Chapter VIII: Special Forms of the Lathe (3)

Let Fig. 722 represent a lathe of this kind provided with a frame A affording journal bearing to the shaft of the drum B, which is driven by the pulleys C. Let E represent a pulley receiving motion from B by the belt D. The cutting tools are carried by the head F which is rotated by pulley E. Let the carriage or frame carrying the shaft of E carry a dull pointed tracer, with continuous contact with the _former_ H by means of a weight or spring, the carriage being so connected to the way N on which it traverses that it is capable of rocking motion, and if H be rotated the carriage will, by reason of the tracing point, have a motion (at a right angle to the axis of H) that will be governed by the shape of H; hence since G rotates equally with H, the form of the blank work G will be similar to that of H, but modified by reason of the tracing point being at a greater distance than F from the centre of rocking motion.

All that is necessary to render this motion positive throughout the lengths of G and H is to connect them together by gears of equal diameter, and traverse the carriage along N for the full length of the pieces. But the effect will be precisely the same if the frame carrying G and H be pivoted below, capable of a rocking motion, and H be kept against the tracing point by means of a spring or weight, in which case the carriage may travel in a straight line upon N and without any rocking motion. This would permit of the carriage operating in a slide way on N enabling it to traverse more steadily.

To maintain continuous contact between the tracing point and the _former_ H, the rotations of H are slow, the necessary rapidity of tool cutting action being obtained by means of the rapid rotation of the head and cutters F.

Since motion from the line shaft to the machine is communicated at C it is obvious that the gears or devices for giving motion to H and G may be conveniently derived from the shaft carrying C and B, for which purpose it extends beyond the frame at one end as shown. Lathes of this kind are made in various forms, but the principles of action in all are based upon the principles above described.

BACK KNIFE GAUGE LATHE.--This lathe, Fig. 723, has a carriage similar to that described with reference to Fig. 718, and carries similar tools upon the tailstock. It is further provided, however, with a self-acting feed traverse to the carriage, and by means of a rope and a weight, with a rapid carriage feed back or from left to right on the bed, and also with a knife at the back. This knife stands, as seen in the engraving, at an angle, and is carried (by means of an arm at each end) on a pivoted shaft that can be revolved by the vertical handle shown. The purpose of this knife is first to shape the work and then to steady and polish the wood or work. Obviously when the knife is brought over upon the work its cutting edge meets it at an angle and cuts it to size and to shape; the surface behind the cutting edge having no clearance rubs against the work, thus steadying it while polishing it at the same time. These lathes are used for turning the parts of chairs, balusters, and other parts of household furniture, the beads or other curves or members being produced on the work by suitably shaped knives, which obviously cut the work to equal shape and length as well as diameter, and it is from this qualification that the term "gauge" is applied to it.

Fig. 724 represents the Niles Tool Works special pulley turning lathe, in which motion from the cone spindle to the live spindle is conveyed by means of a worm on the cone spindle and a worm-wheel on the live spindle. Two compound slide rests are provided, the tool on the rear one being turned upside down as shown. These rests may be operated singly or simultaneously, and by hand or by a self-acting motion provided as follows:--A screw running parallel to the cone spindle is driven by suitable gearing from the cone spindle. At each end of this screw it gears into a worm-wheel having journal bearing on the end of the slide rest feed screw as shown. By a small hand wheel on the end of the slide rest feed screw the worm-wheel may be caused to impart motion to the feed screw by friction causing the slide rest to feed. But releasing this hand wheel or circular nut releases its grip upon the feed screw, and permits of its being operated by the handle provided at the other end. The rail carrying the slide rest is adjustable in and out to suit varying diameters of pulleys, being secured in its adjusted position by the bolts shown.

