Fig. 587.
A simple attachment for regulating on a slide rest the depth of tool cut in screw cutting or for adjusting the cut to a requisite diameter when a number of pieces are to be turned to diameter by a finishing cut, is shown in [Fig. 587], in which b represents the slide rest carriage, and e the cross slide on which the slide rest a is traversed by means of the cross feed screw f. A screw is screwed into the rest, as shown, carrying the two circular milled edge nuts r p; the screw passes an easy fit through the piece c, which is capable of being fixed in any position along the slide e by means of the set screw s; the nut r is set in such a position on the screw that it will abut against c when the tool is clear of the work surface (for the back traverse) while p may be used in two ways:—First it may be set so that when it comes against c the thread is cut to the required depth, and thus act as stop to give the thread depth without trying the gauge: or it may be used to answer the same purpose and in the same way as the ring c in [Fig. 585].
The use of this device as a stop to gauge the thread depth is confined to such lengths of work as enable the tool to cut several pieces without requiring regrinding, because when the tool is removed to grind it, it is impracticable to set it exactly the same distance out from the tool post, hence the adjustment of p becomes destroyed. It is better, therefore, in most cases where a number of threads of equal pitch and diameter are to be cut, to rough them all out, cutting the threads a little above the gauge diameter so as to leave a finishing cut to be taken. In roughing out, however, the nut p may still be used to regulate the depth.
For the finishing cut the tool may be ground and p adjusted to give the requisite depth of cut, taking a single traverse over each thread to finish it. This, of course, preserves the tool and enables it to finish a larger number of threads without regrinding, and the consequent readjustment of p.
It is obvious that the nut p may be employed in the same manner to turn a number of plain pieces to an equal diameter.
Fig. 588.
It is preferable in a device of this kind, however, to employ the two adjusting nuts p and q in [Fig. 588], q being a clamp nut that can be closed by a screw so as to firmly grip the threaded stud. q is adjusted so that when p abuts against it the tool will cut to the correct diameter when it is moved in as far as nuts p q will permit. The use of the second nut p is as follows:—Suppose a first cut has been taken and p may be screwed up to just meet the face of clamp c. Then while the carriage is traversing, p may be screwed back towards q sufficiently to put on the next cut, and so on, so that p is used to adjust the depths of the roughing, and q that of the finishing cut.
Sometimes a feed motion to a slide rest is improvised by what is known as the star feed, the principle of action of which is as follows: Upon the outer end of the feed screw of the boring bar or slide rest, as the case may be, is fastened a piece of iron plate, which, from having the form in which stars are usually represented, is called the star. If the feed is for a slide rest a pin is fastened to the lathe face plate or other revolving part, in such a position that during the portion of the revolution in which it passes the star it will strike one of the star wings, and move it around sufficiently to bring the next wing into position to be struck by the pin during its succeeding revolution. When the feed is applied to a revolving boring bar the construction is the same, but in this case the pin is stationary and the star revolves with the feed screw of the bar.