Modern Machine-Shop Practice Part 124

A very old but a very excellent device for hand drilling when no drilling machine is at hand is the drilling frame shown in Fig. 1791, which consists of two upright posts A, and two B, placed side by side with s.p.a.ce enough between them to receive and guide the fulcrum lever and the lifting lever. The fulcrum lever is pivoted at C, and has an iron plate at E, and suspends a weight at its end which serves to put on the feed. The lifting lever is pivoted at D, and at F hooks on to the fulcrum lever. At its other end is a rope and eye G, and it is obvious that the effect of the weight upon the fulcrum lever is offset by any pressure applied to G, so that by applying the operator's foot at G the weight of drill feed may be regulated to suit the size of hole and strength of drill being used. The work is rested on a bench, and a drill crank or other device such as a ratchet brace may be used to drive the drill. This drill frame is capable of drilling holes up to about two inches in diameter, but it possesses the fault that the upper end of the brace or drilling device moves as the drill pa.s.ses into the work in an arc H of a circle, of which the pin C is the centre. The posts A are provided with numerous holes for the pin C, so that the fulcrum lever may be raised or lowered at that end to suit the height of the work above the work bench. Another objection to this device is, it takes up a good deal of shop room.

Ratchet braces are employed to drill holes that are of too large a bore to be drilled by tread drills, and that cannot be conveniently taken to a drilling machine.

[Ill.u.s.tration: Fig. 1792.]

In Fig. 1792 is represented a self-feeding ratchet brace. A is the body of the brace, having a taper square hole in its end to receive the square shank of the drill. L is a lever pivoted upon A, and having a pawl or catch B, which acts upon ratchet teeth provided upon A. When the lever L is moved backward the pawl B being pivoted rides over the ratchet teeth, but when L is pulled forward B engages the ratchet teeth and rotates A and therefore the drill. At F is a screw threaded into A, its pointed end ab.u.t.ting against some firm piece, so that uns.c.r.e.w.i.n.g F forces the drill forward and into its cut. These features are essential to all forms of ratchet braces, but the peculiar feature of this brace consists in its exceedingly simple self-feeding devices, the feed screw F requiring in ordinary braces to be operated by hand when the drill requires to be fed.

The construction and operation of the self-feeding device is as follows: The feed screw F is provided with a feather way or spline and with a feed collar C, operated by the pawl E. The feed-collar C has at D a groove, into which a f.l.a.n.g.e on pawl E fits, and on its side face there is a groove receiving an annular ring on the face of lever L, these two keeping it in place. The pawl E is a double one, and may be tripped to operate C in opposite directions to feed or release the drill, as the case may be, or it may be placed in hind position to throw the feed off--all these operations being easily performed while the lever L is in motion. Collar C is in effect a double ratchet, since its circ.u.mference is provided with two sets of notches, one at _g_ and the other at _h_.

Each set is equally s.p.a.ced around the circ.u.mference, but one set or circle is coa.r.s.er s.p.a.ced than the other, while both are finer s.p.a.ced than is the ratchet operated by pawl B. Suppose, now, that the lever L is at the end of a back stroke, and pawl E will fall into one of the notches on side _g_ of the feed-ratchet, and when lever L is moved on its forward stroke it will operate the feed ratchet and move it forward, A standing still until such time as pawl B meets a tooth of the ratchet on A. The feed screw F is provided with a left-hand thread, and the feed ratchet has a feather projecting into the spline in the feed screw; hence moving the feed ratchet at the beginning of the forward motion of L and before A is operated, puts a feed on, and the amount of this feed depends upon how much finer the notches into which pawl E falls are than those into which B falls. The feed takes place, be it noted, at the beginning of the lever stroke, and ceases so soon as pawl B operates A and the drill begins to cut.

