[10]The MS. book containing the above was kindly lent to the author of the present work by Mr. Hoblyn, of Bishop's Stortford, who is the inventor of a new form of slide rest, which will be introduced in a later page. The chuck in question is, however, commonly attached to the watchmakers' lathes of the present day, and therefore may not, as the writer supposed, be actually the invention of the late Mr. Wilcox.
[10]The MS. book containing the above was kindly lent to the author of the present work by Mr. Hoblyn, of Bishop's Stortford, who is the inventor of a new form of slide rest, which will be introduced in a later page. The chuck in question is, however, commonly attached to the watchmakers' lathes of the present day, and therefore may not, as the writer supposed, be actually the invention of the late Mr. Wilcox.
It now becomes necessary to speak of another addition to the lathe, by means of which the use of the slide rest is considerably extended. We mean the overhead motion. Of this there are several patterns, and we have sketched three of these. As to their respective merits we can hardly venture to speak. They all answer equally well the purpose for which they have been designed, and the turner must select according to his fancy, or, if he please, design a better for himself. A,Fig. 144, represents the lathe bed. From the left-hand standard rises a round iron rod, not less than one inch in diameter. This is not generally fixed, but is attached to the standard by two staples,a, b, which hold it securely in an upright position, but allow it to turn with its projecting bar F, F,after the manner of a crane. It may thus be turned back, out of the way, or brought into any desired position. The part F, F is made to slide up and down on the part B, and is fixed by a clamping screw D. Thus, if the cord should break and require to be shortened, the arm can be brought nearer to the bed of the lathe. Upon F, F slide two rings, or rather short pieces of tube, from which depend two India rubber springs (door springs), E, E, now procurable at any ironmonger's at one shilling each. From these hang double pulleys, or better still two single ones. These pulleys, with their attachments, are adjustable at any position on the arm F, F, which may be round or square. If considered desirable, a second standard can be added, so as to uphold both ends of this bar; but it is hardly necessary, as the latter is seldom required of greater length than half that of this lathe-bed. It is evident that the above addition to the lathe can be made complete for a few shillings. The following are more expensive, but more general, the writer having devised the above to suit his own fancy, and for his own use. InFig. 145A represents the standard as before, the topof which is forked, as shown at E, and sustains the ring, free to revolve in its arms, as seen in the sketch. Through this passes a bar, B B, with a heavy ball C cast on its end to act as a counterbalance to the longer arm and its connections, and to keep the cord stretched. By sliding this bar either way through the ring which supports it, the tension of the cord can be increased or lessened.[11]When in position it must be very securely fixed by the screw T, which should not simply press against it, but enter one of a row of depressions made for the purpose. The pulleys C, D are double, as in the previous plan of overhead, and are likewise adjustable at any position on the bar B, B. The only drawback to this pattern is the danger of the heavy ball slipping out and falling. We prefer to hang a weight from the end of the lever, as shown by the dotted line. This may be within a few inches of the floor, and if it should fall no harm can ensue. The third pattern,Fig. 146, is the most expensive, but although it is of a more finished appearance, and wears an aspect more stiff and stable, it is not practically any better than the last. From a standard A, with overhanging bar F, F, is suspended a frame H, by means of two coiled springs in brass boxes B, B, which keeps up the necessary tension on the cord, or rather cords, for in this case two are needed—one from the flywheel to the small pulley, and a second from the roller to the slide rest.
[11]Sometimes the bar is merely hung on pivots, and the weight is made to slide upon it.
[11]Sometimes the bar is merely hung on pivots, and the weight is made to slide upon it.
Fig. 144.
Fig. 144.
Fig. 145.
Fig. 145.
Figs. 143, 146, 148.
Figs. 143, 146, 148.
