_TURNING LATHES._
In machinery the ruling form is cylindrical; in structures other
than machinery, those which do not involve motion, the ruling form
is rectangular.
Machine motion is mainly rotary; and as rotary motion is
accomplished by cylindrical parts such as shafts, bearings, pulleys
and wheels, we find that the greater share of machine tools are
directed to preparing cylindrical forms. If we note the area of
the turned, bored and drilled surface in ordinary machinery, and
compare with the amount of planed surface, we will find the former
not less than as two to one in the finer class of machinery, and
as three to one in the coarser class; from this may be estimated
approximately the proportion of tools required for operating on
cylindrical surfaces and plane surfaces; assuming the cutting tools
to have the same capacity in the two cases, the proportion will be
as three to one. This difference between the number of machines
required for cylindrical and plane surfaces is farther increased,
when we consider that tools act continually on cylindrical surfaces
and intermittently on plane surfaces.
In practice, the truth of this proposition is fully demonstrated by
the excess in the number of lathes and boring tools compared with
those for planing.
An engine lathe is for many reasons called the master tool in
machine fitting. It is not only the leading tool so far as
performing a greater share of the work; but an engine lathe as an
organised machine combines, perhaps, a greater number of useful
and important functions, than any machine which has ever been
devised. A lathe may be employed to turn, bore, drill, mill, or
cut screws, and with a strong screw-feed may be employed to some
extent for planing; what is still more strange, notwithstanding
these various functions, a lathe is comparatively a simple
machine without complication or perishable parts, and requires no
considerable change in adapting it to the various purposes named.
For milling, drilling or boring ordinary work within its range, a
lathe is by no means a makeshift tool, but performs these various
operations with nearly all the advantages of machines adapted to
each purpose. An ingenious workman who understands the adaptation
of a modern engine lathe can make almost any kind of light
machinery without other tools, except for planing, and may even
perform planing when the surfaces are not too large; in this way
machinery can be made at an expense not much greater than if a full
equipment of different tools is employed. This of course can only
be when no division of labour is required, and when one man is to
perform all the several processes of turning, drilling, and so on.
The lathe as a tool for producing heliacal forms would occupy
a prominent place among machine tools, if it were capable of
performing no other work; the number of parts of machinery which
have screw-threads is astonishing; clamping-bolts to hold parts
together include a large share of the fitting on machinery of all
kinds, while screws are the most common means for increasing power,
changing movements and performing adjustments.
A finisher's engine lathe consists essentially of a strong
inflexible shear or frame, a running spindle with from eight to
sixteen changes of motion, a sliding head, or tail stock, and a
sliding carriage to hold and move the tools.
For a half century past no considerable change has been made in
engine lathes, at least no new principle of operation has been
added, but many improvements have been made in their adaptation and
capacity for special kinds of work. Improvements have been made in
the facilities for changing wheels in screw cutting and feeding,
by frictional starting gear for the carriages, an independent feed
movement for turning, arrangements to adjust tools, cross feeding
and so on, adding something, no doubt, to the efficiency of lathes;
but the improvements named have been mainly directed to supplanting
the skill of lathemen.
A proof of this last proposition is found in the fact that a
thorough latheman will perform nearly as much work and do it as
well on an old English lathe with plain screw feed, as can be
performed on the more complicated lathes of modern construction;
but as economy of skill is sometimes an equal or greater object
than a saving of manual labour, estimates of tool capacity should
be made accordingly. The main points of a lathe, such as may most
readily affect its performance, are first--truth in the bearings
of the running spindle which communicates a duplicate of its shape
to pieces that are turned,--second, coincidence between the line
of the spindle and the movement of the carriage,--third, a cross
feed of the tool at a true right angle to the spindle and carriage
movement,--fourth, durability of wearing surfaces, especially the
spindle bearings and sliding ways. To these may be added many other
points, such as the truth of feeding screws, rigidity of frames,
and so on, but such requirements are obvious.
To avoid imperfection in the running spindles of lathes, or any
lateral movement which might exist in the running bearings, there
have been many attempts to construct lathes with still centres at
both ends for the more accurate kinds of work. Such an arrangement
would produce a true cylindrical rotation, but must at the same
time involve mechanical complication to outweigh the object gained.
It has besides been proved by practice that good fitting and good
material for the bearings and spindles of lathes will insure all
the accuracy which ordinary work demands.