As shown in the figure, the feed collar is set for large drills (which will stand a coa.r.s.er feed than small ones), because the notches are finer s.p.a.ced at _g_ than at _h_. For small drills and finer feeds the collar is slipped off the screw and reversed so that side _h_ will fall under E, it being obvious that the finer the notches are s.p.a.ced the more feed is put on per stroke. The s.p.a.cings are made to suit very moderate feeds, both for large and small drills, because the operator can increase the feed at any stroke quite independently of the s.p.a.cings on the feed ratchet. All he has to do is to give the lever handle a short stroke and more feed is put on; if still more feed is wanted, another short stroke may be made, and so on, the least possible amount of feed being put on when the longest strokes are made. In any event, however, there will be a certain amount of average feed per stroke if equal length of strokes is taken, the s.p.a.cing being made to suit such ordinary variations of stroke as are met within every-day practice. When it is desired to stop feeding altogether, or to release the drill entirely from the cut, all that is necessary is to trip the feed-pawl E (without stopping the lever motion), and it will operate the feed screw in the opposite direction sufficiently to release the drill in a single backward stroke of the lever. The range of feed that is obtainable with a single feed ratchet is sufficient for all practical purposes, although it is obvious that if any special purpose should require it, a special feed ratchet may be made to suit either an unusually fine or coa.r.s.e rate of feed. The feed screw is not provided with either a squared head or with the usual pin holes, because the feed ratchet is so readily operated that these, with their accompanying wrench or pin, are unnecessary.

[Ill.u.s.tration: Fig. 1793.]

[Ill.u.s.tration: Fig. 1794.]

Figs. 1793 and 1794 represent a self-feeding ratchet brace for hand drilling in which the feed is obtained as follows: The inside or feed sleeve B, which screws upon the drill spindle, is fitted with a friction or outer sleeve A, in the head of which is secured a steel chisel-shaped pin C, the lower end of which is pointed and rests upon a hardened steel bearing D, fixed in the head of the inner sleeve B. This sleeve, with its bearing D, revolves upon the point of the pin C, and within the friction sleeve A. Having thus described its construction, we will now describe the operation of the self-feeding device. The head of the pin C being chisel-shaped, prevents the pin and the outer sleeve A from revolving. If the thumb or friction screw F is unscrewed, it will permit the inner sleeve B to rotate freely upon the bearing of pin C, and within the friction sleeve A. As the screw F is tightened, the friction upon the inner sleeve B is increased, causing it to remain stationary, and consequently causing the screw on the drill spindle to feed the drill until the friction on the drill becomes greater than the friction on the sleeve B. This then commences to rotate again within the outer sleeve A, and continues until the chip which the drill has commenced to cut is finished, when the same operation is repeated, thus giving a continuous feed, capable of being instantly adjusted to feed fast or slow as desired, by tightening or loosening the friction screw F, thereby causing a greater or less friction upon the inside or feed sleeve B.

[Ill.u.s.tration: Fig. 1795.]

To afford a fulcrum or point of resistance for the chisel-piece C, or the pointed centre used in the common forms of ratchet brace feed screws, various supporting arms, or stands are employed. Thus Fig.

1795[30] represents a boiler sh.e.l.l _a_, to which is attached an angle frame or knee _b_, carrying the angle piece _c_ (which may be adjusted for vertical height on _b_ by means of the bolt shown) affording a fulcrum for the feed sleeve _d_. This sleeve is sometimes made hexagonal on its outside to receive a wrench or to be held by the hand when feeding, or it may have holes near its centre end to receive a small pin or piece of wire; _e_ is a chain to pa.s.s around the boiler to secure _b_ to it, which is done by means of the device at _f_.

[30] From _The American Machinist_.

For many purposes a simple stand having an upright cylindrical bar carrying an arm that may be set at any height and set to its required position on the bar by a set-screw is sufficient, the base of the stand being secured to the work by a clamp or other convenient device.

Fig. 1796 represents a flexible shaft for drilling holes inaccessible to a drilling machine, and in situations or under conditions under which a ratchet brace would otherwise require to be used. It consists of a shaft so constructed as to be capable of transmitting rotary motion though the shaft be bent to any curve or angle. A round belt driven from a line shaft rotates the grooved pulley, and the shaft transmits the rotary motion to bevel-wheels contained in a portable drilling frame, the fulcrum for the feed being afforded by a drilling post after the manner employed in ratchet drilling. The shaft is built up of several layers of wire (as shown in the view to the left), the number of layers depending upon the size and strength of shaft required, wound one upon the other helically. The layers are put on in groups of three to eight wires, parallel to each other, each successive layer containing groups of varying numbers of wires, thus giving a different pitch to the helices for each layer, the direction of each twist or helix being the reverse of the one upon which it is wound. When the shaft is laid up in this manner, the wires at each end for a short distance are brazed solidly together, and to these solidified ends the piercers are secured for the attachment of the pulley and tool which it is to drive.