The cut is put on by means of the upper part of the compound rest. To turn a crowning pulley the rails carrying the slide rests are set at an angle, the graduations shown on the edge of the ways to which they are bolted being to determine the degree of angle. When the pulley surface of the pulley is to be "straight" both tools may commence to operate on one edge of the pulley surface, the advance tool taking a roughing and the follower tool a finishing cut; but for crowning pulleys the tools may start from opposite edges of the pulley, the cuts meeting at the middle of the face; hence the angles at which the respective rails are set will be in opposite directions.

The pulleys to be turned are placed upon mandrels and driven by two arms engaging opposite arms of the pulley. To drive both arms with an equal pressure, as is necessary to produce work cylindrically true, an equalizing driver on Clements' principle (which is explained in Fig. 756, and its accompanying remarks) is employed.

For driving the pulleys to polish them after they are turned the cone spindle is hollow at the rear end and receives a mandrel. The high speed at which the cone spindle runs renders this possible, which would not be the case if wheels and pinions, instead of worm-gear, were employed to communicate motion from the cone to the live spindle. A wheel shown in position for polishing is exhibited in the cut, the pivoted arm in front affording a rest for the polishing stick or lever.

BORING AND TURNING MILLS.--The boring and turning mill patented in England by Bodmer in 1839, has developed into its present improved form in the United States, being but little known in other countries. It possesses great advantages over the lathe for some kinds of turning and boring, as wheels, pulleys, &c.

The principal advantages of its form of construction are:--

1st. That its work table is supported by the bed at its perimeter as well as at its centre, whereas in a lathe the weight of the chuck plate as well as that of the work overhangs a journal of comparatively small diameter, and is therefore more subject to spring or deflection and vibration.

2nd. It will carry two slide rests more readily adjustable to an angle, and more readily operated simultaneously, than a lathe slide rest.

3rd. It is much more easy to chuck work on a boring mill table than on a lathe, because on the former the work is more readily placed upon the table, and rests upon the table, so that in wedging up or setting any part of the circumference of the work to the work table, there is no liability to move the work beneath the other holding plates; whereas in a lathe the work standing vertical is apt when moving or setting one part to become unset at other points, and furthermore requires to be held and steadied while first being gripped by the chucking dogs, plates, or other holding devices.

Figs. 725, 726, 727, 728, and 729 represent the design of the Niles Tool Works (of Hamilton, Ohio), boring and turning mill. In this design provision is made to raise the table so that it takes its bearing at the centre spindle only when used upon small work where a quick speed of rotation is necessary, or it may be lowered so as to take its circumferential bearing for large heavy work where slower speeds and greater pressure are to be sustained.

The bearing surfaces are, in either case, protected from dust, &c., and provided with ample means of lubrication. Each tool bar is so balanced that the strain due to the balancing weights is in a line parallel to the bar axis in whatever position and at whatever angle to the work table the bar may be set. This prevents the friction that is induced between the bar and its bearings when the balancing strain is at an angle to the bar axis, and consequently pulls the bar to one side of or in a line to twist the bar. The bar is therefore more easily operated, and the feed gear is therefore correspondingly relieved of strain and wear.

The general construction of the machine is shown in Fig. 725. It consists of a base or bed, affording journal bearing and support to a horizontal work table, rotated by devices carried upon the bed. To each side of the bed are attached uprights or standards, forming a rigid support to a cross slide or rail for the two sliding heads carrying the tool bars.

