Modern Machine-Shop Practice Part 111

[Ill.u.s.tration: Fig. 1565.]

Fig. 1565 represents a planer by William Sellers and Co., of Philadelphia, Pennsylvania. This planer is provided with an automatic feed to the sliding head, both horizontally and vertically, and with mechanism which lifts the ap.r.o.n, and therefore the cutting tool, during the backward stroke of the work table, and thus prevents the abrasion of the tool edge that occurs when the tool is allowed to drag during the return stroke. The machine is also provided with a quick return motion, and in the larger sizes with other conveniences to be described hereafter.

The platen or table is driven by a worm set at such an angle to the table rack as to enable the teeth of the rack to stand at a right angle to the table length, and as a result the line of thrust between the worm and the rack is parallel to the [V]-guideways, which prevents wear between the [V]s of the table and of the bed.

The driving pulleys are set at a right angle to the length of the machine, their planes of revolution being, therefore, parallel to the plane of revolution of the line or driving shaft overhead, and parallel with the lathes and other machines driven from the same line of shafting, thus taking up less floor s.p.a.ce, while the pa.s.sage ways between the different lines of machines is less obstructed.

By setting the worm driving shaft at an angle the teeth of the worm rotate in a plane at a right angle to the length of the work-table rack, and as a result the teeth of the worm have contact across the full width of the rack teeth instead of in the middle only, as is the case when the axis of a worm is at a right angle to the axis of the wheel or rack that it drives.

Furthermore, by inclining the worm shaft at an angle the teeth of the rack may be straight (and not curved to suit the curvature of the worm after the manner of worm-wheels), because the contact between the worm and rack teeth begins at one side of the rack and pa.s.ses by a rolling motion to the other, after the manner and possessing the advantages of Hook's gearing as described in the remarks made with reference to gear-wheel teeth.

By inclining the worm shaft, however, the side thrust incidental to Hook's gearing is avoided, the pressure of contact of tooth upon tooth being in the same direction and in line with the rack motion. As the contact between the worm teeth and the rack is uniform in amount and is also continuous, a very smooth and uniform motion is imparted to the work table, and the vibration usually accompanying the action of spur-gearing is avoided.

The worm has four separate spirals or teeth, hence the table rack is moved four teeth at each worm revolution, and a quick belt motion is obtained by the employment of pulleys of large diameter.

It is desirable that the belt motion of a planing machine be as quick as the conditions will permit, because the amount of power necessary to drive the machine can thus be obtained by a narrower belt, it being obvious that since the driving power of the belt is the product of its tension and velocity the greater the velocity the less the amount of tension may be to transmit a given amount of power.

The mechanism for shifting the belt to reverse the direction of table motion is shown in Fig. 1566 removed from all the other mechanism.

To the bracket or arm B are pivoted the arms or belt guides C and D and the piece G. In the position occupied by the parts in the figure the belt for the forward or cutting stroke would be upon the loose pulley P', and that for the quick return stroke would be upon the loose pulley P, hence the machine table would remain at rest. But suppose the rod F be moved by hand in the direction of arrow _f_, then G would be moved upon its pivot X, and its lug _h_ would meet the jaw _i_ of C, moving C in the direction of arrow _a_, and therefore carrying the belt from loose pulley P' on to the driving pulley P", which would start the machine work table, causing it to move in the direction of arrow W until such time as the stop A meets the lug R, operating lever E and moving rod F in the direction of arrow _d_. This would move G, causing its lug _h_ to meet the jaw _j_, which would move C from P" back to the position it occupies in the figure, and as the motion of G continued its shoulder at _g'_ would meet the shoulder or lug T of K (the latter being connected to D) and move arm D in the direction of _b_, and therefore carrying the crossed belt upon P, and causing the machine table to run backward, which it would do at a greater speed than during the cutting traverse, because of the overhead pulley on the countershaft being of greater diameter than that for the cutting stroke.

It is obvious that since each belt pa.s.ses from its loose pulley to the fast one, the width of the overhead or countershaft pulleys must be twice as wide as the belt, and also that to reverse the direction of pulley revolution one driving belt must be crossed; and as on the countershaft the smallest pulley is that for driving the cutting stroke, its belt is made the crossed one, so as to cause it to envelop as much of the pulley circ.u.mference as possible, and thereby increase its driving power. The arrangement of the countershaft pulleys and belts is shown in Fig. 1567, in which S is the countershaft and N, O the fast and loose pulleys for the belt from the line shaft pulley; Q' is the pulley for operating the table on the cutting stroke (with the crossed belt), while Q is the pulley for operating the table on its return stroke. The difference in the speed of the table during the two strokes is obviously in the same proportions as the diameters of pulleys Q' and Q.

