Chapter V: Turret Lathe Practice (1)

Turret lathes are adapted for turning duplicate parts in quantity. The characteristic feature of a turret lathe is the turret which is mounted upon a carriage and contains the tools which are successively brought into the working position by indexing or rotating the turret. In many instances, all the tools required can be held in the turret, although it is often necessary to use other tools, held on a cross-slide, for cutting off the finished part, facing a radial surface, knurling, or for some other operation. After a turret lathe is equipped with the tools needed for machining a certain part, it produces the finished work much more rapidly than would be possible by using an ordinary engine lathe, principally because each tool is carefully set for turning or boring to whatever size is required and the turret makes it possible to quickly place any tool in the working position. Turret lathes also have systems of stops or gages for controlling the travel of the turret carriage and cross-slide, in order to regulate the depth of a bored hole, the length of a cylindrical part or its diameter; hence, turning machines of this type are much more efficient than ordinary lathes for turning duplicate parts, unless the quantity is small, in which case, the advantage of the turret lathe might be much more than offset by the cost of the special tool equipment and the time required for "setting up" the machine. (See "Selecting Type of Turning Machine.")

=General Description of a Turret Lathe.=--The turret lathe shown in Fig. 1 has a hexagonal shaped turret _A_ with a hole in each side in which the tools are held. This turret is mounted on a slide _B_ which is carried by a saddle _C_ that can be moved along the bed to locate the turret slide with reference to the length of the tools in the turret and the room required for indexing. The turret slide can be moved longitudinally by turning the pilot wheel or turnstile _D_, or it can be fed by power. Ordinarily, the hand adjustment is used for quickly moving the carriage when the tools are not cutting, although sometimes the hand feed is preferable to a power feed when the tools are at work, especially if the cuts are short. After a turret tool has finished its cut, the turnstile is used to return the slide to the starting point, and at the end of this backward movement the turret is automatically indexed or turned one-sixth of a revolution, thus bringing the next tool into the working position. The turret is accurately located in each of its six positions by a lock bolt which engages notches formed in a large index ring at the turret base. A binder lever _E_ at the top of the turret stud is used to clamp the turret rigidly to the slide when the tools are cutting.

The forward movement of the slide for each position of the turret is controlled by stops at _F_, which are set to suit the work being turned. When parts are being turned from bar stock, the latter passes through the hollow spindle of the headstock and extends just far enough beyond the end of the spindle to permit turning one of the parts. The bar is held while the turning tools are at work, by a chuck of the collet type at _G_. This chuck is opened or closed around the bar by turning handwheel _H_. After a finished part has been cut off by a tool held in cross-slide _J_, the chuck is released and further movement of wheel _H_ causes ratchet feed dog _K_, and the bar which passes through it, to be drawn forward. This forward movement is continued until the end of the bar comes against a stop gage held in one of the turret holes, to insure feeding the bar out just the right amount for turning the next piece. On some turret lathes, the lever which operates the chuck also controls a power feed for the bar stock, the latter being pushed through the spindle against the stop.

The machine illustrated has a power feed for the cross-slide as well as for the turret. The motion is obtained from the same shaft _L_ which actuates the turret slide, but the feed changes are independent. The cross-slide feed changes are varied by levers _M_ and those for the turret by levers _N_. For many turret lathe operations, such as turning castings, etc., a jawed chuck is screwed onto the spindle and the work is held the same as when a chuck is used on an engine lathe. Sometimes chucks are used having special jaws for holding castings of irregular shape, or special work-holding fixtures which are bolted to the faceplate. The small handle at _O_ is for moving the cross-slide along the bed when this is necessary in order to feed a tool sidewise.

This particular machine is driven by a motor at the rear of the headstock, connection being made with the spindle through gearing. The necessary speed changes are obtained both by varying the speed of the motor and by shifting gears in the headstock. The motor is controlled by the turnstile _P_ and the gears are shifted by the vertical levers shown.

While many of the features referred to are common to turret lathes in general, it will be understood that the details such as the control levers, arrangement of stops, etc., vary on turret lathes of different make.

=Example of Turret Lathe Work.=--The diagrams Figs. 2 and 3 show a turret lathe operation which is typical in many respects. The part to be turned is a hub casting for an automobile and it is machined in two series of operations. The first series is shown by the plan view, Fig. 2. The casting _A_ is held in a three-jaw chuck _B_. Tool No. 1 on the cross-slide is equipped with two cutters and rough faces the flange and end, while the inner and outer surfaces of the cylindrical part are rough bored and turned by combination boring and turning tool No. 2. This tool has, in addition to a regular boring-bar, a bracket or tool-holder which projects above the work and carries cutters that operate on the top surface. Tools Nos. 3 and 4 next come into action, No. 3 finishing the surfaces roughed out by No. 2, and No. 4 finish-facing the flange and end of the hub. The detailed side view of Tool No. 3 (which is practically the same as No. 2), shows the arrangement of the cutters _C_ and _D_, one of which turns the cylindrical surface and the other bevels the end of the hub. The hole in the hub is next finished by tool No. 5 which is a stepped reamer that machines the bore and counterbore to the required size within very close limits. The surfaces machined by the different tools referred to are indicated by the sectional view _E_ of the hub, which shows by the numbers what tools are used on each surface.

