Modern Machine-Shop Practice Part 109

[Ill.u.s.tration: Fig. 1541.]

The ram or bar is provided with a rack (Z, Fig. 1545) which engages with a pinion S, Fig. 1541, H being the driving shaft driven by the belt cones A and B. These two cones are driven by separate belts, but from the same counter-shaft, one being an open and the other a crossed belt.

The open belt drives either the largest step of pulley B, giving a cutting speed suitable for steel, or the smaller step, giving a cutting speed for softer metals, as cast iron, &c. The crossed belt drives, in either case, the pulley A for the quick-return stroke, and this pulley revolve upon a sleeve or hub C, which revolves upon the shaft H. The sleeve or hub C is in one piece with a pulley C, whose diameter is such as to leave an annular opening between its face and the bore of the largest step of cone pulley B, and pulley A is fast to the hub or sleeve C. It will be seen that as the driving belts from the counter-shaft are one open and one crossed, therefore pulley A runs constantly in one direction, while pulley B runs constantly in the other, so that the direction of motion of the driving shaft H depends upon whether it is locked to pulley A or to pulley B.

[Ill.u.s.tration: Fig. 1542.]

[Ill.u.s.tration: _VOL. I._ =SHAPING MACHINES AND TABLE-SWIVELING DEVICES.= _PLATE XVI._

Fig. 1536.

Fig. 1537.

Fig. 1538.

Fig. 1539.]

In the annular s.p.a.ce left between the face of pulley C and the cone B is a steel band G, Fig. 1542, forming within a fraction a complete circle, and lined inside and out with leather, and this band is brought, by alternately expanding and contracting it, into contact with either the bore of the largest cone step of B or with the outside face of pulley C.

The ends of this band are pivoted upon two pins F, which are fast in two arms E and D, in Fig. 1542. Arm E is fastened to the driving shaft H, and its hub has two roller studs K, Fig. 1541, these being diametrically opposite on the said hub. The hub of arm D is a working fit upon the hub of E, and has two slots to admit the above rollers. Hub D is also provided with two studs and rollers placed midway between the studs K.

These latter rollers project into the spiral slots K' of the ring in Fig. 1543, this ring enveloping the hub of D and being enveloped by the sleeve M, which contains two spiral grooves diametrically opposite, and lying in an opposite direction to grooves K', Fig. 1543. Sleeve M is prevented from revolving by rollers on the studs O, which are screwed into the bearing bush R, and carry rollers projecting into the slots in M.

[Ill.u.s.tration: Fig. 1543.]

It is evident that if the ring L, Fig. 1543, is moved endways with M, then the arms E, D, together with the band G, will be expanded or contracted according to the direction of motion of the ring, because the motion of M, by means of its spiral grooves, gives a certain amount of rotary motion to the ring L, and the spiral grooves in the ring give a certain amount of rotary motion to the arms D and E, Fig. 1542. When this rotary motion is in one direction the band is expanded; while when it is reversed it is contracted, and the direction of motion of shaft H is reversed.

[Ill.u.s.tration: Fig. 1544.]

[Ill.u.s.tration: Fig. 1545.]

The outer sleeve M carries the rod T, Figs. 1544 and 1545, which is connected to the lever U, the upper arm of which is operated by the tappets or dogs X on the ram or sliding bar, and it is obvious that when U is vibrated sleeve M is operated in a corresponding direction, and the ring L also is moved endwise in a corresponding direction, actuating the band as before described, the direction of motion being governed, therefore, by the direction in which U is moved by the tappets or dogs.

A certain degree of friction is opposed to the motion of lever U in order to keep it steady, the construction being shown in Fig. 1546, where it is seen that there is on each side of its nut a leather washer, giving a certain amount of elasticity to the pressure of the nut holding it in place on the shaft U.

[Ill.u.s.tration: Fig. 1546.]

The mechanism for actuating the feed at the end of the return stroke only, is shown in Fig. 1547. The shaft V (which is also seen in a dotted circle in Fig. 1545) carries a f.l.a.n.g.e _c_, on each side of which is a leather disk, so that the pressure of the bolts which secure _b_ to the sleeve _a_ causes _c_ to revolve under friction, unless sleeve _a_, slotted bar _b_, and f.l.a.n.g.e _c_ all revolve together, or, in other words, _c_ revolves under friction when it revolves within _a_ _b_.

[Ill.u.s.tration: Fig. 1547.]

[Ill.u.s.tration: Fig. 1548.]

Fig. 1548 is an end view of Fig. 1547.

[Ill.u.s.tration: Fig. 1549.]