The feed rod, and feed screw, and rope for lifting the tool on the back stroke are operated as follows:--

Fig. 1568 is an end view of the mechanism viewed from the front of the machine, and Fig. 1569 is a side view of the same.

The shaft of the driving pulleys (P P' and P", Fig. 1567) drives a pinion operating the gear wheel W, upon the face of which is a serrated internal wheel answering to a ratchet wheel, and with which a pawl engages each time the direction of pulley revolution (or, which is the same thing, the direction of motion W) reverses, and causes the pawl and the shaft, to which the plate P, Fig. 1569, is fast, to make one-half a revolution, when the pawl disengages and all parts save the wheel W come to rest.

From this plate P the feed motions are actuated, and the tool is lifted during the back traverse of the work table by the following mechanisms.

[Ill.u.s.tration: Fig. 1566.]

[Ill.u.s.tration: Fig. 1567.]

[Ill.u.s.tration: Fig. 1568.]

[Ill.u.s.tration: Fig. 1569.]

[Ill.u.s.tration: Fig. 1570.]

Referring to Fig. 1570, upon the plate P is pivoted a lever Q, carrying a universal joint at Z, and a nut pivoted at V, and it is obvious that at each half-revolution of P, the rod R is moved vertically. This rod connects to a universal joint J (shown in Fig. 1571) that is pivoted in a toothed segment (K, in the same figure) which engages with a pinion on the feed screw, this pinion being provided with a ratchet and feed pawl (of the usual construction) for reversing the direction of the feed or throwing it out of action.

The amount of feed is regulated as follows:--

Referring to Figs. 1569 and 1570, the amount of vertical motion of rod R is obviously determined by the distance of the universal joint Z from the centre of the plate P, and this is set by operating the hand wheel T, which revolves the screw Y in the nut V.

For lifting the tool during the return motion of the work and work table, there is provided in the plate P, Fig. 1570, a pin which actuates the rod B, which in turn actuates the grooved segment C.

[Ill.u.s.tration: Fig. 1571.]

From this segment a cord is stretched pa.s.sing over the grooved pulley D, Fig. 1571, thence over pulley E, and after taking a turn around the pulley F, Fig. 1571, it pa.s.ses to the other end of the cross slide, where it is secured.

This pulley F is therefore revolved at each motion of the plate P, Figs.

1569 or 1570, or in other words each time the work table reverses its motion.

In reference to Figs. 1571 and 1572, F, Fig. 1571, is fast upon a pin _g_, at whose other end is a pinion operating a gear-wheel _h_. Upon the face of this gear-wheel is secured a steel plate shown at _m_ in Fig.

1572, which is a vertical section of the sliding head. In a cam groove in _m_, projects a pin that is secured to the sleeve _n_, which envelops the vertical feed screw O. This sleeve _n_ has frictional contact at _p_ with the bar _q_, whose lower end receives the bell crank _r_, which on each return stroke is depressed, and thus moves the tool ap.r.o.n _s_, and with it the tool, which is therefore relieved from contact with the cut upon the work.

The self-acting vertical feed is actuated as follows:--

Referring to Figs. 1571 and 1572 the gear segment K operates a pinion upon the squared end of the feed rod L, this pinion L having the usual pawl and ratchet for reversing the direction of rod revolution.

The splined feed rod L actuates the bevel pinion M, which is in gear with bevel pinion N, the latter driving pinion P, which is threaded to receive the vertical feed screw O; hence when P is revolved it moves the feed screw O endways, and this moves the vertical slide R upon which is the ap.r.o.n box T and the ap.r.o.n _s_. To prevent the possibility of the friction of the threads causing the feed screw O to revolve with the pinion P, the journal _e_ of the feed screw O is made shorter than its bearing in R, so that the nut _f_ may be used to secure the feed screw O to the slide R.

PLANER SLIDING HEADS.--In order that the best work may be produced, it is essential that the sliding head of a planer or planing machine be constructed as rigid as possible, and it follows that the slides and slideways should be of that form that will suffer the least from wear, resist the tool strain as directly as possible, and at the same time enable the taking up of any wear that may occur from the constant use of the parts.