For the second series of operations, the position of the hub is reversed and it is held in a spring or collet type of chuck as shown by the plan view Fig. 3. The finished cylindrical end of the hub is inserted in the split collet _F_ which is drawn back into the tapering collet ring by rod _G_ (operated by turnstile _H_, Fig. 1) thus closing the collet tightly around the casting. The first operation is that of facing the side of the flange and end of the hub with tool No. 6 on the cross-slide, which is shown in the working position. A broad cutter _H_ is used for facing the flange and finishing the large fillet, and the end is faced by a smaller cutter _I_. When these tools are withdrawn, tool No. 7 is moved up for rough turning the outside of the cylindrical end (preparatory to cutting a thread) and rough boring the hole. These same surfaces are then finished by tool No. 8. The arrangement of tools Nos. 7 and 8 is shown by the detailed view. Tool _J_ turns the part to be threaded; tool _K_ turns the end beyond the threaded part; and tool _L_ bevels the corner or edge. The reaming tool No. 9 is next indexed to the working position for finishing the hole and beveling the outer edge slightly. At the same time, the form tool No. 10, held at the rear of the cross-slide, is fed up for beveling the flange to an angle of 60 degrees. The final operation is that of threading the end, which is done with die No. 11. The boring-bars of tools Nos. 2, 3, 7 and 8 are all provided with pilots _N_ which enter close fitting bushings held in the spindle, to steady the bar while taking the cut. This is a common method of supporting turret lathe tools.

The feed of the turret for both the first and second series of operations is 1/27 inch per revolution and the speeds 60 revolutions per minute for the roughing cuts and 90 revolutions per minute for the finishing cuts. The total time for machining one of these castings complete is about 7-1/2 minutes, which includes the time required for placing the work in the chuck.

=Machining Flywheels in Turret Lathe.=--Figs. 4 to 6, inclusive, illustrate how a gasoline engine flywheel is finished all over in two cycles of operations. First the flywheel is turned complete on one side, the hole bored and reamed, and the outside of the rim finished; in the second cycle the other side of the flywheel is completed.

During the first operation, the work is held by the inside of the rim by means of a four-jaw chuck equipped with hard jaws. The side of the rim, the tapering circumference of the recess, the web, and the hub are first rough-turned, using tools held in the carriage toolpost. The hole is then rough-bored by bar _C_, which is supported in a bushing in the chuck, as shown in Fig. 4. The outside of the wheel rim is rough-turned at the same time by a cutter held in the extension turret tool-holder _T_ (Fig. 5), and the taper fit on the inside of the flywheel is turned by means of cutter _A_ (Fig. 4) held in a tool-holder attached to the turret.

The outside of the wheel rim is next finish-turned with cutter _V_ (Fig. 5) held in an extension turret tool-holder the same as the roughing tool _T_. At the same time, the bore is finished by a cutter in boring-bar _D_ (Fig. 4). The side of the rim and the hub of the wheel are also finished at this time by two facing cutters _H_ and _K_, held in tool-holders on the face of the turret. When the finishing cuts on the rim and hub are being taken, the work is supported by a bushing on the boring-bar which enters the bore of the wheel, the boring cutter and facing tools being set in such relation to each other that the final boring of the hole is completed before the facing cuts are taken.

The web of the wheel is next finish-faced with the facing cutter held in the holder _E_, and the taper surface on the inside of the rim is finished by the tool _L_, at the same time. While these last operations are performed, the work is supported by a bushing on a supporting arbor _J_, which enters the bore of the wheel. The bore is finally reamed to size by a reamer _F_ held in a "floating" reamer-holder. When the reaming operation is completed, a clearance groove _N_ is cut on the inside of the rim, using a tool _G_ held in the carriage toolpost. The first cycle of operations on the flywheel is now completed.

The flywheel is then removed from the chuck, turned around, and held in "soft" jaws for the second cycle of operations, the jaws fitting the outside of the wheel rim. (Soft unhardened jaws are used to prevent marring the finished surface of the rim.) The operations on this side are very similar to those performed on the other side. First, the side of the rim, the inside of the rim, the web, and hub are rough-turned, using tools held in the carriage toolpost. The inside of the rim and the web are then finished by a cutter held in a tool-holder at _P_, Fig. 6, which is bolted to the face of the turret. The work is supported during this operation by a bushing held on a supporting arbor _U_, having a pilot which enters a bushing in the chuck. Finally, the rim and hub are finished, by the facing cutters _R_ and _S_, the work being supported by an arbor, as before.

These operations illustrate the methods employed in automobile factories, and other shops where large numbers of engine flywheels, etc., must be machined.