This roller, which may be grooved or plain, may be replaced by a second small pulley, which is capable of being slid along the round bar which forms its axle and turns with it between the centre screws. In this case the bar is made with a groove or channel along its length,Fig. 147, and a pin projecting from the central hole in the pulley enters this groove. Thus the two will turn together, and at the same time the position of the pulley is adjustable at pleasure. The tension of the springs B, B, is increased or lessened by a turn or twoof the nuts C, C, just in the same manner as the spring of the safety valve is adjusted on the boiler of a locomotive.[12]The application of the overhead motion is plainly shown in Figs.143and144. The work is fixed in the lathe, and the mandrel kept stationary. The cord passes to the overhead directly from the fly wheel, and thence to a pulley on the screw of the slide rest, as in143, to a drill, as in 144, or to any other apparatus at pleasure. To fit the work in the lathe in such a manner as to enable any point in the face or side to be operated upon, a division plate and index are required. The first is a round plate of brass or gun metal,1/16thof an inch thick, drilled with holes in concentric circles. The index is a steel spring with a projecting point, which, entering any one of the holes, retains the plate, and with it the pulley, on the face of which it is fixed immovable. There are generally four circles of holes, the number in each selected with reference to its divisibility by the greatest possible number of divisions—thus 360, which is usually on the outside or largest circle, can be divided without a remainder by 2, 3, 4, 5, 6, 8, or 9—144 is a good number for the second circle, being divisible by 2, 3, 4, 6, 8, 9. The other two may be 112 and 96. The uses of the division plate are many. Eccentric cutting and drilling could not bedone without its aid, and wheel cutting for clocks, or for making cycloidal and other chucks, is entirely dependent on this contrivance.
[12]The frontispiece shows another superior form of overhead, with balance-weights hanging close to the ground behind the lathe.
[12]The frontispiece shows another superior form of overhead, with balance-weights hanging close to the ground behind the lathe.
Fig. 147.
Fig. 147.
The division plate and index are shown inFigs. 147, A, B, C. The part B is a knob with a round hole through it, to take the tail of the spring C. This is shown again at C on a larger scale, to bring to view a slot which allows of some slight adjustment of the point, to suit the different-sized circles. The milled-headed screw clamps the point at any desired part of this slot. When not in use, the spring is drawn back so as to release it from the division plate, and turned down in the position shown by the dotted line.
The overhead apparatus is applicable not only to revolving cutters in the slide rest, but to other contrivances available where the latter is not possessed. There are many cases in which the back poppet may be brought into use to hold revolving drills, or stationary tools, if the resistance to their action is not too great. The only damage that can well happen in this method is to the pin working in the slot on the outside of the spindle. If this is of steel and tolerably stout and strong (being very short it cannot be subject to any very injurious strain), no harm is likely to result. With this it is possible to drill very neatly, and also to do a little eccentric cutter work, under certain conditions to be described. Screwing, or rather chasing, may likewise be done very passably. We do not, of course, advise this course when the more perfect slide rest is at hand, but there are many who are obliged to put up with all sorts of homemade contrivances, and to press into service for divers operations apparatus and tools not precisely meant to be thus used, and it is as well to learn how to act under an emergency, even if the practice is not intended to be carried out generally. The drill stock is shown inFig. 148, in which the screw A fits into the spindle of the back poppet. If the hole in the latter is conical instead of cut with a screw thread, the drill must be made accordingly.
The pulley of brass has a hole drilled through it, and the screw is also drilled, the hole in the latter being rather larger than that in the pulley (which is tapped). The steel pin H, shown white in the sectional drawing, passes truly through the centre of the screw in which it revolves, and is screwed into the pulley. The socket ofsteel to hold the drill is then screwed or soldered into the opposite face of the pulley. The black part shows a flange on the screw which abuts against the cylinder of the poppet into which it is screwed. Thus the pulley, being attached to the cord from the overhead apparatus, is free to revolve upon the steel pin with great rapidity, and will carry round any drills or cutters placed in the socket B. The simple straight drill being thus worked and advanced by the leading screw of the poppet, will suffice for holes in any work held on the mandrel. On page 335, vol. ii., of theEnglish Mechanic, this method is mentioned, and also a similar pattern of slide tool to be used for boring, being, in fact, the head and spindle of the back poppet, but instead of the standard and sole, a pin, like that of aT, to fit the socket of the rest. This has the advantage of the back poppet, but must be made on purpose, and if any special slide tool is added, a proper slide rest is by far the best. Our present purpose is to describe the use of the back poppet as a substitute for a better tool.
