_PLANING OR RECIPROCATING MACHINES._
The term planing should properly be applied only to machines that
produce planes or flat surfaces, but the technical use of the term
includes all cutting performed in right lines, or by what may be
called a straight movement of tools.
As no motion except rotary can be continuous, and as rotary
movement of tools is almost exclusively confined to shaping
cylindrical pieces, a proper distinction between machine tools
which operate in straight lines, and those which operate with
circular movement, will be to call them by the names of rotary and
reciprocating.
It may be noticed that all machines, except milling machines, which
act in straight lines and produce plane surfaces have reciprocating
movement; the class includes planing, slotting and shaping
machines; these, with lathes, constitute nearly the whole equipment
of an ordinary fitting shop.
It is strange, considering the simplicity of construction and the
very important office filled by machines for cutting on plane
surfaces, that they were not sooner invented and applied in metal
work. Many men yet working at finishing, can remember when all flat
surfaces were chipped and filed, and that long after engine lathes
had reached a state of efficiency and were generally employed,
planing machines were not known. This is no doubt to be accounted
for in the fact that reciprocal movement, except that produced by
cranks or eccentrics, was unknown or regarded as impracticable for
useful purposes until late years, and when finally applied it was
thought impracticable to have such movements operate automatically.
This may seem quite absurd to even an apprentice of the present
time, yet such reciprocating movement, as a mechanical problem, is
by no means so simple as it may at first appear.
A planing machine platen, for instance, moves at a uniform rate
of speed each way, and by its own motion shifts or reverses the
driving power at each extreme of the stroke. Presuming that there
were no examples to be examined, an apprentice would find many
easier problems to explain than how a planing machine can shift
its own belts. If a platen or table disengages the power that is
moving it, the platen stops; if the momentum carries it enough
farther to engage or connect other mechanism to drive the platen in
the opposite direction, the moment such mechanism comes into gear
the platen must stop, and no movement can take place to completely
engage clutches or shift belts. This is a curious problem that will
be referred to again.
Reciprocating tools are divided into those wherein the cutting
movement is given to the tools, as in shaping and slotting
machines, and machines wherein the cutting movement is given to
the material to be planed, as in a common planing machine. Very
strangely we find in general practice that machine tools for both
the heaviest and the lightest class of work, such as shaping, and
butting, operate upon the first principle, while pieces of a medium
size are generally planed by being moved in contact with stationary
tools.
This problem of whether to move the material or to move the tools
in planing, is an old one; both opinion and practice vary to some
extent, yet practice is fast settling down into constant rules.
Judged upon theoretical grounds, and leaving out the mechanical
conditions of operation, it would at once be conceded that a proper
plan would be to move the lightest body; that is, if the tools
and their attachments were heavier than the material to be acted
upon, then the material should be moved for the cutting action,
and _vice versa_. But in practice there are other conditions to
be considered more important than a question of the relative
weight of reciprocating parts; and it must be remembered that in
solving any problem pertaining to machine action, the conditions
of operation are to be considered first and have precedence over
problems of strain, arrangement, or even the general principles of
construction; that is, the conditions of operating must form a base
from which proportions, arrangements, and so on, must be deduced.
A standard planing machine, such as is employed for most kinds of
work, is arranged with a running platen or carriage upon which the
material is fastened and traversed beneath the cutting tools. The
uniformity of arrangement and design in machines of this kind in
all countries wherever they are made, must lead to the conclusion
that there are substantial reasons for employing running platens
instead of giving a cutting movement to the tools.
A planing machine with a running platen occupies nearly twice as
much floor space, and requires a frame at least one-third longer
than if the platen were fixed and the tools performed the cutting
movement. The weight which has to be traversed, including the
carriage, will in nearly all cases exceed what it would be with a
tool movement; so that there must exist some very strong reasons in
favour of a moving platen, which I will now attempt to explain, or
at least point out some of the more prominent causes which have led
to the common arrangement of planing machines.