=Finishing a Flywheel at One Setting in Turret Lathe.=--The plan view _A_, Fig. 7, shows an arrangement of tools for finishing a flywheel complete at one setting. The hole for the shaft has to be bored and reamed and the hub faced on both sides. The sides and periphery of the rim also have to be finished and all four corners of the rim rounded. The tools for doing this work consist of boring-bars, a reamer, facing heads on the main turret, a turret toolpost on the slide rest (carrying, in this case, three tools) and a special supplementary wing rest attached to the front of the carriage at the extreme left.

The casting is held by three special hardened jaws _b_ in a universal chuck. These jaws grip the work on the inner side of the rim, leaving room for a tool to finish the rear face without striking the chuck body or jaws. Three rests _c_ are provided between the chuck jaws. The work is pressed against these rests while being tightened in the chuck, and they serve to locate it so that the arms will run true so far as sidewise movement is concerned. These rests also locate the casting with relation to the stops for the turret and carriage movements. The chuck carries a bushing _r_ of suitable diameter to support the boring-bars in the main turret, as will be described.

In the first operation, boring-bar _m_ is brought in line with the spindle and is entered in bushing _r_ in the chuck. Double-ended cutter _n_ is then fed through the hub of the pulley to true up the cored hole. While boring the hole, the scale on the front face of the rim and hub is removed by tool _j_. Tool _k_ is then brought into action to rough turn the periphery, after which tool _e_, in the wing rest, is fed down to clean up the back face of the rim. As soon as the scale is removed, the hole is bored nearly to size by cutter _n_{1}_ in bar _m_{1}_, and it is finally finished with reamer _q_ mounted on a floating arbor.

The cutters _f_, _g_ and _h_, in the facing head, are next brought up to rough face the hub and rim, and round the corners of the rim on the front side. This operation is all done by broad shaving cuts. The facing head in which the tools are held is provided with a pilot bar _t_ which fits the finished hole in the flywheel hub, and steadies the head during the operation. The cutters _f_, _g_ and _h_ are mounted in holders which may be so adjusted as to bring them to the proper setting for the desired dimensions. This completes the roughing operations.

The periphery of the rim is now finished by cutter _l_ in the turret toolpost which is indexed to the proper position for this operation. The rear face of the rim is finished by the same tool _e_ with which the roughing was done. Tool _e_ is then removed and replaced with _d_ which rounds the inner corner of the rim. Tool _d_ is also replaced with a third tool for rounding the outer corner of the rear side. For finishing the front faces of the rim and hub and rounding the corners of the rim, a second facing head, identical with the first one, is employed. This is shown in position in the illustration. Cutters _f_{1}_, _g_{1}_ and _h_{1}_ correspond with the cutters _f_, _g_ and _h_, previously referred to, and perform the same operations.

The remaining operation of finishing the back of the hub is effected by cutter _p_. This cutter is removed from the bar, which is then inserted through the bore; the cutter is then replaced in its slot and the rear end of the hub is faced by feeding the carriage away from the headstock. This completes the operations, the flywheel being finished at one setting.

=Finishing a Webbed Flywheel in Two Settings.=--The plan views _B_ and _C_, Fig. 7, show the arrangement of tools for finishing a webbed flywheel which has to be machined all over. This, of course, requires two operations. In the first of these (see sketch _B_) the rough casting is chucked on the inside of the rim with regular inside hard chuck jaws _b_. The cored hole is first rough bored with cutter _n_ attached to the end of boring-bar _m_, and guided by the drill support _d_ pivoted to the carriage. Next, the boring-bar _m_{1}_ is brought into position, the drill support being swung back out of the way. This bar is steadied by its bearing in bushing _r_ in the chuck. Two cutters, _n_{1}_ and _n_{2}_, are used to roughly shape the hole to the desired taper, the small end being finished to within 0.002 inch of the required diameter. While boring with the bar _m_{1}_, the scale is broken on the web and hub of the casting by the tool _k_ in the turret toolpost. The latter is then shifted to bring the tool _j_ into position for removing the scale on the periphery of the wheel. Next, the hole is reamed with taper reamer _q_, the pilot of which is supported by bushing _r_.

The first of the facing heads is now brought into action. This facing head carries a guide _t_ which is steadied in a taper bushing _c_, driven into the taper hole of the hub for that purpose. The top cutter _f_ turns the periphery, cutter _g_ turns the hub and faces the web, and cutter _h_ faces the rim. A fourth cutter _e_ on the under side of the head faces the hub. This casting is now machined approximately to size.

For finishing, similar cutters, _e_{1}_, _f_{1}_, _g_{1}_ and _h_{1}_, in the other facing head are used, the latter being supported by the taper bushing _c_ in the same way. A very light cut is taken for finishing. Tool _l_ in the carriage turret is used to round the outer and inner corners of the rim, which completes the work on this face of the casting.