With the following modification of the eccentric cutter, a fair amount of good ornamental work may be done.Fig. 149represents the back poppet with the apparatus in position. The various parts are shown separately in A, B, C, D. A is a small frame, two or three inches long, and ¾ to 1 inch wide, cast in iron, with a circular piece at the back, 1½ or 2 inches diameter. This circular piece is to be accurately divided on the edge with any even number, which is divisible as above explained. This is turned and drilled through thecentre. B is a flange-shaped piece to screw to the poppet as before, and this is also accurately drilled, and the two are attached by a central steel pin, so that the two flanges can turn face to face, the outer one with frame revolving against the other. The pin must be put in its place in B, and turned at the same time as the face of B, that it may be truly central. The divisions on the edge of the outer flange are to be drilled like the division plate of the lathe, or cut into cogs, and in either case can be held at any point by a spring detent or index attached. C shows the front of the frame, with a screw down its centre and traversing slidef. The head of this screw is divided, and a small brass index marks its position; one side of the frame may likewise be divided. The small brass pulley and drill socket are fixed to revolve in the traversing nut or slidef, as shown in a side view at D. Drills of all shapes, as E 1, 2, 3, 4, may be fixed in the socket at pleasure.
Fig. 149.
Fig. 149.
Fig. 149E.
Fig. 149E.
Figs. 150, 151.
Figs. 150, 151.
Figs. 152, 153.
Figs. 152, 153.
It is evident that the cutter frame last described having a division plate of its own dispenses with the necessity of one on the pulley of the mandrel. Indeed it was the impossibility of supplying the latter on the face of the cogwheel of the author's back-geared lathe that necessitated the substitution of that alluded to. The latter may consequently be omitted if applied to a lathe already fitted with division plate and index. With crank-form drills the cutting edges of which may be as E 1, 2, 3, 4, or other form, such patterns can be cut, as shown in Figs. 149,150,151,152, which are merely samples of the simplest combinations of circles intersecting in straight, spiral, or other lines. Although not now treating of eccentric work, we may state in passing that to produce the pattern round the edge of150the division plate of the instrument only is required. Select a crank-form drill which by its revolution will produce the required size of circle. Turn the divided screw head until you find upon trial that the circle will come at the required distance from the edge, and putting the lathe in motion cut the first circle. The work on the mandrel is understood to be stationary, the mandrel being fixed so as not to turn. The cutter frame is now turned on its axis, one, two, or more of its divisions, and fixed by its detent, and a second circle cut intersecting the first. This is repeated till the whole circle iscompleted. The straight line of circles inFig. 151is cut as follows:—The first circle is made as before, but the cutter-frame should be placed so as to stand either horizontally or vertically. Between each cut the divided screw-head is moved so many points according to the proposed fineness of the pattern, and no movement of the cutter frame on its axis is to be made. The spiral,152, is simply the result of a combination of the two movements. Start from the centre by turning the screw head until the shank of the crank-form drill is coincident with the axis of the mandrel. Cut the central circle. Thence for each successive circle turn the screw head and the frame an equal number of points, and the pattern will be formed. If the divided screw head is made to take off and a handle substituted, stars formed of channelled lines, or radial flutes can be made as Fig 153, but in this case the crank form drill is replaced by a plain straight tool with a rounded end like149 E 4. Set the tool as for pattern 150 straight across the face of the work, put the drill in rapid motion, let it be advanced by the leading screw of the poppet so as to penetrate slightly, and with the handle that has been attached to the screw of the cutter frame cause it to traverse the face of the work as it revolves. When one flute is thus cut, turn the frame on its axis as many points as required, and proceed with number two, and so on to the completion of the star. It is not necessary at present to describe other patterns.Fig. 153is therefore merely given without special details of the method of producing it.