The various motions of the machine are as follows: There are 16 speeds of work table, 8 with the single, and the same with the back gear. The cross slide is capable of being raised or lowered, to suit the height of the work, by an automatic motion. Both tool rests are capable of hand or automatic feed motion at various rates of speed, in a line parallel to the surface of the work table. Both are also capable of automatic or hand feed motion, either vertically or at any required angle to the work table, and have a quick return motion for raising them, while each may be firmly locked while taking radial or surfacing cuts, thus preventing spring or vibration to the tool bar. In addition to this, however, there is provided, when required, a tailstock, carrying a dead centre after the manner of a lathe, so that the work may be steadied from above as well as by the work table. In Figs. 726 and 727 are shown the devices for raising the work table and those for actuating the feed screws and the feed rod; thus operating the sliding heads horizontally and the tool bars vertically. A is the base or bed supporting the work carrying table B´, and affording its spindle journal bearing at D´. A step within and at the foot of D´ rests upon the wedge F´ so that when the wedge is caused to pass within D´ it lifts the step, which in turn lifts the table spindle, and hence the table, sufficiently to relieve its contact with the outer diameter of the bed. F´ is operated as follows: The lever G´ is pivoted at E´ and carries at its upper end a nut H´, operated by a screw on the end of the bolt I´; hence rotating I´, operates wedge F´.

For operating the automatic feed motions, _f_ is a disc upon a shaft that is rotated by suitable gears beneath the work table; _g_ is a disc composed of two plates, having a leather disc between them, the perimeter of the disc having sufficient frictional contact with _f_ to cause _g_ to rotate when _f_ does so: _g_ drives the vertical spindle _i_, which has a worm at J´ driving a worm-wheel which rotates the gears upon the feed spindles V, F, W, in the figures; _f_ rotates in a continuous direction, but the spindle _i_ is caused to rotate in either direction, according to whether it has contact with the top or bottom of the face of _f_, it being obvious that the motion of _f_ above its centre is in the opposite direction to that below its centre of rotation. The means of raising and lowering _g_ to effect this reversal of rotative direction is as follows: It is carried on a sleeve _g´_ which is provided with a rack operated by a pinion that is rotated by means of hand wheel _h_; hence, operating _h_ raises or lowers _g´_, and therefore _g_; _h´_ is a hand wheel for locking the pinion, and hence detaining the rack (and therefore _g_) in its adjusted position. This design is an excellent example of advanced American practice for obtaining a variable rate of feed motion in either direction, it being obvious that _g_, being driven by the radial face of _f_, its speed of rotation will be greater according as it is nearer to the perimeter of _f_ and less as it approaches the centre of _f_, at which point the rotary motion of _g_ would cease. Here, then, we have a simple device, by means of which the direction and rate of feed may be governed at will with the mechanism under continuous motion, and conveniently situated for the operator, without his requiring to move from the position he naturally occupies when working the machine.

The means of raising or lowering the height of the rail R on the side standards Z are as follows: K is a pulley driven by belt from the countershaft and operating pinion _l_, which operates pinion _n_, driving _m_. O is a gear on the shaft driving the pinions _p_, _p_, which operate the gears _q_, _q_, on the vertical screws which engage with nuts attached to R; _m_ and _n_ are carried on a bell-crank _r_ pivoted on the shaft of pulley K. Pinion _n_ is always in gear with pinion _l_, and pinion _m_ is always in gear with pinion _n_ (and not with pinion _l_). With the bell-crank in one position, motion passes from _l_ to _n_ and to O; but with it in the other position, motion passes from _l_ to _n_, thence to _m_, and from it to O. The motion of _m_, therefore, is always in a direction opposite to that of _n_; hence O, and gears _p_ and _q_, may be operated in either direction by regulating which of the two gears _n_, _m_ shall drive O, and this is accomplished as follows: The bell-crank _r_ is connected by an arm to rod _s_, and the latter is connected by a strap to an eccentric _t_, operated by the handle shown. When this handle stands horizontally, both _m_ and _n_ are disengaged from pinion O; but if the handle be raised, rod _s_ is raised, and _m_ is brought into gear with O. If, however, it be lowered from the horizontal position, _n_ is brought into gear with O, and _m_ becomes an idle wheel.

