Modern Machine-Shop Practice Part 216

We may now consider the means employed to drive the rolls, first remarking that the upper rolls F and D, are given a motion slightly quicker than the lower ones, so as to cause them to clean themselves (from particles of wood that might otherwise cling to them), by a sort of rubbing action which is due to their velocity being greater than the lower rolls and the work. This rubbing action is due to the fact that the work has the slower motion of the lower rollers, resisting the quicker motion of the upper ones, and as a result there is a certain amount of slip between the upper rollers and the work.

Another and important feature, is that the upper delivery roller (D, Fig. 3260), is placed from 1/4 to 1/2 inch nearer to the cutter head than the bottom delivery roll, which a.s.sists in keeping the work down upon the table.

The mechanism for driving the feed rolls is shown in Figs. 3163, 3164 and 3165, in which L, L are the pulleys which receive motion from a countershaft, and drive the cutter head, being fast upon its shaft, as is also the pulley S, which connects by belt and drives pulley T, on whose shaft is the stepped pulley U, which connects by a crossed belt to pulley V, which drives the feed gear through the medium of the pinion _a_. The two steps on pulleys U and V, obviously give two rates of feed.

The pinions O and O', both receive motion from the gear wheel E, this part of the gearing consisting of gears _a_, _b_, _c_, _d_ and E, and as both pinions receive motion from the same gear, their revolutions are equal. The lower feed roll is driven by the pinion _p_, which gears with and is driven by wheel _d_, whose face is broad enough to meet _p_, which sits nearer to the frame than pinion O does, so that the teeth of _p_ may escape those of O.

Now the velocities of all the wheels O, O', E, _d_ and _p_, will be equal at the pitch circles, because they const.i.tute a simple train of gearing. Thus if _d_ moves through a part of a revolution equal to the pitch E, then O and O' will move through the same distance, because the wheels are in continuous gear. Now as _d_ drives _p_, therefore the velocity of _p_ must at the pitch circle be the same as _d_, let the numbers of teeth in the respective wheels be what it may, and it follows that the velocities of O, E, _d_ and _p_ are at the pitch circles equal.

But by making the diameter of the upper roll greater than the pitch circle of its gear O, and the diameter of the lower roll correspondingly less than the diameter of the pitch circle of its pinion _p_, the velocity of the circ.u.mference of the upper roll will be greater than that of the lower roll, and the rubbing action before referred to with reference to the upper roll will thus be induced.

[Ill.u.s.tration: Fig. 3164.]

Referring now to the lower delivery roll, its pinion _x_ receives motion through gear _w_, which is also driven by gear E, which has a broad face so as to gear with _x_, which is behind and below gear O'. In this case the circ.u.mstances are the same, as will be seen from the following.

An inch of motion of the pitch circle of E will produce an inch of motion at the pitch circles of O' and of _w_ and _x_, hence the velocities of the pitch circles will be equal, and if the diameters of the upper and lower rolls are equal, or the same as the pitch circles, the velocities of the circ.u.mferences of the respective rolls will be equal, but by making the diameter of the upper delivery roll greater than that of the pitch circle of its pinion, and that of the lower roll less, a rubbing action is induced between the roll and the work, and this rubbing action will keep the roll clear of any dust, etc., that might otherwise cling to it.

The cutter head is formed triangular, as in Fig. 3166, carrying three knives. The knives are set at an angle to the axis of the cutter bar or cutter head. When the knives are at an angle, they take their cut gradually, and the cutting action is more continuous, which diminishes the vibration of the machine, and causes the finished surface to be smoother. Furthermore, the knives take a shearing cut, and therefore cut more easily and freely.

In some practice the knives are made spiral, but spiral knives are difficult to bed properly to the cutter head, and also difficult to grind. The cutter head is made of a solid mild centre steel forging, and runs in phosphor bronze journals, in which it has about 1/8 inch end play, which tends to distribute the oil along the bearing. It is driven by a pulley at each end, the pulleys seating on a cone.