Strains caused by cutting action, in planing or other machines,
fall within and are resisted by the framing; even when the tools
are supported by one frame and the material by another, such
frames have to be connected by means of foundations which become a
constituent part of the framing in such cases.
Direct action and reaction are equal; if a force is exerted in
any direction there must be an equal force acting in the opposite
direction; a machine must absorb its own strains.
Keeping this in view, and referring to an ordinary planing machine
with which the reader is presumed to be familiar, the focal point
of the cutting strain is at the edge of the tools, and radiates
from this point as from a centre to the various parts of the
machine frame, and through the joints fixed and movable between the
tools and the frame; to follow back from this cutting point through
the mechanism to the frame proper; first starting with the tool and
its supports and going to the main frame; then starting from the
material to be planed, and following back in the other direction,
until we reach the point where the strains are absorbed by the main
frame, examining the joints which intervene in the two cases, there
will appear some reasons for running carriages.
Beginning at the tool there is, first, a clamped joint between the
tool and the swing block; second, a movable pivoted joint between
the block and shoe piece; third, a clamped joint between the shoe
piece and the front saddle; fourth, a moving joint where the front
saddle is gibed to the swing or quadrant plate; fifth, a clamp
joint between the quadrant plate and the main saddle; sixth, a
moving joint between the main saddle and the cross head; seventh,
a clamp joint between the cross head and standards; and eighth,
bolted joints between the standards and the main frame; making in
all eight distinct joints between the tool and the frame proper,
three moving, four clamped, and one bolted joint.
Starting again from the cutting point, and going the other way from
the tool to the frame, there is, first, a clamped and stayed joint
between the material and platen, next, a running joint between
the platen and frame; this is all; one joint that is firm beyond
any chance of movement, and a moving joint that is not held by
adjustable gibs, but by gravity; a force which acts equally at
all times, and is the most reliable means of maintaining a steady
contact between moving parts.
Reviewing these mechanical conditions, we may at once see
sufficient reasons for the platen movement of planing machines;
and that it would be objectionable, if not impossible, to add a
traversing or cutting action to tools already supported through the
medium of eight joints. To traverse for cutting would require a
moving gib joint in place of the bolted one, between the standards
and main frame, leading to a complication of joints and movements
quite impracticable.
These are, however, not the only reasons which have led to a
running platen for planing machines, although they are the most
important.
If a cutting movement were performed by the tool supports, it
would necessarily follow that the larger a piece to be planed, and
the greater the distance from the platen to the cutting point,
the farther a tool must be from its supports; a reversal of the
conditions required; because the heavier the work the greater
the cutting strain will be, and the tool supports less able to
withstand the strains to be resisted.
It may be assumed that the same conditions apply to the standards
of a common planing machine, but the case is different; the upright
framing is easily made strong enough by increasing its depth; but
the strain upon running joints is as the distance from them at
which a force is applied, or to employ a technical phrase, as the
amount of overhang. With a moving platen the larger and heavier
a piece to be planed, the more firmly a platen is held down; and
as the cross section of pieces usually increases with their depth,
the result is that a planing machine properly constructed will act
nearly as well on thick as thin pieces.
The lifting strain at the front end of a platen is of course
increased as the height at which the cutting is done above its
top, but this has not in practice been found a difficulty of any
importance, and has not even required extra length or weight of
platens beyond what is demanded to receive pieces to be planed and
to resist flexion in fastening heavy work. The reversing movement
of planing machine platens already alluded to is one of the most
complex problems in machine tool movement.
Platens as a rule run back at twice the forward or cutting
movement, and as the motion is uniform throughout each stroke, it
requires to be stopped at the extremes by meeting some elastic or
yielding resistance which, to use a steam phrase, "cushions" or
absorbs the momentum, and starts the platen back for the return
stroke.