We must now return from this digression to speak of other applications of the slide rest. It is evident that when connection is made between the overhead and a pulley on the screw of the slide rest, the latter becomes self-acting. The speed is, however, far too great, and in addition, the mandrel is stationary. The above connection, therefore, is not practically possible, and the overhead is only connected with the pulleys of revolving drills and cutters fixed in the tool-holders of the rest. It is, however, very important to be able to form some such connection between the lathe mandrel and the screw of the rest, for the purpose of cutting screws or spirals. A little consideration will show the principle of this arrangement, from which some practical plan is not difficult to design. If, for instance, the tool simply remains in contact with a cylinder while the latter revolves with the mandrel, a simple line will be cut round its circumference; but if, while the mandrel revolves once, motion is given to the screw of the rest by which the tool is made to traverse a distance of one-eighth ofan inch, the commencement of a spiral having that pitch will be made. A perfectly smooth surface, as it leaves the lathe, in which a slide rest has been used with a point tool, is in reality cut with a very fine screw thread readily discernable under the microscope. We have, therefore, only to devise some method of giving regular motion to the screw of the rest while the work revolves as usual, in order to turn plain surfaces, screws, or spirals. For the purpose of plain turning a plan is sketched by Nasmyth in the last chapter of "Baker's Mechanics," in Weale's series. A spur wheel is represented fixed to the slide rest screw, the teeth of which are alternately caught at every turn of the work, by an arm fixed to the latter, after the manner of a lathe carrier. This plan is simple, and might be to some extent used, but for one defect, due to the fact that the slides of ordinary rests are the reverse of what is required to make this plan available. The screw which advances the tool towards the work is generally underneath that which moves the tool along the surface of the work. The result is that, when the tool-holder is advanced to take the deeper cut the spur wheel is brought nearer to the arm which acts upon it, and greater traverse is thus given to the screw. This is shown inFig. 154. A is the spur wheel, B the cylinder to be turned, C the arm or carrier. The arrow shows the direction of the movement. Now, if the lathe is put in motion, the arm will remain in contact with one tooth of the wheel until both arrive atb, giving a certain amount of motion to the screw, and thence to the tool-holder. After one cut is thus taken, the lower screw of the rest is turned to advance the tool nearer to the work, the effect of which is to cause the arm to extend further over the wheel. Suppose its position represented by the dotted line, it will remain in contact with the tooth till both arrive atc, having thus traversed a larger arc, and given more movement to the tool. Now if the frames of the slide rest were made to cross in the contrary direction so that the screw to advance the tool towards the work was above that which gave the traverse in the direction of the bed, this objection would no longer hold, and the above gearing would answer very well, since the necessary advance of the tool would not affect the relative position of the spur-wheel and carrier. In Fig. 5 of the same book, in which the gearing is effected by two cogwheels, this alteration in the rest appears in the drawing. In this case the work and rest are connected for screw cutting, and the arrangement is satisfactory andsimple, and for the amateur especially is the simplest and best that can be devised. The range of screw pitches is however limited, and the rest must have a left-handed screw, or the result will be a left-hand thread to that which is cut. Hence another device has been arranged, represented inFig. 155, A, B, C. A shows the apparatus complete. B is an arm of iron or brass which is about ¼ inch thick or rather more. This is first slipped over the mandrel screw in front of the poppet and fixed in any desired position by a screw passing through the slota, into the face of the poppet. This slot allows the arm to be raised or lowered at pleasure and adjusted, as will be presently described. In the slot formed in the long arm B, pins D with nuts, fit, on the rounded part of which cogwheels,b,c,d, are made to revolve and to gear with each other, and with a similar wheel attached to the back of the chuck, C. The centre of the outside wheel, whether one, two, or three are used, is connected to the screw of the slide-rest. For the production of a right-handed screw, the intermediate wheel comes into play, simply to reverse the direction of the motion imparted to the screw of the slide rest. The number of teeth which it may contain is of no importance, the calculation of the change wheel teeth being only necessary with the first and last. The central one is called an idle wheel, though its work is equal to that of the rest. Thus, suppose the wheel on the chuck to contain 40 teeth, and the third wheel 20, while the former revolves once, the third will, if in immediate contact with it, revolve twice, introduce an idle wheel with 10 teeth between these two. The wheel, with 40 teeth, revolving once, the idle wheel will revolve 4 times—the third wheel twice, just as if the idle wheel was not in use. In any train of wheels, if we regard relative speed, any number between the first and last become similarly idle wheels, and the ultimate result is the same as if the first and last were in immediate contact.
Fig. 154.
Fig. 154.
Fig. 155.
Fig. 155.