The amount of skew is about 3/4 inch for a cutter head carrying a knife 30 inches long, and about 3/8 inch for a cutter head whose knives are 10 or 12 inches long.

[Ill.u.s.tration: Fig. 3165.]

[Ill.u.s.tration: Fig. 3166.]

[Ill.u.s.tration: Fig. 3167.]

Figs. 3167 and 3168 represent a machine in which there are three feed rolls and one delivery roll, all being driven.

First there is the pair of feed rolls the bottom roll of which is set sufficiently above the surface of the table to relieve the work of friction upon the table.

The work next meets an upper feed roll that acts to force the work down to the table surface (there being in this case no lower feed roll).

After pa.s.sing the knives, the work is carried out by a delivery roll that also acts to keep the work down to the table face.

All three upper rolls are provided with rubber springs in the casings H, H'.

P, P, are the pulleys for the cutter head and B, those for the feed works, which have two speeds. The feed is thrown in and out by the lever _d_, which moves the pinion D endways and engages or disengages it from its gear wheel.

[Ill.u.s.tration: Fig. 3168.]

[Ill.u.s.tration: Fig. 3169.]

Figs. 3169, 3170, 3171 and 3172 represent a pony planer, by P. Pryibil.

[Ill.u.s.tration: Fig. 3170.]

Referring to the sectional view Fig. 3170, the work table slides in vertical slideways S, in the side frames, the elevating screw being operated by the bevel gears at G, which receive motion from the hand wheel M in Figs. 3170 and 3171. There are four upper rolls, marked 1, 2, 3 and 4 respectively, and of these the first two are fluted in the usual way. There are two lower rolls, marked respectively 5 and 6. The fluted feed rolls 1 and 2 are weighted, the weight lever acting on the rod R, which at its upper end connects to the cap Y, which covers the bearings of feed rolls 1 and 2. By this construction the two rolls are acted upon by the same weights and levers, the rolls being of course weighted at each end, or in other words on both sides of the machine.

The delivery rolls 3 and 4 receive their pressure by the construction shown in Fig. 3172, the bearings of the rolls being held down by rubber cushions receiving pressure from the cap E, screwed down by the bolt and nut.

The rolls 5 and 6 are idle rolls, and are set to just relieve the work from undue pressure on the work table.

By this construction of feed mechanism the following ends are attained.

First, sufficient feed power for heavy cuts is obtained without driving the lower rolls. Second the work is held to the table on both sides of the cutter head, hence there will not be left on the end of the work the step that is left when but two upper and two lower rolls are used, and which occurs because the work falls after leaving the feed rolls, whereas, in this machine the work is held to the table by rolls 2 and 3.

The cutter head H, Fig. 3170, has in front of it the pressure bar P, whose lever is shown at L and the weight at W. On the delivery side of the cutter head is a pressure bar _r_, which is acted upon by a spiral spring in the box C. In the engraving to the right of Fig. 3170 the knife K is shown in action on a piece of work, and it is seen that the end of the pressure bar P coming close to the edge of the knife prevents the pressure of the cut from splitting or splintering off the end of the work at _a_, and therefore acts as what is termed a _chip break_.

Furthermore, the sides of the cutter head between the knives being hollowed out gives the shavings _s_ room to curl in and prevent the work from splintering at the end when the cut is terminating.

BALANCING CUTTER HEADS AND KNIVES.--Planer knives must be balanced as accurately as possible, in order that they may run steadily and smoothly, and therefore produce smooth work.

The first requisite for proper balancing is that the cutter head itself be properly balanced, and in order that this may be the case the faces forming the knife seats must be equidistant from the axis of the cutter head, and the journals must run true, being best tested on dead centres.

The holes for the cutter bolts should all be drilled to the same depth, and tapped equally deep. The faces or seats for the knives should be parallel one to the other, and this may be tested by a pair of straight edges, one pressed to each face and the width between them measured at each end, or if a long surface plate is at hand, one face of the head may be rested on the surface plate, and the straight edge ruled on the other face, and its distance measured from the surface plate at each end, with a pair of inside callipers delicately adjusted.