There are two feed screws--one for operating each boring bar-head, and a spindle for operating the vertical feeds of the bars in the sliding heads. Fig. 728 shows the arrangement for engaging and disengaging the feed nuts of these heads. A is the slide that traverses the rail. It carries a nut made in two halves, N and N´, which are carried in a guide or slide-way, and which open from or close upon the screw F when the handle O is operated in the necessary direction. Each half of the nut is provided with a pin projecting into eccentric slots _x_ in the face of a pivoted plate (shown dotted in), to which the handle O is attached. W, W represent bearings for the vertical feed spindle W in Fig. 726. _a_ is the annular groove for the bolts _b_ in Fig. 729.

Fig. 730.

Fig. 731.]

For a quick hand traverse for the head the ratchet, P is provided, operating a pinion _s_, which engages with a rack T, running along the underneath side of the cross-rail R. To adjust the fit of A to the rail the gibs _y_ and _y´_ and the wedge _x_ are employed.

Fig. 729 represents the automatic feed motion within the head for operating the tool bars vertically. R is the cross rail on which slides A carrying B, and permitting it to swivel at any angle by means of bolts _b_, whose heads pass within an annular groove, _a_ in A. In B is carried the boring bar G, having the rack shown. P is a pinion to operate the rack. W is the feed-rod driving the worm H, which drives the worm-wheel I. This worm-wheel is provided with a coned recess, into which the friction plate C fits, so that when the two are forced together rotary motion from I is communicated to C, and thence to C´ (which is a sleeve upon C), where it drives pinion P by means of pin P´. _i_ rotates upon and is supported by the stud J, which is threaded into C^{2} (the latter being also a continuation of C); hence when hand-wheel K is operated in one direction, C^{2} acting as a nut causes J to clamp I to C, and the tool bar to therefore feed. Conversely, when K is operated in the opposite direction, I is released from C, and may, therefore, rotate while C remains at rest. For feeding the tool bar G by hand, or for moving it rapidly, the hand-wheel M is provided, being fast to the sleeve at its section C^{2}, and, therefore, capable of rotating pinion P. D affords journal bearing to C at its section C´. The chain from the weights which counterbalance the bars G pass over sheaves which are fixed to the piece B in which the bar slides, so that they occupy the same position with relation to the axis of the bar at whatever angle the latter may be set, and thus the counterbalancing weight is delivered upon the bar in a line parallel to its axis. As an example of the efficiency of the machine, it may be mentioned that at the Buckeye Engine Co.'s Works, at Salem, Ohio, a pulley 12 feet in diameter, weighing 8860 pounds, and having a 27-inch face, was bored and turned on one of these machines in 17 hours, taking three cuts across the face, turning the edge of the rim facing off the hub and recessing the bore in the middle of its length for a distance of several inches, the bore being in all 18 inches deep. The machine is made in different sizes, and with some slight variations in each, but the main features of the design, as clearly shown in our engravings, are common to all sizes.

Fig. 730 represents a lathe for turning chilled rolls such as are used for paper calendering machines, and is constructed by the J. Morton Poole Company of Wilmington, Delaware.

In the figure a roll is shown in position in the lathe. The journals of the rolls are first turned in a separate lathe, and form the guide by which the body of the roll is turned in the lathe shown in the figure. The lathe consists of a bed plate P, at one end of which is mounted the driving head. Upon this bed plate are also mounted three standards or vertical frames, to the two end ones of which are pivoted the binder arms shown. These frames hold the bushes at L and N, in which the journals of the roll revolve. They also carry the bar G, secured to the arm W of the frame by clamps _a_, _a_, _a_. Upon the bar G are two slide rests, consisting of a tool rest E, a tool clamp A, and a feed yoke B, which is screwed up by a wrench applied to the nuts as shown on the right-hand tool rest in the figure. The binder arm is adjusted to hold the bushings L N (which are varied to suit the size of the roll journal) a fair working fit upon the roll journals, the bolts S holding the binder arms firmly against the enormous pressure due to the cut. It is obvious that the frames W may be adjusted anywhere along the bed plate P to suit the length of roll to be turned, and that the slide rests may be moved to any required position along the bar G. Further details of the construction are as follows. Fig. 731 is an end, and Fig. 732 is a top view of the tool rest; A is the tool clamp securing the tool to the rest E, R representing a section of the roll, B is the feed yoke, which to put on a cut is screwed inwards by operating the nuts D. The pins C are fast in B, and their ends abut against the tool, which is fed in under the full pressure of the clamp A. The tool is shown at F in figure, and also at F in Fig. 733, which is a view of the rest with the clamp A removed. The form of tool employed is shown in Fig. 734, its length varying from five to six inches. As the tool feeds in and does not traverse along the roll it is obvious that it cuts along its entire length, the cuttings coming off like a bundle of fine ragged needles.

