If, then, f be unscrewed, c may be rotated and set to the required angle, in which position screwing up f will lock it through the medium of e. g is the feed nut for the upper slider h, which operates along a slide way provided on c, the upper feed screw having journal bearing at c′. i is the tool post, having a stepped washer j, by means of which the height of the tool k may be regulated to suit the work.
Suppose, now, that it be required to turn a shaft having a parallel and a taper part; then the carriage may be traversed to turn the parallel part, and the compound slide c may be set to turn the taper part, while the lower feed screw operating in n may be used to turn radial faces.
The object of making a and l in two pieces is to enable the boring and insertion of b, which is done as follows:—The front end of l as l′ is planed out, leaving in it a groove equal in diameter and depth to the diameter and depth of b, so that b may be inserted laterally along this groove to its place in l. The segment e is then inserted and a piece is then fitted in at l′ and held fast to a by screws. It is into this piece that the set screw f is threaded.
Various forms of construction are designed for compound rests, but the object in all is to provide an upper sliding piece carrying the tool holder, such sliding piece being capable of being so set and firmly fixed that it will feed the tool at an angle to the line of the lathe centres.
Another and valuable feature of the compound rest is that it affords an excellent method of putting on a very fine cut or of accurately setting the depth of cut to turn to an exact diameter; this is accomplished by setting the upper slide at a slight angle to the line of centres and feeding the tool to the depth of cut by means of the screw operating the upper slide. In this way the amount of feed screw handle motion is increased in proportion to the amount to which the tool point moves towards the line of lathe centres, hence a delicate adjustment of depth of cut may be more easily made.
Suppose, for example, that a cut be started and that it is not quite sufficiently deep, then, while the carriage traverse is still proceeding, the compound rest may be operated to increase the cut depth, or if it be started to have too deep a cut the compound rest may be operated to withdraw the tool and lessen its depth of cut. Or it may be used to feed the tool in sharp corners when the feed traverse is thrown out, or to turn the tops of collars or flanges when the tailstock is set over to turn a taper.
It is obvious, however, that comparatively short tapers only can be conveniently turned by a compound slide rest; but most tapers, however, are short.
To turn long tapers the tailstock of the lathe is set over as described with reference to the Putnam lathe, but for boring deep holes the slide rest must either be a compound one or a taper turning former or attachment must be employed.
When, however, the tailstock is set over, the centres in the work are apt to wear out of true and move their location (the causes of which will be hereafter explained).
In addition to this, however, the employment of a taper turning attachment enables the boring of taper holes without the use of a compound slide rest, thus increasing the capacity of the lathe not having a simple or single rest.