This section is from the book "Lathe Design, Construction And Operation, With Practical Examples Of The Lathe Work", by Oscar E. Perrigo. Also available from Amazon: Lathe Design: Construction And Operation.
Special forms of turned work. Attachment for machining concave and convex surfaces. Attachment for forming semicircular grooves in rolling mill rolls. Device for turning balls or spherical work. Turning curved rolls. A German device for machining concave surfaces. A similar device for convex surfaces. Making milling cutters. Backing off or relieving attachment. Operation of the device. Cross-feed stop for lathes. Grinding attachments. The "home-made" attachment. Electrically driven grinding attachment. Center grinding attachment. Large grinding attachment. The Rivett-Dock thread-cutting attachment.
While an engine lathe will readily turn straight and taper work, and will "face" work at right angles to the center line of the lathe, or by means of the compound rest will turn or face at any angle, no means is provided for turning curved contours, as spheres, curved rolls smallest in the center, or largest in the center, as the case may be, or to "face up" convex or concave surfaces. These and many other forms must be made by the aid of some kind of a device built for the special purpose and usually known under the general name of a "lathe attachment."
There are, of course, a great variety of jobs that can be economically performed on a lathe if we are provided with the proper tools and a suitable "attachment" for handling them.
It is not proposed to give here a complete list of these ever varying kinds or types of lathe attachments or to exhaust the list of forms of work that may be machined by one or another of these devices, yet it may be interesting to present a few of the attachments that are most likely to be needed, and in a general way those that the machinist may easily make for himself.
It often happens that a large number of concave or convex surfaces have to be machined to accurate spherical forms, the pieces being of such dimensions or material that the usual forming tools are impractical, or that the variety of dimensions would render them too expensive.
In these cases a special device must be designed which will properly fulfil the conditions and be capable of adjustment within a reasonable range of diameters of the work and the radii of the curves to be machined.
Fig. 150. - Plan of Lathe Attachment for Forming Concave and Convex Surfaces.
This may be accomplished by a special device attached to almost any ordinary lathe having a compound rest with a circular base, such as are now nearly always designed and built. The author has had occasion to design several of these devices, and the general form and arrangement of them has been as shown in the accompanying engravings, in which Fig. 150 is a plan of the lathe carriage showing the circular feeding device; Fig. 151 is a front elevation showing the method of varying the feed to suit the material to be machined; and Fig. 152 shows a modification of the device for a different form of work.
The various forms of work required to be done by a device of this kind are most frequently for concaving vertical step bearings of various diameters from three inches up, and of radii varying in proportion, for forming concave and convex surfaces for ball and socket joints, for turning large spherical surfaces, and for forming semicircular grooves in rolls for rolling iron and steel bars.
The construction and application of this device, as arranged on an ordinary lathe, is as follows. A machine steel ring A is forged, turned up, bored to a force fit to the circular portion of the compound rest. Its outer surface is properly formed for a worm-gear, its teeth cut and hobbed and it is forced on, and pinned if thought necessary. Engaging the teeth of this is the worm B, fixed to a shaft C, journaled in the brackets D, D, which are fixed to the carriage as shown. Upon the front end of the shaft C is the gear E, which may be removed and another size substituted for varying the rate of feed. Upon the front end of the cross-feed screw F is fitted a removable gear G. Connecting the gears E and G is the intermediate gear H, carried upon a movable stud located in the stud-plate J, which is pivoted upon a projecting sleeve formed upon the front bracket D and held in any desired position by the clamping screw K. By this arrangement the rate of feed may be conveniently changed to suit different diameters of work and materials of varying degrees of hardness, the same as the usual change-gears of a lathe. By releasing the stud-plate J, the gears G and H may be thrown out of engagement temporarily, while the stud-plate J and the brackets D, D may be easily removed altogether, if it is desired to use the lathe for ordinary turning for any length of time. As the feed for this device is derived from the cross-feed screw F, it is necessary to replace the usual solid nut in the shoe by a split nut (not shown) with the usual lever or eccentric device for opening and closing it as may be desired. A clutch device on the front end of the cross-feed screw F may be adopted, if desired, by having the cross-feed pinion N formed upon a sleeve projecting through to the front of the carriage and the gear G mounted upon it, and connected with, or disconnected from, the cross-feed screw F by sliding a double-faced clutch.
Fig. 151. - Front Elevation of Attachment.
In using this device for concave work held in a chuck or strapped to the face-plate, care must be taken to have the compound rest so adjusted on the carriage that when set parallel with the center line of the lathe its center will be exactly under that line, and that the horizontal distance from the point of the tool to the center of the compound rest will be the exact radius of the curve to be produced.
If convex work is to be done the compound rest tool block must be drawn back far enough past the center to give the required radius of the convex curve.