In the second cycle of operations, shown at _C_, the casting is chucked on the outside with the soft jaws _b_, which are bored to the exact diameter of the finished rim. The work is further supported and centered by sliding bushing _c_, which is tapered to fit the finished hole in the hub, and has an accurate bearing in bushing _r_ in the chuck. This bushing is provided with a threaded collar for forcing it into the work and withdrawing it. The scale on the web and the inside and face of the rim is first broken with the tool _k_ in the turret toolpost. These surfaces are then roughed off with cutters _f_, _g_ and _h_, in the facing head. This latter is steadied by a pilot _t_ which enters the hole in the sliding bushing _c_ on which the work is supported. A light cut is next taken with cutters _f_{1}_, _g_{1}_ and _h_{1}_, in the finishing facing head, which completes the operation.

=Tools for Turret Lathes.=--The operation of a turret lathe after the tools have been properly arranged is not particularly difficult, but designing and making the tools, determining what order of operations will give the most efficient and accurate results, and setting the tools on the machine, requires both skill and experience. For some classes of work, especially if of a rather complicated nature, many of the tools must be specially designed, although there are certain standard types used on turret lathes which are adapted to general turning operations. Some of the principal types are referred to in the following.

=Box-tools.=--Tools of this type are used for turning bar stock. There are many different designs, some of which are shown in Figs. 8, 9 and 10. Box-tools are held in the turret and they have back-rests opposite the turning tools, for supporting the part being turned. The box-tool shown at _A_, Fig. 8, is for roughing. The cutter _a_ is a piece of high-speed steel beveled on the cutting end to produce a keen edge. It takes a shearing tangent cut on top of the bar and the latter is kept from springing away by means of the adjustable, hardened tool-steel back-rest _b_. This tool is considered superior to a hollow mill whenever a fair amount of stock must be removed. If considerable smoothness and accuracy are necessary, the finishing box-tool shown at _B_ should follow the roughing box tool, but in most cases, especially if the part is to be threaded by a die, a finishing cut is unnecessary.

The finishing box-tool _B_ is also used to follow a hollow mill if special accuracy or smoothness is desired. This tool is only intended for light finishing cuts, the allowances varying from 0.005 inch to 0.015 inch in diameter. The cutters are made of square tool steel of commercial size, and are ground and set to take a scraping end cut. This particular tool has two tool-holders which permit finishing two diameters at once. If a larger number of sizes must be turned, extra tool-holders can be applied.

The single-cutter box-tool shown at _C_ is bolted directly to the face of the turret instead of being held by a shank in the turret hole, and it is adapted for heavy cuts such as are necessary when turning comparatively large bar stock. The tool-holder _a_ swivels on a stud, thus allowing the cutter to be withdrawn from the work while being returned, which prevents marring the turned surface. The high-speed steel cutter is ground to take a side cut on the end of the bar. The latter is supported by hardened and ground tool-steel rolls _b_ which revolve on hardened and ground studs. These rolls are mounted on swinging arms which have a screw adjustment for different diameters. They can also be adjusted parallel to the bar, thus enabling them to be set either in advance of or back of the cutter. The opening in the base allows the stock to pass into the turret when it is not larger than the turret hole.

The box-tool shown at _D_ is similar to the one just described, except that it has two or more cutters and roller back-rests, thus enabling different diameters to be turned simultaneously. The cutters are ground to take a side cut. Ordinarily this gives a satisfactory finish, but if special accuracy and smoothness are desired, two tools should be used, one for roughing and one for finishing, the latter being ground to take a light scraping end cut.

The taper-turning box-tool shown at _E_ is designed for accurately turning tapers on brass or cast-iron parts, when there is a small amount of stock to be removed. The taper is obtained by cross motion imparted to the cutter slide as the turret advances. The taper-turning box-tool shown at _F_, instead of having a single-point cutter, is provided with a wide cutter _a_. This tool is designed to turn tapering parts of small or medium diameter, requiring the use of a support which cannot be provided with a straight forming tool and holder mounted on the cut-off slide. The cutter is backed up by the screws shown, which also provide adjustment for different tapers within a limited range. The bar is supported by the three back-rests shown, which also have screw adjustment.

=Examples of Box-tool Turning.=--Box-tools are not only used for cylindrical and taper turning on the end of a bar, but for many other operations. Figs. 9 and 10 show a number of box-tools of different designs, with examples of the work for which each is intended. While these tools are designed for some specific part, they can, of course, with slight modifications be adapted to other work.

A box-tool of the pilot type that is used for finishing, after the surplus stock has been removed by roughing tools, is shown at _A_, Fig. 9. The work, which is the cone for a ball bearing, is shown at _a_ by the dotted lines and also by the detail view to the right. The pilot _b_ enters the work before either of the cutters begins to operate on its respective surface. The inverted cutter _c_, which sizes the flange of the cone, is held in position by a clamp _d_, which is forced down by a collar-head screw. The cutter is further secured against a beveled shoulder at _g_ by the set-screws _f_, and it is adjusted forward by the screw _e_. By loosening the screws _f_ and the collar-head screw, the cutter may be removed for sharpening. The cutter _h_ is adjusted to cut to the proper diameter, by the screws _l_, after which the clamp _k_ is made level by the screw _j_. The collar-screw _m_ is then used to secure the tool in place. The cutter is made from drill rod and it is slightly cupped out on the cutting end to give keenness to the cutting edge. The adjusting screw _o_, which passes through plate _p_, prevents the cutter from backing away from the work. This adjusting screw plate has its screw holes slotted to avoid removing the screws when it becomes necessary to remove the plate and cutter for sharpening. Pilot _b_ is held firmly to the tool body by set-screw _r_. The hole _s_ through the shank makes it easy to remove the pilot, in case this is necessary.