This construction, it will be readily seen, produces a shaft which will have considerable transverse elasticity, while it must necessarily offer great resistance to torsional strain, the reversed helices forming a kind of helical trussing, which effectually braces it against torsion.

The case within which it turns is simply an elastic tube of leather or other suitable material, within which is wound a single helix of wire fitting its inside tightly, the inside diameter of the helix being a little greater than the outside diameter of the shaft, and wound in a contrary direction to the outer helices of the shaft. This forms a continuous bearing for the shaft; or at least serves as a bearing at the points of contact between the shaft and case which are brought about in the various bending of the whole when in use.

[Ill.u.s.tration: Fig. 1796.]

In order to give to the instrument all the transverse elasticity possible, that end of the shaft carrying the pulley is made with a feather so that it may slide endways in the pulley, while the latter is secured to the case, the case, however, not rotating with it. It will be readily seen that this is a necessary precaution, inasmuch as in the varying curves given to the instrument in use a difference will occur in the relative lengths of the shaft and tube.

It might be supposed that the friction of the shaft within the tube would be so considerable as to militate against the success of the apparatus; but in practice, and under test for the determination of this, it has been found that the friction generated by running it when bent at a right angle does not exceed that when used in a straight line more than 15 per cent. of the latter.

In the running of it in a bent position, not only will there be friction between the shaft and tube, but there must also be some little motion of the layers of wire one upon another in the shaft itself; and to provide against the wear and friction which would otherwise occur in this way, provision is made for not only oiling the bearings at the ends, but also for confining a small quant.i.ty of oil within the tube, by which all motion of the wires upon one another, or the shaft upon the interior of the tube, is made easy by its being well lubricated.

In the figure the shaft is shown complete with a wood-boring auger in place at the shaft end. Shafts of similar but very light construction are employed by dentists for driving their dental drills and plugging tools, many of them having ingenious mechanical movements derived from the rotary motion of the shaft.

[Ill.u.s.tration: Fig. 1797.]

In Fig. 1797 is represented a drilling device in position for drilling a hole from the inside of a steam boiler. A represents a base piece made with a journal stud _b_. This base piece is provided with radial arms _a_, with threaded ends and nuts made with conical projecting ends, as shown at _a_^{2}. One of these pieces is used at each end of the machine when convenient, their use for centring and holding the frame being apparent. When not convenient to use two of them, one end of the frame is sustained as shown in the engraving, or in some other manner that may suggest itself. The casting B is made in two pieces, and is provided with a bearing for the pin _b_, and holds the ends of the rods C C. The actuating shaft G carries the bevel-wheel _g_, more clearly seen in the figure at side, which drives the drill spindle, whose ends are of different lengths, for convenience in reaching to different distances.

The cross-head E may be slid along as required on the rods, and the revolving frame and drill turned around to different positions.

[Ill.u.s.tration: Fig. 1798.]

Fig. 1798 represents a small hand drilling machine to be fastened to a work bench. A suitable frame affords journal bearing to the upright spindle, upon which is a bevel-gear G, which is driven by a gear upon the same shaft as the wheel W. The spindle is threaded at S and is fed by the hand wheel F, which is threaded upon the screw S and has journal bearing in the cap C.

Fig. 1799 represents a hand drilling machine for fixture against a post, the larger wheel serving as a fly-wheel and the smaller one being to feed with.

SLOTTING MACHINE.--In the slotting machine the cutting tools are carried in a ram or slide that operates vertically, and the work table lies horizontal and beneath the ram.

Fig. 1800 represents a slotting machine, and Fig. 1801 is a sectional view of the same machine.

The cone spindle shaft has a pinion which drives a spur-wheel upon an horizontal shaft above. Upon the inside face of this spur gear is a cam groove for operating the feed motions, at the other end of the shaft is a Whitworth quick-return motion, such as has already been described with reference to shaping machines. The connecting rod from the quick-return motion attaches to the ram, which operates on a guide pa.s.sing through a way provided at the upper end of the main frame, and bolting to the front face of the main body of the frame. The object of this arrangement is that by adjusting the height of this guide to suit the height of the work, the ram will be guided as close to the top of the work as the height of the latter will permit; whereas when the guide for the ram is fixed in position on the frame the ram pa.s.ses as far through the guide when doing this as it does when doing thick work, and is therefore less closely guided than is necessary so far as the work is concerned.

