Part 13 (1/2)
[Ill.u.s.tration: Fig. 10. Examples of Box-tool Designs]
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 bus.h.i.+ng _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 bus.h.i.+ng 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 bus.h.i.+ng _d_, so that the latter may be easily removed by pa.s.sing 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 bus.h.i.+ng 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.
[Ill.u.s.tration: Fig. 11. (A) Hollow Mill and Holder. (B) Spring Screw-threading Die and Releasing Die-holder]
=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 bra.s.s) 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 finis.h.i.+ng 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 bra.s.s, it should be on the center-line.
[Ill.u.s.tration: Fig. 12. Geometric Adjustable Hollow Milling Tool]
Hollow mills are also made adjustable. The design shown in Fig. 12 is especially adapted for bra.s.s finis.h.i.+ng. It can also be used for taking light cuts on cast iron or steel but its use in place of roughing or finis.h.i.+ng 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 pa.s.ses 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.
[Ill.u.s.tration: Fig. 13. Geometric Self-opening and Adjustable Screw-cutting Die Head]
=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.
[Ill.u.s.tration: Fig. 14. Geometric Collapsing Tap]
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 r.e.t.a.r.ded, 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 s.c.r.e.w.i.n.g 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 finis.h.i.+ng 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 finis.h.i.+ng 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 cla.s.ses 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 bus.h.i.+ng 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.
[Ill.u.s.tration: Fig. 15. Various Types of Tools for the Turret Lathe]
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 s.p.a.ced 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 princ.i.p.al 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 quant.i.ties, the rough castings are made with extra projections so arranged as to a.s.sist 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.