A pilot box-tool for finishing another type of ball bearing cone is shown at _B_. The shape of the work itself is indicated by the dotted lines _a_ and by the detail view. This tool is somewhat similar in its construction to the one just described. The cutters _b_ and _c_ are inverted and are used to face the flange at _d_ and to turn it to the proper diameter. These cutters are held by the clamp _f_ and screws _g_ and are adjusted forward by the screw _h_. The cutter _j_, which operates on top of the stock, rests on a bolster, of the proper angle and is adjusted up or down by the screws _k_. The clamp _l_, which binds against this tool, is beveled to correspond with the angle of the tool. This clamp is secured by the collar-screw shown and it is leveled by set-screws _s_. The adjusting screw _p_ prevents the cutter from slipping back. The holes in the adjusting-screw plate are also slotted in this case so that it will not be necessary to remove any screws when the cutter has to be taken out of the holder.

A box-tool for finishing a treadle-rod cone for a sewing machine is shown at _C_. This tool is also of the pilot type. The cutters in it operate on opposite sides of the cone _a_. The inverted cutter _b_ sizes the cylindrical part of the cone, while the front cutter _d_ is set at the proper angle to finish the tapered part. The rear cutter _b_ is held in place by the clamp _g_ and a collar screw. It is adjusted forward by the screw _h_ in the plate _i_ which is held by screws as shown. The pilot is retained by a set-screw, and it is easily removed by inserting a small rod in the hole _l_ which passes through the shank. The cutter _d_ is held by clamp _m_ and is adjusted by screw _n_ which passes through a tapped hole in plate _o_. The screw holes in both the adjusting plates _i_ and _o_ are slotted to facilitate their removal.

The box-tool illustrated at _A_, Fig. 10, is used for finishing the bushing of a double-taper cone bearing _a_. The cutters are so arranged that they all cut on the center; that is, the cutting edges lie in a horizontal plane. The inverted cutter _b_ at the rear forms the short angular surface, and the cutter _c_ in front forms the long tapering part of the bearing. The large diameter is turned, to size by cutter _d_. The pilot _e_ has a bearing in the bore nearly equal to the length of the work and it is provided with oil grooves, as shown. The taper shank of this pilot is tapped for the screw _i_ which extends the whole length of the shank and is used to draw the pilot back to its seat. It is not necessary to remove adjusting-screw plate _k_ to take out the cutter _b_, as the latter can be drawn out from the front after the collar-screw _m_ is loosened. The cutter _c_ is removed by taking off the adjusting-screw plate _s_ after loosening the collar-screw _n_. The cutter _d_ is held in a dove-tailed slot by two headless set-screws _q_. It is also backed up by an adjusting screw in the plate _s_. These adjusting screws should all have fine threads, say from 32 to 40 per inch, and be nicely fitted so they will not loosen after being adjusted.

The box-tools shown at _B_ and _C_, Fig. 10, are for turning the sides of a loose pulley for a sewing machine. This pulley (shown by the dotted lines) is finished in two operations. The box-tool for finishing the side of the pulley on which the hub projects beyond the rim, is shown at _B_. The inverted cutter _a_, which faces the end of the hub, is held by a clamp _c_ (clearly shown in the end view) from the under side and it has no adjustment. The collar-screw _d_ is tapped into this clamp, which is prevented from getting out of place by the dowel-pin _f_. The pilot _g_ is made small in the shank, so that tool _a_ can be so placed as to insure the removal of all burrs around the bore of the hub. The pilot is held by a set-screw and it is provided with oil grooves. The cutter _j_ sizes the outside of the hub, and the cutter _k_ faces the side of the pulley rim. These cutters are both held by the clamp _l_ and the collar-screw _m_. No side plates are used on this tool, and the cutters are all easily removed.

Sketch _C_ shows the box-tool used for the second operation. As the hub is flush with the rim on the side for which this tool is intended, it needs only one cutter to face both. This is done by the wide cutter _a_ which is held in a dove-tailed slot in the front of the tool and is fastened by the clamp _b_ and collar-screw _c_. The bushing _d_, in which the end of the work arbor is supported, is held by the collar-screw _e_, and to obtain the necessary compression, the body of the tool is slotted as far back as _f_. This bushing is provided with oil grooves and one side is cut away to clear the cutter _a_. The pilot end of the arbor on which the work is mounted is 1/16 inch smaller than the bore of the pulley, which allows the cutter to be set in far enough to prevent any burr which might form at the edge of the bore. A disk _i_ is inserted back of bushing _d_, so that the latter may be easily removed by passing a rod through the hollow shank. The special chuck used for this second operation on the loose pulley is screwed onto the spindle, and the work is mounted on a projecting arbor and driven by the pins engaging holes in the pulley web. The arbor is made a driving fit for the work, and the end or pilot is a running fit in the bushing of the box-tool. A counterbore in the arbor hub provides clearance for the hub of the pulley which projects beyond the rim on one side.

