The following article is reprinted from Work, an English hobby magazine. It originally appeared in the December 4, 1920 issue, which C. H. Wendel furnished from his collection.
A LATHE, whilst primarily intended to cut articles to a circular cross section, may be adapted to turn any geometrical cross-sectional shape, whether regular or irregular. Any turner who has manipulated a lathe set up for relieving or placing the clearance angles on a hob (the tool used in the milling machine for cutting gears) will have noticed the peculiar action of the slide-rest in cutting in to a predetermined amount, and then suddenly jumping back to clear the cutting lip of the next tooth. This action, of course, is obtained by the use of cams and return springs, the setting out of which is a specialised branch of draughtsmanship.
It is not intended to deal here with the difficult and strictly mechanical methods of turning ovals, hexagons, spheres, etc., but to indicate some 'short cuts' or tricks which produce the same result.
Turning Ellipses.-Figs. 1 and 2 show an attachment easily made and fitted to any lathe for the purpose of turning a section tapering from round to oval, a section often desirable in making ornaments and other fancy articles. The change in section is, moreover, quite a symmetrical one and pleasing to handle. The ring A (Fig. 1) is elliptical, the difference in diameter being equal to the difference in diameter of the ellipse it is desired to turn. The arm B is provided with a roller C, held tightly against the ring by means of the spring D. The shaft E (Fig. 2) is secured to the lever B at one end, the other being supported by the upright piece F. Two rollers G are secured to the cross slide on each side of the shaft E, and work the feed of the cutting tool as the work revolves. When the carriage is disposed at the left end of E nearly all of the motion on the roller C is carried to the cutting-tool, while at the right-hand end of its travel the tool will have no motion, and that portion therefore will be round.
Turning a Ball in the Milling Machine.-It is an exacting job to turn a true ball in the lathe. A good wrinkle where wood is concerned is to bore a piece of steel the diameter of the required ball and harden and temper it. The ball is roughly cut to shape by compound action on the slide-rest, and finally trued by working the bored piece of steel all over it and finally letting it part it off. The hand-rest should be used for this job.
In metal, however, it is a tedious job to turn a ball in the lathe. It is not generally known that it may easily be done in the milling machine without the need for any special fixture. All that is required is a simple tool to fit in the fly-cutter head. This is, as shown in Fig. 3, made from square-section steel suitable to the hole in the milling-head, a portion being bent at right angles to make the cutting tool shown. Mount the stock from which the ball is required in the centers, so that it may be held by the lugs A. The tool is rotated in the ordinary way, feeding the stock round in the direction of the arrow. The result is a nearly perfect ball, save for the centre lugs A, which can easily be finished by hand to conform to the contour. They can be turned down very small before removing the ball by the tool.
Turning Scalloped Circumference.- Hand-wheels and similar machine parts often have their periphery serrated or scalloped, so that a better grip on them may be obtained (see Fig. 4). The ordinary method is to mill them, afterwards cleaning up to a smooth hand-grip with a file. The whole operation may easily be done in the lathe by means of the tool arrangement shown by Fig. 5.
The tool shown by Fig. 6 is made from cast-steel to the diameter of the required serrated circumference. The roller (Fig. 7) is of tool steel, and of a diameter to suit the scallops in the work to be turned. Remove the feed screw from the cross-head of the lathe, and fix a strong spring from the cross-head to the carriage as shown. Also rig up the double tool post, shown in Fig. 5, on the cross of the lathe carriage. Now mount the work on the mandrel of the forming tool and place between centres, bringing the poppet centre hard up. Place the roller in the tool post, as shown at B (Fig. 5), and secure a lathe tool in the opposite tool post C. Next start the lathe, and the roller, as shown by Fig. 5, will roll over the formed scallops and consequently feed the lathe tool into the work. This apparatus can be used for a variety of other work, such as polygons, cams, eccentrics, triangles, spokes, etc.
Turning a Hexagon, Octagon, etc.- For turning hexagons, octagons, etc., it is best to use a former in the chuck made as an exact model of the required work. The slide-rest is freed from its feed screw, and is pulled over against the copy plate by a heavy weight. A roller makes contact with the plate, carrying the tool with it, and therefore making the work a counterpart of the former-plate, that is, hexagon, octagon, as the case may be.
Cutting Left-Hand Screws.-In making a left-hand screw it is not always necessary to cut from headstock to tailstock. If the tool is placed to cut the back edge instead of the front, the cut may be from tailstock to head-stock, as in cutting a right-hand thread.