Other formats

    TEI XML file   ePub eBook file  


    mail icontwitter iconBlogspot iconrss icon

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

Tools Of Steel — Part VIII

page 52

Tools Of Steel

“Our existence as a nation depends upon our manufacturing capacity and production.“—Joseph Chamber'ain.

What is a Turret Lathe?

The turret lathe is an improved engine lathe fitted with a compact set of tools capable of doing either bar or chuck work without the necessity for constantly changing the tools and fixtures.

The labour-saving devices of the turret lathe are legion, and the more this machine tool is used in competition with its predecessor, the engine lathe, the more conspicuous does this fact become. It is incorrect to regard this machine as a unit for repetition work solely. Its universal adoption by the most progressive manufacturers whose production shops have to cater for new work, and repair work in small batches, demonstrates the wide range of its utility.

(Photo W. W. Stuwart) Welding a flange on a locomotive wheel at Newmarket shops.

(Photo W. W. Stuwart)
Welding a flange on a locomotive wheel at Newmarket shops.

A Glimpse into History.

The efficiency of the turret lathe did not add to its popularity with tradesmen of the old school. When first introduced to the engineering fraternity its reception was not too cordial. From a distinct labour point of view, this was a blunder which can be best appreciated by those who have moved with the times.

It would be interesting to know what Henry Maudslay would say if he could make a tour of our new railway workshops when the work therein is in full swing, and see some of the latest bar and combination lathes in operation. Henry Maudslay, just over 125 years ago, invented the first slide rest lathe with an improvised screw-cutting attachment, which to-day can be seen in the South Kensington Museum, London. His first plain slide rest lathe is now the property of the British Machine Tool Trades’ Association, and it was exhibited by that Association at the 1924 Machine Tool Exhibition.

The lathe of Henry Maudslay was made before the dawn of the planing machine. The present-day fitter sometimes pictures to himself that old-time craftsman toiling with his chisel and hammer, file, callipers, and wooden rule—with occasional reference to an old shilling that was used as a sort of step gauge.

Maudslay had very little schooling. He commenced work in the Woolwich Arsenal at the age of twelve, in the year 1769. He was a born craftsman, however, and with his skilll and dexterity he combined an intuitive power of mechanical analysis and a fine sense of proportion. At the age of nineteen he was a workshop foreman, and at twenty-six a workshop manager — drawing the handsome salary of 30/- per week.

Evidently not too satisfied with a big position and a small salary, Maudslay started in business for himself, founding the world-famed firm of Maudslay, Son and Fields. This firm subsequently produced some of Great Britain's finest engineers, including Sir Joseph Whitworth, who served the first years of his apprenticeship under this grand old champion.

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.)