When the tool has been fed in cutting the roll to the required diameter the rest is moved along the bar G, a distance equal to the length of the tool, and the operation is repeated until the full length of the roll has been turned. It is obvious that to feed the tool in parallel, both nuts D of the tool rest are operated. The tool is held as close in to the rest as the depth of cut to be taken will permit, and is used at a cutting speed varying from about 2-1/3 feet to 5 feet per minute according to the hardness of the roll. The tool has four cutting edges, and each cutting edge will carry in at least one cut, and may sometimes be used for a second one. The tools are used dry and the amount of clearance is just sufficient to clear the roll and no more.

The rolls are driven by a socket bolted to the lathe face plate, and containing a square hole, in which fits loosely the square end of the roll. The object of this arrangement is to permit the roll to be guided entirely by the bearings in which it rotates, uninfluenced by the guiding effect that accompanies the use of centres in the ordinary method of turning.

Fig. 735 represents a lathe designed and constructed by the American Tool and Machine Company, of Boston, Mass. This class of lathe is strictly of American origin, and has become the most important tool in the brass finishing shop.

In its design the following advantages are obtained:--

1st. The front of the lathe is entirely unobstructed by the ordinary lathe carriage and slide rest, hence the work may be more easily chucked and examined, while in the case of work requiring to be ground together, while one part is in the chuck, the trouble of moving the slide rest out of the way is entirely obviated.

2nd. In place of the single cutting tool carried in a slide rest and of the tailstock of the ordinary lathe, there is provided, what is known as a turret, or turret rest, carrying 6 tools, each of which can be successively brought into action upon the work by the simple motion of a lever or handle.

3rd. The rest for traversing single pointed screw cutting tools or chasers (for internal threads) is at the back of the lathe where it is out of the way.

4th. In place of the usual change wheels required to operate the lead screw, the chasing bar is operated by a single threaded collar or hob, which is more easy of application and removal.

5th. The slide rest carrying the screw cutting tool is capable of such adjustment, that the tool will thread successive pieces of duplicate work to an exactly equal diameter, so as to obviate the necessity of either measuring or trying the work after the tool has been accurately set for the first piece.

6th. When the threading tool has traversed to the end of its cut it may be lifted from the same and pulled back by hand, ready to take a second cut, thus avoiding the loss of time involved in traversing it back by a lead screw or its equivalent.

7th. Each of the tools in the turret may be set so as to operate to an equal depth and diameter upon successive pieces of work.

In the particular lathe shown in our example, there is another and special advantage as follows:--

In lathes operating upon small work and upon the softer metals, as composition, brass, &c., the time occupied in traversing the cutting tool is comparatively short, and from the comparative softness of the metal the speed of lathe rotation is quick, and the tool motions must be correspondingly quick. In addition to this the work being so much more quickly performed, changes and readjustments of the parts are necessarily more frequent, hence the rests traverse the bed more rapidly as well as more frequently and the wear of the [V]s on the lathe, and the corresponding [V]-grooves in the tool rest, slide rest, or turret, is increased; as a result, tools carried in the tailstock or the turret, as the case may be, which tools should for a great many purposes stand axially true with the live spindle, stand below it, and hence instead of boring a hole equal to their own diameter, bore one of larger diameter. In the case of tools, however, which, as in the case of drills, endeavour to find their own centre in the work, this action takes place to some extent as the tool enters the work, and as a result the hole is made a taper, whose largest diameter is at the mouth. This induces another evil in that it dulls the advance edge of the drill flute, and wears away the clearance which is of such vital importance to the free action of the drill.