It may be noticed that the carriages of some lathes move on what
are termed V tracks which project above the top of lathe frames,
and that in other lathes the carriages slide on top of the frames
with a flat bearing. As these two plans of mounting lathe carriages
have led to considerable discussion on the part of engineers, and
as its consideration may suggest a plan of analysing other problems
of a similar nature, I will notice some of the conditions existing
in the two cases, calling the different arrangements by the names
of flat shears and track shears.
These different plans will be considered first in reference to
the effect produced upon the movement of carriages; this includes
friction, endurance of wear, rigidity of tools, convenience of
operating and the cost of construction. The cutting point in both
turning and boring on a slide lathe is at the side of a piece,
or nearly level with the lathe centres, and any movement of a
carriage horizontally across the lathe affects the motion of the
tool and the shape of the piece acted upon, directly to the extent
of such deviation, so that parallel turning and boring depend
mainly upon avoiding any cross movement or side play of a carriage.
This, in both theory and practice, constitutes the greatest
difference between flat top and track shears; the first is arranged
especially to resist deviation in a vertical plane, which is of
secondary importance, except in boring with a bar; the second is
arranged to resist horizontal deviation, which in nine-tenths of
the work done on lathes becomes an exact measure of the inaccuracy
of the work performed.
A true movement of carriages is dependent upon the amount or
wearing power of their bearing surface, how this surface is
disposed in reference to the strain to be resisted, and the
conditions under which the sliding surfaces move; that is, how kept
in contact. The cutting strain which is to be mainly considered,
falls usually at an angle of thirty to forty degrees downward
toward the front, from the centre of the lathe. To resist such
strain a flat top shear presents no surface at right angles to the
strain; the bearings are all oblique, and not only this, but all
horizontal strain falls on one side of the shear only; for this
reason, flat top shears have to be made much heavier than would be
required if the sum of their cross section could be employed to
resist transverse strain. This difficulty can, however, be mainly
obviated by numerous cross girts, which will be found in most lathe
frames having flat tops.
A carriage moving on angular ways always moves steadily and
easily, without play in any direction until lifted from its
bearing, which rarely happens, and its lifting is easily opposed
by adjustable gibs. A carriage on a flat shear is apt to have play
in a horizontal direction because of the freedom which must exist
to secure easy movement. In the case of tracks, it may also be
mentioned that the weight of a carriage acts as a constant force to
hold it steady, while with a flat shear the weight of a carriage
is in a sense opposed to the ways, and has no useful effect in
steadying or guiding. The rigidity and steadiness of tool movement
is notoriously in favour of triangular tracks, so much so that
nearly all American machine tool-makers construct lathes in this
manner, although it adds no inconsiderable cost in fitting.
It may also be mentioned that lathes constructed with angular
guides, have usually such ways for the moving heads as well as for
the carriages; this gives the advantage of firmly binding the two
sides of the frame together in fastening the moving head, which in
effect becomes a strong girt across the frame; the carriages also
have an equal and independent hold on both sides of a shear. In
following this matter thus far, it may be seen how many conditions
may have to be considered in reasoning about so apparently simple a
matter as the form of ways for lathe carriages; we might even go on
to many more points that have not been mentioned; but what has been
explained will serve to show that the matter is not one of opinion
alone, and that without practical advantages, machine tool-makers
will not follow the most expensive of these two modes of mounting
lathe carriages.
Lathes in common use for machine fitting are screw-cutting engine
lathes, lathes for turning only, double-geared, single-geared,
and back-geared lathes, lathes for boring, hand-lathes, and
pulley-turning lathes; also compound lathes with double heads and
two tool carriages.
These various lathes, although of a widely varied construction
and adapted to uses more or less dissimilar, are still the engine
lathe either with some of its functions omitted to simplify and
adapt it to some special work, or with some of the operative parts
compounded to attain greater capacity.
In respect to lathe manipulation, which is perhaps the most
difficult to learn of all shop operations, the following hints are
given, which may prove of service to a learner: At the beginning
the form of tools should be carefully studied; this is one of the
great points in lathe work; the greatest distinction between a
thorough and indifferent latheman is that one knows the proper
form and temper of tools and the other does not. The adjustment
and presenting of tools is soon learned by experience, but the
proper form of tools is a matter of greater difficulty. One of the
first things to study is the shape of cutting edges, both as to
clearance below the edge of the tool, and the angle of the edge,
with reference to both turning and boring, because the latter
is different from turning. The angle of lathe tools is clearly
suggested by diagrams, and there is no better first lesson in
drawing than to construct diagrams of cutting angles for plane and
cylindrical surfaces.
