Modern Machine-Shop Practice Part 56

[Ill.u.s.tration: Fig. 734.]

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.

[Ill.u.s.tration: Fig. 735.]

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, bra.s.s, &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 bra.s.s-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.

[Ill.u.s.tration: Fig. 736.]

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:--

[Ill.u.s.tration: Fig. 737.]

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 pa.s.ses through the cone spindle. This tool holding or driving spindle is threaded at its back end, pa.s.sing 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 bus.h.i.+ng which may be changed to suit the diameter of the boring bar.

[Ill.u.s.tration: Fig. 738.]

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 pa.s.ses through it a feed screw, which operates a nut on _h_, which nut pa.s.ses 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.

[Ill.u.s.tration: Fig. 739.]

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 finis.h.i.+ng. 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 coa.r.s.e finis.h.i.+ng 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 circ.u.mferential 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.

[Ill.u.s.tration: Fig. 740.]

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.

[Ill.u.s.tration: Fig. 741.]

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

[Ill.u.s.tration: Fig. 742.]

[Ill.u.s.tration: Fig. 743.]

Fig. 742 ill.u.s.trates 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.

[Ill.u.s.tration: Fig. 744.]

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 finis.h.i.+ng-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 finis.h.i.+ng-knives are brought into operation (the roughing-knives receding). By suitable stops, the motion of the roughing and finis.h.i.+ng-knives respectively are arrested when those knives have cut the blanks to the desired diameter, the finis.h.i.+ng-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 finis.h.i.+ng-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 pa.s.sed 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.

CHAPTER IX.--DRIVING WORK IN THE LATHE.

The devices employed to drive work that is suspended between the lathe centres are shown in the following ill.u.s.trations.

They are termed lathe dogs, drivers, or carriers. It is to be observed, however, that since the term dog is also applied to a device for holding work to the lathe face plate, as well as to the jaws of chucks, either the term driver or the English term carrier is preferable to the term dog.

[Ill.u.s.tration: Fig. 745.]