The manner in which these advantages are obtained is as follows:--

In place of the ordinary tailstock a back head is provided which has a cross slide operating after the manner of the ordinary slide rest; this carries an upper slide, thus forming a compound slide rest. On the top of this rest is carried a rotating head or turret head, serving the same purpose as the head shown in Fig. 694, and carrying a series of tool holders. These tool holders may be operated by the feed screw of the compound rest, or may be operated by the hand lever shown standing horizontally. In addition to the ordinary back gear for reducing the live spindle speed there is provided on the live spindle a second small pinion, driving at the back of the lathe head a shaft, on the left-hand end of which is a seat for collars or hobs, operating a bar running along the back of the lathe, and forming what is termed the screw apparatus, whose operation is as follows:--

This bar carries the slide rest shown, a handle or lever for partly rotating the slide rest, spanning the bed of the lathe. When this handle is lifted, the bar at the back of the lathe rotates in its journals. On this bar is an arm which carries a segment of a circle, containing a thread corresponding in pitch to the thread on the collar or hob. When the lever is raised the segment moves away from the hob, and the bar may be moved laterally by hand, but when the lever is lowered the arm falls, and the segment comes into contact with the hob thread, which therefore feeds the bar; all that is necessary for thread cutting is, therefore, to place on the lathe a hob having the required pitch for the thread to be cut, and place in the slide rest a chaser or single-pointed threading tool, and set the tool to the work by means of the slide rest, depressing the lever to cause the tool to feed forward, and elevating it to move the bar back by a lateral hand pressure. To put on successive cuts the slide rest is operated, the lever always being lowered till it meets the surface of the lathe bed. To cause the slide rest to cut successive threads to the same diameter, a suitable stop motion is provided to the slide rest, and when the rest has been operated as far as the stop will permit it, the thread is cut to the required depth and diameter.

A stop motion is also provided to the lateral motion of the turret, so that the tools being set to enter the work to their respectively required distances, all pieces will be turned to equal depths or lengths.

To enable the centres of the tool holders to maintain true alignment with the live spindle, notwithstanding the wear of the lathe bed and back head, the bed is made in two parts. One of them carries the headstock, and on the vertical face of this part is a slide in which the end of the second part fits, so that by means of adjusting screws the second part may be elevated to effect the true alignment when necessary.

Fig. 736 represents a square arbor brass-finisher's lathe. The object of the square arbor or tail spindle is to enable it to carry cutting tools in place of the dead centre. A cross slide is provided to the tailstock, and upon this slide the head of the tailstock is pivoted so as to bore taper holes; the tailstock thus virtually becomes a compound slide rest. This lathe is provided at the back of the bed with a bar carrying a slide rest, operated in the same way and for the same purpose as that described with reference to Fig. 735. Both these lathes are furnished with separate compound slide rests, and with a hand rest.

When work of considerable weight requires to be bored with holes of moderate diameter, it is more convenient that it remain fixed upon a table, and that the boring tools rotate, and a machine constructed by the Ames Manufacturing Company for this purpose is shown in Fig. 737; a standard occupies the position of the ordinary tailstock. It carries an horizontal table, or angle plate, on which the work may be chucked. This table is capable of a vertical and a cross shear movement, so that when the work is chucked upon it, holes whose axes are parallel, but situated in different locations upon the same surface, may be drilled or bored by so moving the table as to bring each successive hole into line with the live spindle. The feed motions are as follows:--

At the back of the smallest step on the cone and fast on the cone spindle is a gear-wheel gearing into a pinion, which drives the lower shaft shown behind the back bearing, and on this shaft are two pinions. One drives the upper feed cone, shown at the back of the back bearing, which cone connects by belt to the feed cone below, which operates a traverse feed for the work table; the other drives the tool holding spindle which passes through the cone spindle. This tool holding or driving spindle is threaded at its back end, passing through a nut which causes it to self-feed from left to right, or in other words, towards the work table. To throw this feed out of operation the pinion on the end of the lower or feed driving spindle is moved laterally out of gear with the pinion driving it.

