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.