Fig. 1549 gives a cross-sectional view of the shaft sleeve, &c. The sleeve _a_ is provided with two pins _i_, _i_, and a pin _k_ is fast in the frame of the machine, and it is seen that _a_ and V may revolve together in either direction until such time as one of the pins _i_ meets the stationary pin _k_, whereupon the further revolving of _a_ will be arrested and V will revolve within _a_, and as f.l.a.n.g.e _c_, Fig.

1547, revolves with V, it will do so under the friction of the leather washers. The pins _i_ and the pin _k_ are so located that _a_ can have motion only when the ram or sliding-bar is at the end of the return stroke, and the feed-rod _f_, being connected to _b_, is therefore actuated at the same time.

Among the various mechanisms employed to give a quick return to the tool-carrying slide of shaping machines, those most frequently employed are a simple crank, a vibrating link, and the Whitworth quick-return motion, the latter being the most general one.

The principle of action when a vibrating link is employed may be understood from Fig. 1550, in which P is a pinion driven by the cone pulley and imparting motion to D. At L is a link pivoted at C. At A is a link block or die capable of sliding in the slot or opening in the link and a working fit upon a pin which is fast in the wheel D. As D rotates the link block slides in the slot and the link is caused to travel as denoted by the dotted lines. R is a rod connecting the tool-carrying slide S to the upper end of link L, and therefore causing it to reciprocate with L. But S being guided by its slide in the guideway traverses in a straight line.

[Ill.u.s.tration: Fig. 1550.]

Since the rotation of P and D is uniform, the vibrations of the link L will vary in velocity, because while the link block is working in the lower half of the link slot it will be nearer to the centre of motion C of the link, and the upper end of C will move proportionately faster.

The arrangement is such that during this time the tool-carrying slide is moved on its return stroke, the cutting stroke being made while the link block is traversing the upper half of the slot, or in other words, during the period in which the crank pin in A is above the horizontal centre of wheel D.

Now suppose the arrangement of the parts is such that the front of the machine or the cutting tool end of the slide is at the end K of S, then S will be pushed to its cut by the rod R at an angle which will tend to lift S in the slideways. But suppose the direction of rotation of wheel D instead of being as denoted by the arrow at D be as denoted by the arrow at E, then S will be on its back stroke, the front of the machine being at J. In this case rod R will pull S to the cut, and S will, from the angularity of R, be pulled down upon the bed of the slideway guiding it, and will therefore be more rigidly held and less subject to spring, because the tendency to lift is resisted on one side by the adjustable gib only, and on the other by the projecting V, whereas the tendency to be pulled downwards is resisted by the strength of the frame of the machine.

Furthermore, as the pressure on the cutting tool is below the level of the tool-carrying slide it tends to force that slide down upon the slideway, and it will therefore be more rigidly and steadily guided when the force moving the slide and the tool pressure both act in the same direction.

To vary the length of stroke of S pin A is so attached to wheel D that it may be adjusted in its distance from the centre of D.

[Ill.u.s.tration: Fig. 1551.]

The Whitworth quick-return motion is represented in Fig. 1551. At P is the pinion receiving motion from the cone pulley or driving pulley of the machine and imparting motion to the gear-wheel G, whose bearing is denoted by the dotted circle B. Through B pa.s.ses a shaft C, which is eccentric to B and carries at its end a piece A in which is a slot to receive the pin X, which drives rod R whose end Z is attached to the ram of the machine. At D is a pin fast in gear-wheel G and pa.s.sing into a slot in A.

Taking the position the parts occupy in the figures, and it is seen that the axis of B is the centre of motion of G and is the fulcrum from which the pin D is driven, the power being delivered at X. The path of motion of the driving pin D is denoted by the dotted circle H', and it is apparent that as it moves from the position shown in the figure it recedes from the axis of C, and as the motion of G is uniform in velocity therefore D will move A faster while moving below the line M than it will while moving above it, thus giving a quick return, because the cutting stroke of the ram occurs while D is above the line M and the return stroke occurs while D is below M.

In some constructions the pin X and pin D work in opposite ends of the piece A, as shown in Fig. 1552. This, however, is an undesirable construction because the shaft C becomes the fulcrum, and as the power and resistance are on opposite ends of the lever A, the wheel G is therefore forced against its bearing, and this induces unnecessary friction and wear.

We may now consider the tool motion given by other kinds of slide operating mechanism.

In Fig. 1553 is a diagram of the tool motion given when the slide is operated by a simple crank C, the thickened line R representing the rod actuating the slide and line on the line of motion of the cutting tool.

The circle H denotes the path of revolution of the crank pin, and the black dots 1, 2, 3, 4, &c., equidistant positions of the crank pin.

[Ill.u.s.tration: Fig. 1552.]

Line _m_ represents the path of motion of the cutting tool.