To provide a quick hand-feed traverse the shaft or spindle, shown with a hand-wheel, is provided, being connected to the tool driving spindle by gearing.

When employed to operate a boring bar, a bearing to support the bar at the tail or footstock end may be bolted to the table, such bearing carrying a bushing which may be changed to suit the diameter of the boring bar.

Fig. 738 represents a cylinder boring lathe. D is the driving cone, on whose shaft is the worm W, driving the worm-wheel G, which is fast upon the boring bar _g_, having journal bearing in the standards H and H´, the latter of which must be moved out of the way to get the work over the bar. _h_ is a head provided with slots to carry the cutting tools; _h_ is a close sliding fit to the bar _g_, and is traversed along _g_ as follows:--_g_ is hollow and there passes through it a feed screw, which operates a nut on _h_, which nut passes through a longitudinal opening in the bar _g_. At the end of this feed screw is the gear-wheel D. Now fast upon the end of _g_, and therefore rotating with it, is the gear A, driving gear B, which is fast on the same sleeve as C, which it therefore drives; C drives D. The diameter of A is less than that of B, while that of C is less than that of D; hence the rotation of D is slower than that of A, and the difference in the relative velocities of D and A causes the feed screw to rotate upon its axis and feed the head _h_ along the bar. If C be placed out of gear with D, the feed screw (and hence the head H) may be operated by the handle E.

There are several objections to this form of machine, as will be seen when comparison is made with Fig. 739, which represents a special cylinder boring lathe, designed and constructed by William Sellers and Co., of Philadelphia, Pennsylvania. The boring bar is here supported in two heads, and is hollow, the feed screw for traversing the head carrying the boring cutters being within the bar. The feed is effected through the medium of the train of gearing shown at the end. The two face plates shown which drive the boring bar, also carry two slide rests which are used to face off the ends of cylinders while the boring bar is in operation, these slide rests being operated by a star feed, acting on the principle described with reference to Fig. 589. The boring bar in this case being driven from each side of the work the torsion due to the strain of the cut is divided between the two halves of the bar; or in other words, when a boring bar is driven from one end the strain due to the cut falls upon that part of the bar that lies between the boring-head and the point at which the bar is driven; but when the bar is driven from each end then the strain is divided between the two ends, causing a bar of a given strength to operate more steadily and take a heavier cut for roughing, and a smoother one for finishing. A greater advantage, however, is that it gives to the bar a rigidity, enabling it to carry a cutter having a long cutting edge without chattering, thus allowing a very coarse finishing feed, which will finish a bore with less wear to the tool edge (and therefore more parallel) because for a given amount of work the cutting-edge is under duty for a less period of time, the cutting speed remaining the same, or even slower than would be desirable for a fine feed. The driving-cone, which is shown to be below the boring-bar, is so situated to accomplish two objects, which are to operate the two face plates by a shaft having two pinions (within the bed) gearing with the circumferential teeth on the face plates, and to operate at the same time the table (shown on the bed between the face-plates) to which the cylinder is bolted.

In a boring machine it is of the utmost consequence that the bar shall be as free from vibration as possible, while lost motion, or looseness from wear, is especially to be avoided. By carrying the bar in two bearings, as it were, the wear is greatly reduced.