=Hollow Mills.=--A hollow mill such as is shown at _A_ in Fig. 11 is sometimes used in place of a box-tool (especially when turning brass) for short roughing cuts preceding a threading operation. The turning is done by the cutting edges _e_, and the turned part enters the mill and is steadied by it. If this type of tool is used for long, straight cuts, especially on square stock and when making screws with large heads from the bar, it should always be followed by a finishing box-tool to insure accurate work. A hollow mill can be sharpened readily by grinding the ends without materially changing the cutting size. A slight adjustment can be obtained by means of the clamp collar shown to the left, although this is not generally used. When making these mills, they should be reamed out tapering from the rear to give clearance to the cutting edges. For turning steel, the cutting edge should be about 1/10 of the diameter ahead of the center, whereas for brass, it should be on the center-line.

Hollow mills are also made adjustable. The design shown in Fig. 12 is especially adapted for brass finishing. It can also be used for taking light cuts on cast iron or steel but its use in place of roughing or finishing box-tools for general use is not recommended. With the exception of the cutters and screws, the complete tool consists of three parts, _viz._, the holder, cam, and ring. The cam serves to adjust the cutters for different diameters. The adjustment is made by the two screws shown, the amount being indicated by a micrometer scale. When adjusting the cutters for a given diameter, the use of a hardened steel plug of the required size is advisable, the cutters being adjusted against the plug.

=Releasing Die and Tap Holders.=--Threads are cut in the turret lathe by means of dies for external threading, and taps for internal threading, the die or tap being held in a holder attached to the turret. A simple form of releasing die holder is shown at _B_, Fig. 11. This holder was designed for the spring-screw type of threading die shown to the left. The die is clamped in the holder _a_ by the set-screw shown, and the shank _b_ of the holder is inserted in the turret hole. Holder _a_ has an extension _c_ which passes through the hollow shank. When the die is pressed against the end of the work, holder _a_ and its extension moves back until lug _d_ on the holder engages lug _e_ on the shank. The die and holder are then prevented from rotating with the work and the die begins to cut a thread. It continues to screw itself onto the work with the turret following, until the thread has been cut to the required length; the turret is then stopped and as the die and holder _a_ are drawn forward, lugs _d_ and _e_ disengage so that the die simply rotates with the work without continuing to advance. The lathe spindle is then reversed and as the turret is moved back by hand, pin _f_ comes around and enters notch _g_, thus holding the die stationary; the die then backs off from the threaded end. Some tap holders are also constructed the same as this die holder, so far as the releasing mechanism is concerned. There are also many other designs in use, some of which operate on this same principle.

=Self-opening Die Heads.=--The type of die holder shown at _B_ in Fig. 11 is objectionable because of the time required for backing the die off the threaded end; hence, self-opening dies are extensively used in turret lathe work. As the name implies, this type of die, instead of being solid, has several chasers which are opened automatically when the thread has been cut to the required length. The turret can then be returned without reversing the lathe spindle. The dies are opened by simply stopping the travel of the turret slide, the stop-rod for the feed of the turret being adjusted to give the proper amount of travel.

A well-known die head of the self-opening type is shown in Fig. 13. The dies open automatically as soon as the travel of the head is retarded, or they can be opened at any point by simply holding back on the turnstile or lever by which the turret slide is moved. The die is closed again by means of the small handle seen projecting at right-angles from the side of the head. The closing may be done by hand or automatically by screwing a pin into a threaded hole opposite the handle and attaching a small piece of flat steel to the back edge of the turret slide. The latter will then engage the pin as the turret revolves, thus closing the die head. This die head has a roughing and finishing attachment which is operated by handle _A_. When this handle is moved forward, the dies are adjusted outward 0.01 inch for the roughing cut, whereas returning the handle closes and locks the dies for the finishing cut. The die head has a micrometer scale which is used when making slight adjustments to compensate for the wear of the chasers or to make either a tight-or a loose-fitting thread.

=Collapsing Taps.=--The collapsing tap shown in Fig. 14 is one of many different designs that are manufactured. They are often used in turret lathe practice in place of solid taps. When using this particular style of collapsing tap, the adjustable gage _A_ is set for the length of thread required. When the tap has been fed to this depth, the gage comes into contact with the end of the work, which causes the chasers to collapse automatically. The tool is then withdrawn, after which the chasers are again expanded and locked in position by the handle seen at the side of the holder. In all threading operations, whether using taps or dies, a suitable lubricant should be used, as a better thread is obtained and there is less wear on the tools. Lard oil is a good lubricant, although cheaper compounds give satisfactory results on many classes of work.

=Miscellaneous Turret Lathe Tools.=--The chamfering tool shown at _A_, Fig. 15, is used for pointing the end of a bar before running on a roughing box-tool. This not only finishes the end of the bar but provides an even surface for the box-tool to start on. The cutter is beveled on the end to form a cutting edge and it is held at an angle. The back-rest consists of a bell-mouthed, hardened tool-steel bushing which supports the bar while the cut is being taken.