A straight edge rested lengthways along the knife seat of the head and projecting over the journal will show whether each knife seat is equidistant from the journal as it should be, the measurement being taken with a pair of inside callipers adjusted to just sensibly touch the journal and the straight edge. This measurement should be taken at each end of the head.

In all tests made with straight edges, the straight edge should be turned end for end and each measurement repeated, because, if the straight edge is true, turning it end for end will make no difference to the measurement, while if the straight edge is not true the measurement will vary when the straight edge is reversed.

If the cutter head is square, the straight edge tests may be applied to all four of its faces, and they may then be tested with a square, and if the head shows no error under these tests, and the bolt holes or slots are of equal diameter and depths, the head will be correct as far as it can be tested without running it.

[Ill.u.s.tration: Fig. 3171.]

[Ill.u.s.tration: Fig. 3172.]

A cutter head may be roughly tested by placing it between the lathe centres, both centres being oiled and delicately adjusted so as to just prevent end motion of the head without perceptible friction when the head is revolved by hand.

The first thing to test is whether the journals run true, which may be tested by a pointer fastened in the slide seat, and moved up to just touch the journal. The pointer should be soft, and not a cutting tool, unless indeed it be set so high in the slide rest that it cannot cut.

If the journals do not run true, the next thing to test is whether the body of the head runs true to the centres, which may be done by first setting a pointer to just touch the extreme corners of the head at each end and in the middle of its length, and if there is an error in the same direction as the test at the journal shows, then the centres of the head are out of true, and must be corrected before a test of this kind can be proceeded with.

But the body of the head may show true at the corners while the journals do not run true, and if this is the case we may further test the body of the head as follows:

With the lathe slide rest at one end of the head we may set a pointer so that it will just pa.s.s on the flat of the cutter seat and make a mark when the slide rest is traversed along the lathe bed. We then move the slide rest so as to bring the pointer to the journal end of the head; give the head a half a revolution on the centres and try the pointer on the flat of the cutter seat, and if it makes a mark of equal strength, then two faces of the head are equidistant from the axis of the head.

The next thing to do is to make the same test at the other end of the head, and in order to do this without moving the pointer, and therefore without altering its adjustment, we must move the slide rest so as to bring the pointer opposite to the lathe centre, and out of the way of the body of the head, and take the cutter head out of the lathe and turn it end for end, and then repeat the test with the pointer, which will show whether both ends of those two flats are alike.

This test we repeat on the other two faces of the head, and if they show true, then the head is true, except the journal, which must be made true with the head.

This testing will clearly show any want of truth in either the head or the journals, and in what direction correction needs to be made.

Now suppose the above tests do not disclose any error, either in the journals or in the head, and we may continue the tests by revolving the head by hand between the dead centres, and apply the pointer to the journals while the head is revolved as quickly as possible; as, however, the head cannot be revolved very fast in this way, we may adjust the lathe centres as before described, and revolve the head as rapidly as possible by hand, and letting it come to rest mark which side is at the bottom, and if on several tests the same side comes to the bottom of the plane of revolution at each test, that side is the heaviest and must be corrected. If it is found to be a flat side or cutter seat that comes to rest at the bottom, the correction can be made by deepening the bolt holes on that side, measuring to see which bolt hole is the shallowest, and making all as nearly as possible equally deep.

If the head has T slots instead of bolt holes, the slots may be cut or filed out to effect the balance, care being taken to make the slot equal in distance from the edges of the cutter seat face.

The next essential in order to have a properly balanced cutter head is that the bolts and nuts all weigh alike, and that the bolts be of the same length. The bolts should be turned to an equal diameter of equal length and threaded for an equal distance along the body of the bolt, and the nuts should be of equal depth and all fit accurately to the same wrench, and the weight of the bolts and nuts when put together may then be equalized by reducing the heads of the heavy ones.

We now come to the balancing of the knives, which must be made of equal thickness and width throughout, with the slots for the bolts of equal widths and depths.