Between the tool point that receives the cutting strain and the cross bar or cross slide that resists it there are the pivoted joint of the ap.r.o.n, the sliding joint of the vertical feed, and the sliding joint of the saddle upon the cross slide, and it is difficult to maintain a sliding fit without some movements or spring to the parts, especially when, as in the case of a planer head, the pressure on the tool point is at considerable leverage to the sliding surfaces, thus augmenting the strain due to the cut.

The wear on the cross slide is greater at and towards the middle than at the ends, but it is also greater at the end nearest to the operator than at the other end, because work that is narrower than the width of the planing machine table is usually chucked on the side nearest to the operator or near the middle of the table width, because it is easier to chuck it there and more convenient to set the tool and watch the cut, for the reason that the means for stopping and starting the machine, and for pulling the feed motions in and out of operation, are on that side.

The form of cross bar usually employed in the United States is represented in Fig. 1573, and it is clear that the pressure of the cut is in the direction of the arrow _c_, and that the fulcrum off which the strain will act on the cross bar is at its lowest point _d_, tending to pull the top of the saddle or slider in the direction of arrow _e_, which is directly resisted by the vertical face of the gib, while the horizontal face _f_ of the gib directly resists the tendency of the saddle to fall vertically, and, therefore, the amount of looseness that may occur by reason of the wear cannot exceed the amount of metal lost by the wear, which may be taken up as far as possible by means of the screws _a_ and _b_, which thread through the saddle and abut against the gib. The gib is adjusted by these screws to fit to the least worn and therefore, the tightest part of the cross bar slideway, and the saddle is more loosely held at other parts of the cross bar in proportion as its slideway is worn.

[Ill.u.s.tration: Fig. 1572.]

In this construction the faces of the saddle are brought to bear over the whole area of the slideways surface of the cross bar, because the bevel at _g_ brings the two faces at _m_ into contact, and the set-screw _b_ brings the faces in together. Instead of the screws _a_ and _b_ having slotted heads for a screw driver, however, it is preferable to provide square-headed screws, having check nuts, as in Fig. 1574, so that after the adjustment is made the parts may be firmly locked by the check nuts, and there will be no danger of the adjustment altering.

The wear between the slider and the raised slideways S is taken up by gibs and screws corresponding to those at _a_ and _c_ in the Fig. 1575, and concerning these gibs and screws J. Richards has pointed out that two methods may be employed in their construction, these two methods being ill.u.s.trated in Figs. 1575 and 1576, which are taken from "Engineering."

In Fig. 1575 the end _s_ of the adjustment screw _a_ is plain, and is let into the gib _c_ ab.u.t.ting against a flat seat, and as a result while the screw pressure forces the gib _c_ against the bevelled edge of the slideway it does not act to draw the surfaces together at _m_ _m_ as it should do. This may be remedied by making the point of the screw of such a cone that it will bed fair against gib _c_, without pa.s.sing into a recess, the construction being as in Fig. 1576, in which case the screw point forces the gib flat against the bevelled face and there is no tendency for the gib to pa.s.s down into the corner _e_, Fig. 1575, while the pressure on the screw point acts to force the slide _a_ down upon the slideway, thus giving contact at _m_ _m_.

The bearing area of such screw points is, however, so small that the pressure due to the tool cut is liable to cause the screw to indent the gib and thus destroy the adjustment, and on this account a wedge such as shown in Fig. 1577 is preferable, being operated endwise to take up the wear by means of a screw pa.s.sing through a lug at the outer or exposed end of the wedge.

The corners at _i_, Figs. 1575 and 1576, are sometimes planed out to the dotted lines, but this does not increase the bearing area between the gib _c_ and the slide, while it obviously weakens the slider and renders it more liable to spring under heavy tool cuts.

Fig. 1578 represents a form of cross bar and gib found in many English and in some American planing machines. In this case the strain due to the cut is resisted directly by the vertical face of the top slide of the cross bar, the gib being a triangular piece set up by the screws at _a_, and the wear is diminished because of the increased wearing surface of the gib due to its lower face being diagonal.

[Ill.u.s.tration: Fig. 1573.]

[Ill.u.s.tration: Fig. 1574.]

[Ill.u.s.tration: Fig. 1575.]

[Ill.u.s.tration: Fig. 1576.]

On the other hand, however, this diagonal surface does not directly resist the falling of the saddle from wear, and furthermore in taking up the wear the vertical face of the saddle is relieved from contact with the vertical face of the cross bar, because the screws _a_ when set up move the top of the saddle away from the cross bar, whereas in Fig.

1573, setting up screw _b_ brings the saddle back upon the vertical face of the cross bar slideway.