RECOMMENDED LATHE IPRACT~CE-(contd.) account should extra pressure be applied by lengthening the leverage by dubious means such as a piece of piping. Such methods are unnecessary, and cause damage not only to the damping screws, but the entire compound rest. Boring tools may be of one piece solid forged, or may comprise cutter inserts fixed in a boring bar. A point of note in regard to the grinding of boring tools is that the conditions governing the top rake and clearance angles are different from those in turning, so that a secondary dearance is required for the front of the tool to clear the enveloping curve of the bore. DEPTH OF CUT Speeds and feeds.
The cutting speed is expressed in surface feet per minute, and is the speed at which the surface of the work passes the tip of the tool. If D is the diameter of the work in inches, N the work speed in r. p.m., and S the cutting speed in feet per minute, then 3.82 x S 318 x S (Metres/min.) N or N = D mm FIG. 5 DiRECTION OF TOP SLlDE TRAVEL Tables I, 2 and 3 give suitable tool angles, cutting speeds, and feeds for a range of the common engineering materials. The feed rate is expressed in inches per revolution of the headstock spindle. Screwc!.Itting. For cutting vee threads the top of the tool is placed at centre height, having been previously ground to the required shape without any top rake.
Note that if the tool is given top rake the plan angle ofthe tool is not the angle that will be reproduced in the work. The time to complete one cut, in minutes, can be calculated from Length of cut (in.) or (mm) Spindle speed (r.p.m.) x feed (in. per rev.) or (mm per rev.) Fig. 4 shows the use of a setting gauge for both external and internal threading, but when cutting vee threads with the tool set in this manner, slow speeds and light feeds are necessary because the cuttings cannot be free flowing without top rake on the tooL Thus a better method for cutting external vee threads is shown in fig.
5 where the compound slide is swung around so that the tool is fed in at an angle of slightly less than half the included angle of the thread (i.e. approxi- mately 26 deg. from normal, i.e. 64 deg. on cross slide graduations, FIG. 4 19