Take, for example, the following train of five wheels, even numbers being given for the sake of clearness, to represent the circumferences or number of cogs in each,Fig. 156. We have here, as the first and last, 6 in. and 120 in., and, if in contact, the first must revolve 20 times, while the latter revolves once. Interpose the three idle wheels of 10, 30, and 60 in. respectively. During one revolution of the largest wheel, the second will revolve twice, the third four times, the fourth twelve times, the fifth twenty times, the same precisely as if the first and last had been in immediate contact. The range of a slide rest-screw is quite long enough for many purposes of the amateur, and a connection thus made between the mandrel and such screw is what may be termed a miniature of the arrangement adopted in the large self-acting lathes. In the latter, however, a leading screw is added the full length of the bed along which the slide rest travels bodily. We may, therefore, consider the screw of the slide-rest a leading screw, and make use of the rules applied in the case of large lathes to decide the proportions of wheels required to cut a given screw. It is plain that when the pitch of the required screw is greater than that of the leading screw, the revolution of the latter must be at a quicker rate than the former. If, for instance, a spiral is to be cut, like the Elizabethan twist, containing but one perfect thread in two inches, while the leading screw contains twenty threads in the inch, or forty in the 2 in., the latter must be arranged to make forty revolutions while the former makes one, because it takes 40 revolutions to carry the tool along 2 in., which is the pitch of the required spiral. The two outside wheels must therefore bear that proportion to one another. Forty to one, however, would be a practically difficult ratio, to place as described, even a pinion of ten teeth on the leading screw requiring 400 teeth on the chuck. Hence a different arrangement would be necessary if such very great difference exist between the pitch of the leading or rest screw and that to be cut. The same obvious difficulty would occur where a very fine screw is required, and the pitch of the leading screw is coarse. This will have to be referred to again. One example, therefore, of the method of overcoming this difficulty will suffice. A train of wheels is shown inFig. 157, of which A has 60 teeth, B 10 teeth, C,on the same axle and united toC, 30 teeth, D 20 teeth. While A turns once, B will turn six times, C necessarily six times also, D nine times. In this case, if the first and last had geared together D would have made but three turns, while A made one.
Fig. 156.
Fig. 156.
Fig. 157.
Fig. 157.
The following is an easy method of calculating a series of such change wheels:—
Write down the number of threads in the screw to be cut, and also the number of threads in the leading screw; multiply both by any convenient number likely to give such results as to tally with the cogs in the set of change wheels. Suppose it is desired to cut eight threads to the inch, and that the leading screw has two threads in that length. Then:
If you have either couple of these wheels, you can put one on the leading screw, and another on the mandrel, and fill up the intermediate space with dummies.
If the above is inconvenient, or the wheels are not to hand exactly as required, proceed thus. If you have the wheel for the mandrel, and the one you wish to use for the leading screw has only half the proper amount of teeth, it is evident that the leading screw would revolve twice as fast as required, for they are proportioned as two to one—or if I have the proper wheel for the leading screw, and the wheel I wish to use for the mandrel has twice the proper number of teeth, it amounts to the same thing. You can get over the difficulty by using any two wheels which are in the proportion of two to one (say 20 and 40, 30 and 60, 40 and 80, &c.), and coupling the two firmly together, so that the larger wheel of the two works into the mandrel wheel (or dummy working into the mandrel wheel) and the smallest into the screw wheel (or its dummy); if the speed is wrong in the contrary way, so that the case is reversed, the coupled wheels are made to gear in a reversed direction; and whatever may be the amount of error, whether such as to cause either mandrel or screw to revolve ⅛, ¼, or ¾ too slow or too fast, the same arrangement may be pursued, the coupled wheels bearing that proportion to each other. The above method was communicated to theEnglish Mechanicby a working man, James Connor, and is perhaps as easy as any; but tables are published of change wheels for any pitch, with any thread of leading screw. Where it is not possible or inconvenient to apply the above arrangement, and where only a few pitches are likely to be needed, another method can be arranged by connecting the lathe pulley to the overhead motion and thence to the screw of the rest. Such an arrangement is shown inFig. 158. A is the fly wheel, B mandrel pulley, C, D, pulleys on the overhead, E pulley and screw of the slide rest. To facilitate calculation, let diameter of C equal that of the part of the mandrel pulley that drives it, by which it willrevolve in the same time. The calculation of the sizes of pulleys, D and E, will be the same as for the cogwheels of the screw-cutting lathe, circumference and number of cogs being, so far as calculation is concerned, the same thing. Let the leading screw have eight threads to the inch, and let it be required to cut a spiral of two threads to the inch. Proceed as before by dividing the required number of threads to be cut by the number on the leading screw 2-8 = ·25. The pulley on the leading screw will be therefore one quarter the size of that on the overhead (which is virtually that on the mandrel as it revolves at an equal speed with the latter). The overhead pulley may be conveniently twelve inches diameter, and that on the screw three inches. While the mandrel makes one revolution the screw will make four, advancing the tool half an inch, and cutting one thread of a spiral in that distance. The next revolution will advance the cutter a second half inch, cutting a second complete thread of spiral. Two threads will, therefore, have been cut in the space of one inch as desired. By the above method short screws and spirals of divers pitches may be cut at pleasure.The practical difficulty in this plan is due regulation of the various speeds.
Fig. 158.
Fig. 158.