[Ill.u.s.tration: Fig. 1799.]

[Ill.u.s.tration: Fig. 1800.]

The ram, or slotting bar as it is sometimes termed, is counterbalanced by the weighted lever shown, so that the ram is always held up, and there is no jump when the tool post meets the work, because the tool motion is always taken up by the lever.

[Ill.u.s.tration: Fig. 1801.]

The work is held upon a circular table capable of being revolved upon its axis to feed the work to the cut. This table is carried upon a compound slide having two horizontal motions, one at a right angle to the other. The lower of these is operated by a rod running through the centre of the machine, as seen in the sectional view in Fig. 1801. The upper is operated through the larger of the two gear-wheels, seen at the side of the machine in the general view of the machine in Fig. 1800. The upper and smaller of these wheels operates a worm, which engages with worm-teeth cut on the periphery of the circular table to rotate the latter. Either or all of these feed motions may be put in simultaneous action, or all may be thrown out and the feeds operated by hand.

As the tool is in many cases rigid on the ram or bar of a slotting machine, it is preferable that the feed should occur while the tool is at the top of its stroke and before it meets the work, so that it may not rub on the return stroke, and thus become rapidly dulled.

Fig. 1802 represents a slotting machine in which the guideway for the slotting bar or ram is fixed in position, and the feed motions are entirely on the outside of the machine. In this case the worm-gear pinion is on the side of the machine not seen in the engraving.

The cutting tools for slotting machines are carried in one of these ways: first, bolted direct to the slotting bar or ram, in which case they stand vertically; secondly, in a box that is bolted to the end of the ram and standing horizontally; and thirdly, held in a tool bar, in which case the tool may stand either horizontally or vertically.

Fig. 1803 shows a tool B secured in a hole provided in a stout bar A by the set-screw C. The tool in this case being rigidly held the cutting edge is apt to rub against the work during the upward stroke and become rapidly dulled. To avoid this, various devices have been employed, but before describing them it will be well to point out that the shape of the tool has an important bearing upon this point.

In Fig. 1804, for example, is a tool T bolted to the box B at the end of the slide S. W is a piece of work having the cut C taken off it. Now suppose that A is the centre of motion or fulcrum from which the spring of the tool takes place (and there is sure to be a little spring under a heavy cut), then the point of the tool will spring in the direction of the arrow E, and will cut deeper to the amount of its spring; but during the up stroke the tool being released from pressure will not spring, and therefore will partly or quite clear the cut according to the amount of the spring. This desirable action may be increased by giving the face of the tool which meets the cutting a slight degree of side rake, as shown in Fig. 1805, in which S is the slide, T the tool, B the box, and F the direction of the tool spring, which takes place in this case from the pressure of the cutting in its resistance to being bent out of the straight line.

[Ill.u.s.tration: Fig. 1802.]

[Ill.u.s.tration: Fig. 1803.]

[Ill.u.s.tration: Fig. 1804.]

In Fig. 1806 is a device for obviating to some extent this defect. A A is the tool box or bar containing a tool-holding piece pivoted at C, the tool being secured therein by the set-screw E B. A spiral spring sustains the weight of the pivoted piece and of the tool. During the down stroke the spiral spring holds the pivoted piece against the box or bar A, while during the up stroke the pivoted piece allows the tool to swing from the pivot C as denoted by the arrow D. In this case the friction on the tool edge is that due to overcoming the resistance of the spring only.

[Ill.u.s.tration: Fig. 1805.]

[Ill.u.s.tration: Fig. 1806.]

[Ill.u.s.tration: Fig. 1807.]

In round-nose tools that are slight, and which from having a maximum length of cutting edge are very subject to spring, additional strength may be given the tool by swelling it out at the back, as denoted by the dotted line B in Fig. 1807.

[Ill.u.s.tration: Fig. 1808.]

Excessively heavy cuts may be taken by the form of tool shown in Fig.

1808, in which A is the tool, B the tool box, and C the work, the depth of cut being from D to E, which may be made 2-1/3 inches if necessary.