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The New Zealand Railways Magazine, Volume 3, Issue 2 (June 1, 1928)

Speeds and Feeds

Speeds and Feeds.

The old-time engineers did not concern themselves with speeds and feeds. They ran their machines at a steady gait, and what could not be done in twelve hours was done in twenty-four.

The origin of speeds and feeds can not be associated with any particular individual, nor can it be assigned to any definite date. Evolution is the only fair explanation—like Topsy, page 53 they grew—and now, under modern conditions, they occupy an all-important place in the economics of machine shop practice.

Speeds and feeds are determined by the tool endurance and by the length of time it takes to put a sharp tool in the place of a dull one. This does not require a technical table of data; it merely calls for honest attention, a clear head, and a reasonable knowledge of tool steels.

The rapid advance in the use of high speed steel is continually pushing the possible results further forward. Ordinary open hearth steel can be turned at speeds ranging from 50 to 400 feet per minute according to the rate of feed, the cooling medium, and the quality of surface required. A shaft 3 7/8in. diameter (measuring approximately one foot in circumference) and run at 200 revolutions per minute would give a cutting speed of 200 feet per minute. On a shaft of only half that diameter (running at twice that number of revolutions) it would give the same cutting speed in feed per minute.

To enable turret lathe operators to quickly determine the most suitable speeds and feeds, charts are provided by the makers of these machines.

Speeds and feeds are inter-related, and a high cutting speed invariably implies a fine, or slow feed, or vice versa. A lathe cutting at 300 feet per minute with a feed of 100 would not, as might be supposed, be removing more metal in the same time than would another lathe cutting at 160 feet per minute with a feed of 50.
(Photo W. W. Stewart) Outward bound from Auckland. The Rotorua express crossing the Parnell Bridge.

(Photo W. W. Stewart)
Outward bound from Auckland. The Rotorua express crossing the Parnell Bridge.

It is for this reason that machine charts relating to speeds and feeds should, whenever possible, be referred to. Manufacturers have made a long study of this phase of production and have blended feeds and speeds to suit both machine and job, with the object of removing the greatest possible weight of metal per minute.

A third factor has also to be taken into consideration, viz., depth of cut. This is a matter of importance that is, to a very great extent, left to the operator, who must determine it, subject to working conditions, etc.

The feed of a lathe compares exactly with the pitch of a thread. A 20 feed would cut a ¼in. Whitworth thread of 20 to the inch. It is for this reason that a broad-nosed tool is used on a coarse feed, otherwise a thread would be in evidence where a smooth surface is required.

If a perfectly straight turning could be taken off by a turret lathe running at a peripheral speed of 200 feet per minute with a 60 feed, and a cut 3/8in. deep, for every minute of work a shaving or chip 200 feet long, 1/60in. thick, and 3/8in wide, would be the net result. Owing to the stresses and strains set up during the cutting process, distortion takes place and shavings and chips vary in length, form and thickness in spite of a constant speed and feed. It will also generally be found that the thickness of a chip removed on a turret lathe is, owing to distortion, twice as thick as the feed used for its removal.

(To be continued.)