The duty of facing the cylinder ends is sometimes done by facing cutters carried in the head. Such cutters, however, must have a cutting edge equal to the breadth of the surface faced by them, because the cutter cannot be fed radially to its cut. Furthermore, the cut is carried by the bar at a considerable leverage, and as a result it is very difficult indeed to make the radial faces true or even nearly true, the cutter dipping into the softer parts of the iron or into spongy places if there are any. In any event springing away from its cut, resisting it until forced to cut, and then cutting deeper than should be, so that on a finished surface it is often apparent to the eye where the cutter began and left off. When, however, the radial faces are operated upon by a slide rest, as in the Sellers machine, the tool is more firmly held, and may be fed radially to the cut, producing true faces, and saving a great deal of time in making the cylinder cover joints, as well as in the boring and facing operations.

Fig. 740 represents a double boring and facing lathe by G. A. Gray, Junior, of Cincinnati, Ohio. Two driving heads are provided, each having a main spindle, but holding the boring bar after the manner of an ordinary lathe, and within each spindle is another capable of longitudinal traverse. The main spindle is provided with a head corresponding to a slide rest and carrying a cutting tool for facing purposes, the feed being obtained by means of a _star-feed_. The work is bolted to the carriage and fed to the cut for boring purposes. It is provided with an automatic feed and also with hand feed. When facing is to be done the carriage may be firmly locked to the lathe shears.

In boring and facing a steam pump centre, or other similar piece, the casting is fastened to the carriage in a special fixture. The carriage is then moved so that the work will come nearly in contact with tool in the fast head, the loose head is moved up to the work, and both the carriage and loose head are clamped.

Both ends of the casting may be operated upon at the same time or separately, as occasion requires, the object being, however, to work upon as many places at one time as the nature of the work will permit; this being the main point in the economical performance of work. It is evident also that if the machine is true, and the piece is finished at one setting, the work will be true.

In the detail engravings, Fig. 741 represents boring, tapping, and facing steam pump centres, in which operations the carriage is locked.

Fig. 742 illustrates the manner of boring and facing cylinders and similar pieces, the loose head stock being used as a tailstock and the fast headstock as the driver. The facing is done either before or after the boring, all the work obviously being done at one chucking.

Fig. 743 shows a longitudinal cross section of the headstocks showing the main and the internal spindles.

Fig. 744 represents a lathe constructed by the Defiance Machine Works for turning the hubs for carriage and wagon wheels.

The blank from which the hub is turned is driven by a mandrel having a square stem fitting in the live or driving-spindle, this mandrel being supported at the other end by the ordinary dead centre operated by the upper hand-wheel. The bed is provided (between the driving-spindle and tailstock) with the usual raised [V]s on which rests a carriage carrying a cross slide. This cross slide carries, at the back of the lathe, a head or stock containing the roughing-knives, and at the front a table carrying the finishing-knives, hence, by operating the large hand-wheel (which gives transverse motion to the cross slide) in one direction the roughing-knives are brought into operation, while by operating it in the opposite direction the finishing-knives are brought into operation (the roughing-knives receding). By suitable stops, the motion of the roughing and finishing-knives respectively are arrested when those knives have cut the blanks to the desired diameter, the finishing-knives shaping the work correctly by reason of their form of outline. Upon the same cross slide are the equalizing-knives, one on each side of the front table. These knives operate simultaneously with the finishing-knives, cutting the hubs to uniform length. Thus the hubs are cut to exact uniformity of diameter, shape and length, by simply operating the large hand-wheel first in one direction and then in the other.

If it be required to cup the hubs, as in the case of standard wagon hubs, suitable cutters carried in a bar (having sliding motion in a guide way on the tailstock) are caused to do such cupping, the cupper-bar being operated by the left-hand lever.

The live, or driving, spindle is started and stopped by a tight and loose pulley, the belt being passed from one to the other by means of the lever on the right, which simultaneously operates a brake attached to the belt stopper, operating upon the tight pulley. By this means the lathe can be started and stopped more quickly than would be the case with a cone pulley, whose extra weight and inertia would take time to overcome.