A set of lathe tools should consist of all that are required for
every variety of work performed, so that no time will be lost
by waiting to prepare tools after they are wanted. An ordinary
engine lathe, operating on common work not exceeding twenty inches
of diameter, will require from twenty-five to thirty-five tools,
which will serve for every purpose if they are kept in order and in
place. A workman may get along with ten tools or even less, but not
to his own satisfaction, nor in a speedy way. Each tool should be
properly tempered and ground, ready for use 'when put away;' if a
tool is broken, it should at once be repaired, no matter when it is
likely to be again used. A workman who has pride in his tools will
always be supplied with as many as he requires, because it takes no
computation to prove that fifty pounds of extra cast steel tools,
as an investment, is but a small matter compared to the gain in
manipulation by having them at hand.
To an experienced mechanic a single glance at the tools on a lathe
is a sufficient clue to the skill of the operator. If the tools are
ground ready to use, of the proper shape, and placed in order so as
to be reached without delay, the latheman may at once be set down
as having two of the main qualifications of a first-class workman,
which are order, and a knowledge of tools; while on the contrary, a
lathe board piled full of old waste, clamp bolts, and broken tools,
shows a want of that system and order, without which no amount of
hand skill can make an efficient workman.
It is also necessary to learn as soon as possible the
technicalities pertaining to lathe work, and still more important
to learn the conventional modes of performing various operations.
Although lathe work includes a large range of operations which
are continually varied, yet there are certain plans of performing
each that has by long custom become conventional; to gain an
acquaintance with these an apprentice should watch the practice of
the best workmen, and follow their plans as near as he can, not
risking any innovation or change until it has been very carefully
considered. Any attempt to introduce new methods, modes of chucking
work, setting and grinding tools, or other of the ordinary
operations in turning, may not only lead to awkward mistakes, but
will at once put a stop to useful information that might otherwise
be gained from others. The technical terms employed in describing
lathe work are soon learned, generally sooner than they are needed,
and are often misapplied, which is worse than to be ignorant of
them.
In cutting screws it is best not to refer to that mistaken
convenience called a wheel list, usually stamped on some part of
engine lathes to aid in selecting wheels. A screw to be cut is to
the lead screw on a lathe as the wheel on the screw is to the wheel
on the spindle, and every workman should be familiar with so simple
a matter as computing wheels for screw cutting, when there is but
one train of wheels. Wheels for screw cutting may be computed not
only quite as soon as read from an index, but the advantage of
being familiar with wheel changes is very important in other cases,
and frequently such combinations have to be made when there is not
an index at hand.
The following are suggested as subjects which may be studied in
connection with lathes and turning: the rate of cutting movement
on iron, steel, and brass; the relative speed of the belt cones,
whether the changes are by a true ascending scale from the slowest;
the rate of feed at different changes estimated like the threads
of a screw at so many cuts per inch; the proportions of cone or
step pulleys to insure a uniform belt tension, the theory of
the following rest as employed in turning flexible pieces, the
difference between having three or four bearing points for centre
or following rests; the best means of testing the truth of a lathe.
All these matters and many more are subjects not only of interest
but of use in learning lathe manipulation, and their study will
lead to a logical method of dealing with problems which will
continually arise.
The use of hand tools should be learned by employing them on every
possible occasion. A great many of the modern improvements in
engine lathes are only to evade hand tool work, and in many cases
effect no saving except in skill. A latheman who is skilful with
hand tools will, on many kinds of light work, perform more and do
it better on a hand lathe than an engine lathe; there is always
more or less that can be performed to advantage with hand tools
even on the most elaborate engine lathes.
It is no uncommon thing for a skilled latheman to lock the slide
rest, and resort to hand tools on many kinds of work when he is in
a hurry.
(1.) Why does machinery involve so many cylindrical forms?--(2.)
Why is it desirable to have separate feed gear for turning and
screw cutting?--(3.) What is gained by the frictional starting
gearing now applied to the finer class of lathes?--(4.) How
may the alignment of a lathe be tested?--(5.) What kind of
deviation with a lathe carriage will most affect the truth of
work performed?--(6.) How may an oval hole be bored on a common
slide lathe?--(7.) How can the angular ways of a lathe and the
corresponding grooves in a carriage be planed to fit without
employing gauges?--(8.) Give the number of teeth in two wheels to
cut a screw of ten threads, when a leading screw is four threads
per inch?