We here introduce a modification of self-acting lathe for cutting Elizabethan twist described by Mr. Wilcox in his MS. before alluded to. The work is here done by a leading screw and toothed gearing, the principle being that of the ordinary machine lathe. A chuck, A, with cogwheel attached holds the work as usual, the back centre being also required. The cogwheel gears with one of less diameter attached to the end of the guide-screw B. On the latter works the rest C, in which is a nut of the same thread as that on the guide screw, and which holds the tool in a notch or hollow upon its upper part. The tool is then used by hand, but is guided in its course along the surface of the work to be turned. This guide screw, with the rest and cogwheel, is mounted on a board as a separate piece of apparatus, and is, when used, clamped on the lathe bed. As the rest is after all little else than a large nut, it must be prevented from turning round, and must be arranged to bear the pressure of the tool, relieving the long screw from the strain that would be thus caused. This is effected by a long flat bar—like the rest of a chair maker's lathe—extending the full length of the bed shown here atE, and supported by standards F, F. A projecting part of the rest bears upon this, and slides along it with the tool. The work is begun with the rest on the right hand, and some care is necessary as it nears the cogwheel on the left, when the work must be stopped and the whole run back by hand to its starting-place. This is one chief defect in this apparatus, for the rest very quickly traverses the length of the screw, and great delay is caused by having thus constantly to stop the lathe and reverse the motion. The closest attention is also necessary to prevent the rest from overrunning its mark and striking the cogwheel. It is evident that by using different pitches of cogwheels, many screws of varied threads can be cut in the above lathe. There is, however, another defect in the above tool not noticed by the writer of the MS. from which the description is taken, namely, the difficulty (common to all such contrivances for turning wood), of obtaining the requisite speed. If the work is put in rapid motion, without which wood will not be cut clean, the movement of the rest will be so rapid also, from the effect of the multiplying wheel, that the tool will be carried from end to end in a few seconds. We will therefore proceed to describe a modification of this and similar apparatus, which allows the tool a slow traverse lengthwise of the work, but gives it immense rapidity in the necessary direction. The following is applicable to the lathe described above, to the ordinary slide rest worked by hand, or to the large self-acting screw-cutting lathes used in manufactories, and is specially adapted for cutting spirals or other patterns in wood. In theFig. 160, A represents a shank,which may be made of any shape to fit particular patterns of tool holder. This shank is turned up and becomes a cylinder at B, like that of the ordinary revolving cutter. This part is bored, and fitted with a steel spindle, which should be of strength proportionate to the size of stuff likely to be operated on. One end of the spindle is fitted with a brass pulley, from which a cord is to be attached to the overhead apparatus, the other end terminates in a round or hexagonal boss, D, round the margin of which are securely held, by means of bridles B1or other simple contrivance, a pair or more of small sharp gouges. This apparatus is put in the tool holder of the slide rest, set to the angle that corresponds with that of the screw or twist, and put in rapid revolution by means of the overhead apparatus. The whole rest, or merely the upper part, is then put in motion by one of the before-named means, and the tool advanced to make a cut. However slow the movement of the rest may be, the cutters move with such velocity as to make clean and beautiful work.[13]This may be applied to the slide-rest of the twist lathe, just described, or any similar apparatus. In the overhead, a roller, supplying the place of the second pulley, as described in a previous page, will allow the second cord sufficient power of traverse to keep up a proper position in reference to the pulley C. Revolving cutters on the same principle as the above, have of late years come into extensive use in wood-cutting and carving machinery. The steam planes now used in the preparation of flooring boards, the spoke turning lathe, moulding and shaping machines for wood are all thus fitted. The gouges or other cutters used must not be placed radially, but as tangents to the circumference of the boss in which they are fixed. An improvement upon the simple bridle to hold the cutters would be the substitution of Babbage's tool holder, four radial arms being substituted for the metal boss above alluded to. This tool is described and figured in Holtzapffel's "Mechanical Manipulation," to which the reader is referred for an accurate description. It chiefly consists of a shank turned up at the end likeFig. 