such screws, they are spoiled. For heavy threads—seven and eight to
the inch, which is about as hard work as any one wants to do,—it is
the custom of some turners to use a tool with only two teeth, and some
use only a sharp-edged cutter, like Fig. 18, to deepen the thread, the
chaser being used afterward, to rectify the job. There is danger with
this tool, unless it is used by an expert, of digging out the thread,
so that the last end of it will be worse than the first.
[Illustration: Fig. 18.]
Another tool, used in chasing heavy threads, is a doctor. This consists
in having a fac-simile of the thread to be cut on the back of the
chaser, and in applying a short set screw behind, so that, as the iron
is cut away, the chaser may be followed up behind. Fig. 19 is the
doctor, but the follower opposite the chaser is too narrow, and should
be made nearly half a circle to avoid slipping; with this exception it
is all right.
These tools, and the screws made by them, are all inferior to those
made by lathes with traversing mandrels; that is, a mandrel which
slides in and out of the head stock, as in a Holtzapffel lathe.
[Illustration: Fig. 19.]
This lathe has a series of hubs, unlike the one shown previously,
slipped over the back end of the lathe spindle (furthest from the
workman) and a fixed nut on the head-stock, which, being put in
communication with the hub on the mandrel, drives the same in and out,
according to the direction the cone-pulleys are turned. Of course, with
such an attachment as this, there is no danger of making drunken
threads, for the hubs which start the threads, are cut with a train of
gears in an engine lathe, so that it is impossible for them to be
incorrect. Moreover, a square thread, or a V-shaped thread, can be made
with them, which is not the case with common chasers.
In lathes that have traversing mandrels to cut screws, the tool itself
remains stationary, but as this is obviously a disadvantage in many
kinds of work, it is far better to have the tool advance and the
mandrel revolve as usual. By this plan much time is saved, a greater
range of work is possible with the same gear, and a piece that is
chucked, or one that is between the centers, can be cut with equal
facility.
Any common lathe can be rigged to do this by putting a shell on the
back end of the mandrel, between the pulley and the set screw, and
slipping the hub over the shell, with a feather, to keep it from
turning. To take a thread from this hub, a round bar must be set
parallel with the shears, in easy-working guides. The bar must have an
arm at one end, to reach over to the hub, said arm to be fitted with a
piece of hard wood, to match the thread on the hub. The other end of
the bar has the cutting tool in it; of course, at right angles, so as
to run in to the work, and bear on the tool rest. The tool is held in
an arm on the bar by a set screw, so that it can be lengthened or
shortened.
By this arrangement, a true thread can be rapidly generated on any rod,
hollow cylinder, or other kind of work—the pitch depending on the
pitch of the hub.
It is necessary to have as many different hubs, varying in pitch, as
there are different kinds of work to be done, and, although the thread
on the hub is only an inch or half an inch long, perhaps, a screw of
any length may be cut on a rod, by simply shifting the cutter on the
rest. This same bar is also useful for turning, as with a slide rest,
for, by sliding it along gradually, it acts, in a measure, like a fixed
tool in a slide rest.
[Illustration: Fig. 20.]
From these hints the amateur who takes a lathe in hand for the first
time, or is, at best, a neophyte, may learn much to his advantage.
Persons of a mechanical turn only need a hint, when the mind springs to
the conclusion with surprising rapidity.
The little tool, shown in Fig. 20, is very handy in many instances,
particularly for running under the necks of screws when the thread is
cut up to the head. By so making them, the head comes fair down upon
its bed, and holds much better.