Trick-Turning In Wood And Metal

| February/March 1989

Turning Ellipses

Andrew K. Mackey

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.