161, the outside, at B, being rounded to fit hollow gouges such asFig. 161H; against this the hollow of the gouge is laid, and opposite to it, at C, a small piece like D. A band or hoop, E, is now placed over both the above, and between the two a wedge-shaped piece, F, which is intended to bring a strain upon the hoop and tighten it round the gouge. This last piece is attached to the shank or holder by a screw passing through it into the shank. The tighter this screw is worked the lower the central wedge is drawn down, and the tighter the hoop is made to embrace the tool. This holder is also modified to suit flat chisels. The tool cannot possibly slip, but can be released in a few seconds if desired. Such a termination of two, three, or four arms revolving on a spindle in place of the boss would form the best possible circular cutter for shaping lathes. The above lathe for producing Elizabethan twist introduces the reader to self-acting, screw-cutting, and machine lathes, such as are used in all large manufactories. Hand-turning, indeed, except in such light work as turning up the heads of small bolts, and finishing up work which, from peculiarities of form, cannot easily be done by self-acting tools, has become a thing of the past in factories of any pretensions; hand labour, in fact, not only no longer pays, but is quite insufficient to meet the requirements of the present age. Take, for example, a piston-rod requiring to be as "true as a hair," to use a common expression, from end to end. The traverse of an ordinary slide-rest would only enable us to turn a short length at a time, and the result, when accomplished, would not be satisfactory. With a self-acting lathe the tool traverses in a perfectly straight line from end to end, is returned to its starting-point by a quick traverse, and the movement repeated until the proper dimensions are attained. The process is not absolutely rapid, because time is requisite in cutting iron and steel, but the work is executed as speedily as the nature of the metal to be cut will allow, and the execution is perfect. Of late, however, even the above has been improved upon, for two cutters are used at once—one on each side of the bar, so that by one traverse of the rest a cut of double depth is taken, and the tendency of the work to spring away from one tool is counteracted by the operation of the other. But we require something more than speed in the present day. We must have work of absolute truth of measurement. What would our ancestors, or the immortal Watt himself, think of measuring work to the hundredth part of an inch, yet it can be and is done to the thousandth part. I believe I am correct in saying that Whitworth constantly gauges work to that or even a higher degree of nicety. It is not too much to assert that the best engines of the present day really work with the precision of clockwork, and even the bore of an Armstrong or Whitworth gun is executed with no less accuracy and precision. Look again at that ponderous affair, the steam hammer, so ponderous as to require a depth of solid masonry and timbers to sustain the force of its terrific blows. In a few minutes a solid mass of metal is reduced to a flat plate such as would have taken the united strength of a dozen men wielding the heaviest sledge-hammers for an hour at least. The ground beneath the feet of the spectator trembles at every blow of the machine. The work is done, and behold! with a touch the same ponderous concern becomes a nutcracker, not even injuring the kernel when it breaks the shell. Such is one of a hundred specimens of accurate workmanship carrying out in practice the clever designs of the mechanical engineer. Sweep away self-acting machinery, and such work would become a simple impossibility. Again, so long as turning, boring, planing, and suchwork was performed by hand alone (even after the introduction of the slide principle), an attendant was required at each machine. Now that the latter is contrived to regulate its own movements, one man at two or three lathes is sufficient. Thus the same article that was once imperfect and costly, owing to the demand on skilled labour, which was difficult to procure and at best inefficient when procured, has now become cheap, and to all intents and purposes perfect; and although the demand for such work increases year by year, self-acting machinery being constantly improved and simplified, enables the manufacturer to keep pace with the demand.
[13]The spiral chuck for fine work in ivory and costly woods is described in a later page.
[13]The spiral chuck for fine work in ivory and costly woods is described in a later page.
Fig. 159.
Fig. 159.
Figs. 160, 161.
Figs. 160, 161.
The advantages of self-acting machinery are of course chiefly confined to the trade, and it is not often that the mere amateur requires such aid. Indeed, the expense necessarily attendant on the fabrication of these machines deters the great majority from making such a purchase.
