An English authority (Mr. Wilson Hartnell), who made some observations (in different workshops and with different workmen) on this subject, stated that taking the square feet of work surface tooled over in a given time, he had often found as much as from 100 to 200 per cent. difference, and that he had found the rate of tooling small fly-wheels vary from 2 to 8 square feet per hour without any sufficient reason. The author has himself observed a difference of as much as 20 feet of work rotation per minute on work of 18 and less inches in diameter, and as much as 50 per cent. in the rate of tool traverse per lathe revolution.
It is only by keeping the speed rotation at the greatest consistent with the depth of cut, and by exercising a fine discretion in regulating the rotations of feed and cutting speed, that a maximum of duty can under any given conditions be obtained.
It has hitherto been assumed that the workman’s attention is confined to running one lathe, but cases are found in practice where the lathes, having automatic feed and stop motions, one man can attend to several lathes, and in this case the feeds and speeds may be considerably reduced, so as to give the operator time to attend to a greater number of lathes. As an example, in the use of automatic lathes, several of which are run by one man, the following details of the practice in the Pratt and Whitney Company’s tap and die department are given.
Lathe Number 1.—Lathe turning tool steel 3⁄8 inch in diameter and 11⁄4 long, reducing the diameter of the work 1⁄8 inch. Revolutions of work per minute 125. Feed one inch of tool travel to 200 lathe revolutions.
Lathe Number 2.—Turning tool steel 2 inches long and 1⁄2 inch diameter, reducing diameter 1⁄8 inch. Revolutions of work 100 per minute. Feed 200 lathe revolutions per inch of tool travel.
Lathe Number 3.—Turning tool steel 4 inches long and 7⁄8 inch in diameter, reducing the diameter 1⁄8 inch. Revolutions of work 40 per minute. Feed 200 lathe revolutions per inch of tool travel.
Lathe Number 4.—Turning tool steel 6 to 8 inches long and 13⁄16 diameter, reducing work 1⁄8 inch in diameter. Revolutions of work 35 per minute. Feed 200 lathe revolutions per inch of tool travel.
Lathe Number 5.—Turning tool steel 8 to 10 inches long, and 2 inches in diameter, reducing diameter 1⁄8 inch. Lathe revolutions 30 per minute. Feed 200 lathe revolutions per inch of tool travel.
Lathe Number 6.—Turning tool steel 5 inches long and 31⁄2 inches diameter, reducing diameter 3⁄16. Lathe revolutions 19 per minute. Feed 200 lathe revolutions per inch of tool travel.
The power required to drive the work under a given depth of cut varies greatly with the following elements:—