_SCREW-CUTTING._
The tools employed for cutting screw threads constitute a separate
class among the implements of a fitting shop, and it is considered
best to notice them separately.
Screw-cutting is divided into two kinds, one where the blanks or
pieces to be threaded are supported on centres, the tools held
and guided independently of their bearing at the cutting edges,
called chasing; the other process is where the blanks have no axial
support, and are guided only by dies or cutting tools, called
die-cutting.
The first of these operations includes all threading processes
performed on lathes, whether with a single tool, by dies carried
positively by slide rests, or by milling.
The second includes what is called threading in America and
screwing in England. Machines for this purpose consist essentially
of mechanism to rotate either the blank to be cut or the dies, and
devices for holding and presenting the blanks.
Chasing produces screws true with respect to their axis, and is the
common process of threading all screws which are to have a running
motion in use, either of the screw itself, or the nut.
Die-cutting produces screws which may not be true, but are still
sufficiently accurate for most uses, such as clamping and joining
together the parts of machinery or other work.
Chasing operations being lathe work, and involving no principles
not already noticed, what is said further will be in reference to
die-cutting or bolt-threading machines, which, simple as they may
appear to the unskilled, involve, nevertheless many intricacies
which will not appear upon superficial examination.
Screw-cutting machines may be divided into modifications as
follows:--(1) Machines with running dies mounted in what is called
the head; (2) Machines with fixed dies, in which motion is given to
the rod or blank to be threaded; (3) Machines with expanding dies
which open and release the screws when finished without running
back; (4) Machines with solid dies, in which the screws have to be
withdrawn by changing the motion of the driving gearing; making in
all four different types.
If these various plans of arranging screw-cutting machines had
reference to different kinds of work, it might be assumed that all
of them are correct, but they are as a rule all applied to the
same kind of work; hence it is safe to conclude that there is one
arrangement better than the rest, or that one plan is right and
the others wrong. This matter may in some degree be determined by
following through the conditions of use and application.
Between a running motion of the dies, or a running motion of the
blanks, there are the following points which may be noticed.
If dies are fixed, the clamping mechanism to hold the rods has to
run with the spindle; such machines must be stopped while fastening
the rods or blanks. Clamping jaws are usually as little suited
for rotation on a spindle as dies are, and generally afford more
chances for obstruction and accident. To rotate the rods, if they
are long, they must pass through the driving spindle, because
machines cannot well be made of sufficient length to receive
long rods. In machines of this class, the dies have to be opened
and closed by hand instead of by the driving power, which can be
employed for the purpose when the dies are mounted in a running
head.
With running dies, blanks may be clamped when a machine is in
motion, and as the blank does not revolve, it may, when long, be
supported in any temporary manner. The dies can be opened and
closed by the driving power also, and no stopping of a machine is
necessary; so that several advantages of considerable importance
may be gained by mounting the dies in a running head, a plan which
has been generally adopted in late years by machine tool makers
both in England and America.
In respect to the difference between expanding and solid dies it
consists mainly in the time required to run back, and the injury
to dies which this operation occasions. Uniformity of size is
within certain limits insured by solid dies, but they are more
liable to derangement and less easy to repair than expanding or
independent dies.
Another difference between solid and expanding dies, which may be
pointed out, is in the firmness with which the cutting edges are
held. With a solid die, the edges or teeth being all combined in
one solid piece, are firmly held in a fixed position; while with
expanding dies their position has to be maintained by mechanical
devices which are liable to yield under the pressure which arises
in cutting. The result is, that the precision with which a
screwing machine with movable dies will act, is dependent upon the
strength of the 'abutment' behind the dies, which should be a hard
unyielding surface with as much area as possible.
Connected with screw dies, there are various problems, such as
clearance behind the cutting edge; whether an odd or even number
of edges are best; how many threads require to be bevelled at the
starting point; and many other matters about which there are no
determined rules. The diversity of opinion that will be met with on
these points, and in reference to taps, the form of screw-threads,
and so on, will convince a learner of the intricacies in this
apparently simple matter of cutting screw-threads.
(1.) Describe the different modifications of screw-cutting
machines.--(2.) What is gained by revolving the dies instead of
the rod?--(3.) What is gained by expanding dies?--(4.) What is
the difference between screws cut by chasing and those cut on a
screw-cutting machine?