The first requisite for fitting up a lathe for screw-cutting and plain turning is the fitting a guide-screw, and adding a saddle to the slide-rest. But it must be observed that the ordinary mandrel of small foot-lathes, which works in a collar and back centre, is not convertible, and must be replaced by one working through two collars, so that a part may project at the opposite end to that intended for the chucks. On this projection various cogwheels have to be fitted. Now there are divers patterns of mandrel suitable for the above purpose; some are perfectly cylindrical, some have one conical part, some are fitted with a second cone, independent of the mandrel, and which slides upon it, and can be adjusted by means of regulating nuts. This latter being a good pattern, when well constructed, we shall first describe in detail. InFig. 162is shown A, the mandrel complete, with fittings in section, but without any of the change wheels. The coneais forged on the mandrel, which then becomes cylindrical. The cone is figured too large in proportion to size of mandrel; the latter should be represented as much more substantial. Atbthe second cone slides on, the latter shown again at B. It is bored truly, and a slot cut to fit a feather on the mandrel. Thus it will slide along it, but must necessarily turn with it. Beyond D on the left are two screwed parts, one slightly larger than the other, one being made with a left-handed, the other witha right-handed thread. On these screw the two nuts,d, e, which drive the movable cone towards the right, causing it to fit more tightly into its collar, and also by the same movement tightening the fixed cone in its bearings. The mandrel is thus readily adjustable in the collars, and can be made to run very truly and easily without shake or endlong movement. Any pressure, however, against the front, such as would be caused by drilling, would jam the front cone in its collar, and tend to loosen the other. This is counteracted by the partf, g, with its screw,h, against which the end of the mandrel bears. That this form requires good workmanship is evident, for there are three points, or bearing surfaces, to be brought into a correct line, and the slightest deviation will cause the mandrel to jam in some part of its revolution. The nuts screw up in opposite directions to counteract the tendency in either to screw up more tightly, or to become looser by the revolution of the mandrel. On the whole the above is a good form of mandrel; if it has a fault, it is a slight tendency to work heavily. The next form is that of the ordinary mandrel, with single cone, but a second collar is added, which is cylindrical, through which the mandrel passes, and it then abuts on the sustaining screw as before. This is shown inFig. 163, with the addition of the wheel, which is to be connected by intermediate wheels and pinions with the leading screw. If the conical collar is replaced by a cylindrical one, so that two similar bearings are made use of, a shoulder becomes necessary to prevent the mandrel from slipping endwise. The collars must also be split as inFig. 164, so that they can be tightened up as they become worn. Either of the above forms of mandrel can be used; each has its advocates, and not unfrequently all may be found in different machines in the same manufactory. The bed of a self-acting lathe requires to be accurately surfaced, and formed by the planing machine with twoV's or edges bevelled underneath, as165b, b. The saddle of which we have spoken is a flat plate of cast iron fitted withVpieces to match the bevelled edges of the lathe bed, along which it slides truly, its under-edge being planed.To this plate the slide rest is attached securely, either turning when required on a central pin, and being clamped at any desired angle, as before stated when treating of the compound slide rest, or, when this movement is preferred to be given to the upper slide, fixed permanently by nuts and screws. The saddle is represented detached inFig. 166. The principle of the self-acting lathe is very simple. Motion is given to the screw by means of cog wheels geared with the mandrel, a nut fitting the screw is attached to a hanging bracket of the saddle, and this with the rest is thereby carried along the bed. It is necessary, however, to add some contrivance for instantly throwing the screw out of gear, without the necessity of stopping the lathe itself. There are many ways of effecting this, the most common being the use of a split nut, which embraces the leading screw when the two halves are brought together upon it, but which is instantly freed by separating them through the action of levers and cams, or other simple mechanical contrivance. InFig. 167,ais the bottom of the saddle from which depend the brackets E, E. The nut B, B, which is divided across the middle, slides up and down between these brackets, D being the leading screw which they embrace when closed. The movement of the halves of the nut is effected by the leverc, in the form of the letter T moving on a centre pin at K, and having two links, D, D, attached to the halves of the nut at one end, and to the ends of the cross lever at the other. A connecting bar pivoted to the part, L, is attached to a lever and handle, by which motion is communicated to the lever. When this rod is moved in the direction of the arrow the links will cause the nut to close andvice versa. The next form,168 and 169, represents the split nut attached to two arms, A, A, hinged together at E. B, B, are slots in which work the pins attached to the cross head of the levers C, C. A heavy knob of iron keeps the latter in the position to which it may be moved. In168the pins keep the arms and nuts apart. When the lever is thrown over, as in169, the nut is securely closed and held in gear. This form requires to be fixed to the lower part of a bracket attached to the saddle of the slide rest, such as is shown at B, C,Fig. 170. In this figure, A is the bottom of the saddle E, E, theVpiece, that on the left, having an adjusting screw to tighten it on the lathe bed when necessary. AtF is a projecting piece of the saddle fitting accurately between the lathe bed, and kept down by a screw with bed plate underneath. This serves to steady the movement of the saddle and relieve the pressure and strain upon theVpieces. The bed-plate may be cut out to fit the lower part of the bed on which it slides, both being planed for the purpose, as shown at H.