This object is attained in planing machines by the friction of
the belts, which not only cushions the platen like a spring, but
in being shifted opposes a gradually increasing resistance until
the momentum is overcome and the motion reversed. By multiplying
the movement of the platen with levers or other mechanism, and
by reason of the movement that is attained by momentum after the
driving power ceases to act, it is found practicable to have a
platen 'shift its own belts,' a result that would never have been
reached by theoretical deductions, and was no doubt discovered by
experiment, like the automatic movement of engine valves is said to
have been.
It is not intended to claim that this platen-reversing motion
cannot, like any other mechanical movement, be resolved
mathematically, but that the mechanical conditions are so obscure
and the invention made at a time that warrants the supposition of
accidental discovery.
In the driving gearing of planing machines, conditions which favour
the reversing movement are high speed and narrow driving belts. The
time in which belts may be shifted is as their speed and width; to
be shifted a belt must be deflected or bent edgewise, and from this
cause wind spirally in order to pass from one pulley to another. To
bend or deflect a belt edgewise there will be required a force in
proportion to its width, and the time of passing from one pulley to
another is as the number of revolutions made by the pulleys.
Planing machines of the most improved construction are driven by
two belts instead of one, and many mechanical expedients have been
adopted to move the belts differentially, so that both should not
be on the driving pulley at the same time, but move one before
the other in alternate order. This is easily attained by simply
arranging the two belts with the distance between them equal to one
and one-half or one and three-fourth times the width of the driving
pulley. The effect is the same as that accomplished by differential
shifting gearing, with the advantage of permitting an adjustment of
the relative movement of the belts.
Another principle in planing machines which deserves notice is the
manner of driving carriages or platens; this is usually performed
by means of spur wheels and a rack. A rack movement is smooth
enough, and effective enough so far as a mechanical connection
between the driving gearing and a platen, but there is a difficulty
met with from the torsion and elasticity of cross-shafts and a
train of reducing gearing. In all other machines for metal cutting,
it has been a studied object to have the supports for both the
tools and the material as rigid as possible; but in the common type
of planing machines, such as have rack and pinion movement, there
is a controversion of this principle, inasmuch as a train of wheels
and several cross-shafts constitute a very effective spring between
the driving power and the point of cutting, a matter that is easily
proved by planing across the teeth of a rack, or the threads of
a screw, on a machine arranged with spur wheels and the ordinary
reducing gearing. It is true the inertia of a platen is interposed
and in a measure overcomes this elasticity, but in no degree that
amounts to a remedy.
A planing machine invented by Mr Bodmer in 1841, and since improved
by Mr William Sellers of Philadelphia, is free from this elastic
action of the platen, which is moved by a tangent wheel or screw
pinion. In Bodmer's machine the shaft carrying the pinion was
parallel to the platen, but in Sellers' machine is set on a shaft
with its axis diagonal to the line of the platen movement, so
that the teeth or threads of the pinion act partly by a screw
motion, and partly by a progressive forward movement like the
teeth of wheels. The rack on the platen of Mr Sellers' machine is
arranged with its teeth at a proper angle to balance the friction
arising from the rubbing action of the pinion, which angle has
been demonstrated as correct at 5°, the ordinary coefficient of
friction; as the pinion-shaft is strongly supported at each side of
the pinion, and the thrust of the cutting force falls mainly in the
line of the pinion shaft, there is but little if any elasticity, so
that the motion is positive and smooth.
The gearing of these machines is alluded to here mainly for the
purpose of calling attention to what constitutes a new and singular
mechanical movement, one that will furnish a most interesting
study, and deserves a more extended application in producing slow
reciprocating motion.
(1.) Can the driving power be employed directly to shift the
belts of a planing machine?--(2.) Why are planing machines
generally constructed with a running carriage instead of running
tools?--(3.) What objection exists in employing a train of spur
wheels to drive a planing machine carriage?--(4.) What is gained
by shifting the belts of a planing machine differentially?--(5.)
What produces the screeching of belts so common with planing
machines?--(6.) What conditions favour the shifting of planing
machine belts?