The stop gages _B_ and _C_ are used in the turret to govern the length of stock that is fed through the spindle. When a finished piece has been cut off, the rough bar is fed through the spindle and up against the stop gage, thus locating it for another operation. This gage may be a plain cylindrical piece of hardened steel, as at _B_, or it may have an adjusting screw as at _C_; for special work, different forms or shapes are also required. The stop gages on some machines, instead of being held in the turret, are attached to a swinging arm or bracket that is fastened to the turret slide and is swung up in line with the spindle when the stock is fed forward.

The center drilling tool _D_ is designed to hold a standard combination center drill and reamer. This type of tool is often used when turning parts that must be finished afterwards by grinding, to form a center for the grinding machine. The adjustable turning tool _E_ is used for turning the outside of gear blanks, pulley hubs or the rims of small pulleys. The pilot _a_ enters the finished bore to steady the tool, and cutter _b_ is adjusted to turn to the required diameter.

The cutting-off tool-holder _F_ (which is held on the cross-slide of the turret lathe) is usually more convenient than a regular toolpost, as the blade can be set closer to the chuck. The blade is held in an inclined position, as shown, to provide rake for the cutting edge; the inclined blade can also be adjusted vertically, a limited amount, by moving it in or out. The multiple cutting-off tool _G_ holds two or more blades and is used for cutting off several washers, collars, etc., simultaneously. By changing the distance pieces between the cutters, the latter are spaced for work of different widths. The flat drill holder _H_ is used for drilling short holes, and also to form a true "spot" or starting point for other drills.

Knurling tools are shown at _I_ and _J_. The former is intended for knurling short lengths and is sometimes clamped on top of the cut-off tool on the cross-slide, the end being swung back after knurling (as shown by the dotted lines) to prevent interference with the work when the cutting-off tool is in operation. The knurling tool _J_ has a shank and is held in the turret. The two knurls are on opposite sides of the work so that the pressure of knurling is equalized. By adjusting the arms which hold the knurls, the tool can be set for different diameters.

Three styles of drill holders are shown at _K_, _L_ and _M_. Holder _K_ is provided with a split collet (seen to the left) which is tightened on the drill shank by a set-screw in the holder. This holder requires a separate collet for each size drill. The taper shank drill holder _L_ has a standard taper hole into which the shank of the drill is inserted. The adjustable type of holder _M_ is extensively used, especially on small and medium sized machines when several sizes of drills are necessary. This holder is simply a drill chuck fitted with a special shank. For large drills the plain style of holder _K_ is recommended, and if only a few sizes of drills are required, it is more satisfactory and economical than the adjustable type.

The various types of small turret lathe tools referred to in the foregoing for turning, threading, tapping, knurling, etc., are a few of the many different designs of tools used in turret lathe practice. Naturally, the tool equipment for each particular job must be changed somewhat to suit the conditions governing each case. The tools referred to, however, represent in a general way, the principal types used in ordinary practice. Some of the more special tools are shown in connection with examples of turret lathe work, which are referred to in the following.

=Turning Gasoline Engine Pistons in Turret Lathe.=--The making of pistons for gas engines, especially in automobile factories, is done on such a large scale that rapid methods of machining them are necessary. The plan view _A_, Fig. 16, shows the turret lathe tools used in one shop for doing this work. As is often advisable with work done in large quantities, the rough castings are made with extra projections so arranged as to assist in holding them. These projections are, of course, removed when the piece is completed. In this case the piston casting _a_ has a ring about 1-1/4 inch long and a little less in diameter than the piston, at the chucking end. The piston is held in suitable chuck jaws _b_ which are tightened against the inside of this ring. The set-screws in these special jaws are then tightened, thus clamping the casting between the points of the screws and the jaws. This method of holding permits the whole exterior of the piston to be turned, since it projects beyond the chuck jaws. This is the object in providing the piston with the projecting ring by which it is held.

The first operation consists in rough-boring the front end of the piston. The double-ended cutter _n_ is held in boring-bar _m_, which is, in turn, supported by a drill-holder, clamped to one of the faces of the turret. This bar is steadied by a bushing in the drill support _c_ which is attached to the carriage, and may be swung into or out of the operating position, as required. After this cut is completed, the turret is revolved half way around and the casting is finish-bored in a similar manner, with double-ended cutter _n_{1}_ held in bar _m_{1}_, the drill support being used as in the previous case. The support is then turned back out of the way to allow the turning tools in the turret toolpost to be used.

The outside of the piston is next rough-turned with tool _k_ in the turret toolpost, which is revolved to bring this cutter into action. The toolpost is then turned to the position shown, and the outside is finish-turned by tool _j_, which takes a broad shaving cut. The turret tool-holder is again revolved to bring form tool _l_ into position. This tool cuts the grooves for the piston rings. Suitable positive stops are, of course, provided for both the longitudinal and cross movements of the turret toolpost.