If a pair of compa.s.ses be set to the full length of the thick line R, that is from the centre of the crank pin to end B of line R, and these compa.s.ses be then applied to the centre of crank pin position 1, and to the line _m_, they will meet _m_ at a point denoted by line _a_, which will, therefore, represent the position of the tool point when the crank pin was in position 1. To find how far the tool point is moved while the crank pin moves from position 1 to position 2, we place the compa.s.s point on the centre of crank pin position 2 and mark line _b_. For crank position 3 we have by the same process line _c_, and so on, the twelve lines from _a_ to _l_ representing crank positions from 1 to 12.

Now let it be noted that since the path of the crank pin is a circle, the tool point will on the backward stroke occupy the same position when the crank pin is at corresponding positions on the forward and backward strokes. For example, when the crank pin is in position 7 the tool point will be at point _g_ on the forward stroke, and when the crank pin is in position 17 the tool will be at point _g_ on the backward stroke, as will be found by trial with the compa.s.ses; and it follows that the lines _a_, _b_, _c_, &c., for the forward stroke will also serve for the backward one, which enables us to keep the engraving clear, by marking the first seven positions on one side of line _m_, and the remaining five on the other side of _m_, as has been done in the figure.

[Ill.u.s.tration: Fig. 1553.]

Obviously the distances apart of the lines _a_, _b_, _c_, _d_, &c., represent the amount of tool motion during equal periods of time, because the motion of the crank pin being uniform it will move from position 1 to position 2 in the same time as it moves from position 2 to position 3, and it follows that the cutting speed of the tool varies at every instant in its path across the work, and also that since the crank pin operates during a full one-half of its revolution to push the tool forward, and during a full one-half to pull it backward, therefore the speed of the two strokes are equal.

[Ill.u.s.tration: Fig. 1554.]

We may now plot out the motion of the link quick return that was shown in Fig. 1550, the dotted circle H', in Fig. 1554, representing the path of the pin A, and the arc H representing the line of motion of the upper end of link L, and lines N, O, its centre line at the extreme ends of its vibrating motion. In Fig. 1554 the letters of reference refer to the same parts as those in Fig. 1550. We divide the circle H' of pin motion into twenty-four equidistant parts marked by dots, and through these we draw lines radiating from centre C and cutting arc H, obtaining on the arc H the various positions for end Z of rod R, these positions being marked respectively 1, 2, 3, 4, &c., up to 24. With a pair of compa.s.ses set to the length of rod R from 1 on H, as a centre, we mark on the line of motion of the slide line _a_, which shows where the other end of the rod R will be (or, in other words, it shows the position of bolt B in Fig. 1550), when the centre of A, Fig. 1550, is in position 1, Fig.

1554.

From 2 on arc H, we mark with the compa.s.ses line _b_ on line M, showing that while the pin moved from 1 to 2, the rod R would move slide S, Fig.

1550, from _a_ to _b_, in Fig. 1554. From 3 we mark _c_, and so on, all these marks being above the horizontal line M, representing the line of motion, and being for the forward stroke. For the backward stroke we draw the dotted line from position 17 up to arc H, and with the compa.s.ses at 17 mark a line beneath the line M of motion, pursuing the same course for all the other pin positions, as 18, 19, &c., until the pin arrives again at position 24, and the link at O, and has made a full revolution, and we shall have the motion of the forward stroke above and that of the backward one below the line of motion of the slide.

On comparing this with the crank and with the Whitworth motion hereafter described, we find that the cutting speed is much more uniform than either of them, the irregularity of motion occurring mainly at the two ends of the stroke.

In Fig. 1555 we have the motion of the Whitworth quick return described in Fig. 1551, H' representing the path of motion of the driving-pin D about the centre of B, and H' the path of motion of X about the centre C, these two centres corresponding to the centres of B and C respectively in Fig. 1551. Let the line M correspond to the line of motion M in Fig. 1551. Now, since pin D, Fig. 1551, drives, and since its speed of revolution is uniform, we divide its circle of motion H'

into twenty-four equal divisions, and by drawing lines radiating from centre B, and pa.s.sing through the lines of division on H', we get on circle H twenty-four positions for the pin X in Fig. 1551. Then setting the compa.s.ses to the length of the rod (R, Fig. 1551), we mark from position 1 on circle H as a centre, line _a_; from position 2 on H we mark line _b_, and so on for the whole twenty-four positions on circle H, obtaining from _a_ to _n_ for the forward, and from _n_ to _y_ for the motion during the backward stroke. Suppose, now, that the mechanism remaining precisely the same as before, the line M of motion be in a line with the centres C, B, instead of at a right angle to it, as it is in Fig. 1551, and the motion under this new condition will be as in Fig.

1556, the process for finding the amount of motion along M from the motion around H being precisely as before.