In the second operation, the piston _a_ is reversed and held in soft jaws, which are used in place of the hardened jaws _b_ shown in the illustration. These jaws are bored to the outside diameter of the piston, so that when closed, they hold the work true or concentric with the lathe spindle. In this operation the chucking ring by which the piston was previously held is cut off, and the end of the piston is faced true. If the crank-pin hole is to be finished, a third operation is necessary, a self-centering chuck-plate and boring and reaming tools being used. (These are not shown in the illustration.)

=Turning Piston Rings in Turret Lathe.=--One method of turning piston rings is shown at _B_ in Fig. 16. The piston rings are cut from a cast-iron cylindrical piece which has three lugs _b_ cast on one end and so arranged that they may be held in a three-jawed chuck. This cylindrical casting is about 10 inches long, and when the rings are to have their inside and outside surfaces concentric, the casting is held by the lugs in the regular jaws furnished with the chuck. (The arrangement used for turning and boring eccentric rings, which is that shown in the illustration, will be described later.)

The casting _a_, from which the rings are made, is first rough-bored with double-ended cutter _n_ in boring-bar _m_, after which it is finish-bored with cutter _n_{1}_ in bar _m_{1}_. While taking these cuts, the bars _m_ and _m_{1}_ are supported by their extension ends which enter bushing _r_ located in the central hole of the chuck. This furnishes a rigid support so that a heavy cut can be taken.

The outside of the casting is next rough-turned with tool _k_, held in the turret toolpost. This toolpost is then revolved to bring tool _j_ into position, by which the outside is turned true to size, a broad shaving chip being taken. The toolpost is again swung around, to bring the cutting-off tool-holder _l_ into position. This holder contains four blades set the proper distance apart to give rings of the desired width. Each blade, from right to left, is set a little back of the preceding one, so that the rings are cut off one after the other, the outer rings being supported until they are completely severed. After the first four rings are cut off, the carriage is moved ahead to a second stop, and four more rings are severed, this operation being continued until the casting has been entirely cut up into rings.

When the bore of the ring is to be eccentric with the outside, the holding arrangement shown in the illustration is used. The casting a is bolted to a sliding chuck-plate _c_, and the outside is rough-turned with tool _k_ in the toolpost. Finishing tool _j_ is then brought into action, and the outside diameter is turned accurately to size. Then the sliding chuck-plate _c_, carrying the work, is moved over a distance equal to the eccentricity desired, and the work is bored with cutters _n_ and _n_{1}_ as in the previous case. The turret toolpost is next revolved and the tools _l_ are used for cutting off the rings. The reason for finishing the outside first is to secure smooth rings in cutting off, as this operation should be done when the work is running concentric with the bore, rather than with the exterior surface.

It will be evident that this method gives a far greater output of rings than is possible by finishing them in the more primitive way on engine lathes. The faces of the rings may be finished in a second operation if desired, or they may be ground, depending on the method used in the shop where the work is being done, and the accuracy required.

=Piston Turning in Pratt and Whitney Turret Lathe.=--A turret lathe equipped with tools for turning, facing and grooving automobile gasoline engine pistons is shown in Fig. 17. The piston is held on an expanding pin chuck which is so constructed that all of the pins are forced outward with equal pressure and automatically conform to any irregularities on the inside of the piston. Tool _A_ rough-turns the outside, and just as this tool completes its cut, a center hole is drilled and reamed in the end of the piston by combination drill and reamer _B_. The turret is then indexed one-half a revolution and a finishing cut is taken by tool _C_. After the cylindrical body of the piston has been turned, tools held in a special holder _E_ attached to the cut-off slide are used to face the ends of the piston and cut the packing-ring grooves. While the grooves are being cut, the outer end of the piston is supported by center _D_. The center hole in the end also serves to support the piston while being ground to the required diameter in a cylindrical grinding machine. The edge at the open end of the piston may also be faced square and the inner corner beveled by a hook tool mounted on the rear cross-slide, although this is usually done in a separate operation. (This provides a true surface by which to hold this end when grinding.)

This illustration (Fig. 17) shows very clearly the stops which automatically disengage the turret feed. A bracket _F_ is bolted to the front of the bed and contains six stop-rods _G_ (one for each position or side of the turret). When one of these stop-rods strikes lever _H_, the feed is disengaged, the stop being adjusted to throw out the feed when the tool has completed its cut. Lever _H_ is automatically aligned with the stop-rods for different sides of the turret by a cam _J_ on the turret base. A roller _K_ bears against this cam and, through the connecting shaft and lever shown, causes lever _H_ to move opposite the stop-rod for whatever turret face is in the working position. Lever _L_ is used for engaging the feed and lever _R_ for disengaging it by hand.

The indexing of the turret at the end of the backward movement of the slide is controlled by stop _M_ against which rod _N_ strikes, thus disengaging the lock bolt so that the turret can turn. This stop _M_ is adjusted along the bed to a position depending upon the length of the turret tools and the distance the turret must move back to allow the tools to clear as they swing around.