In connection with lathe work, special attachments and tools are often used, especially when considerable work of one class must be turned; however, if a certain part is required in large quantities, it is usually more economical to use some semi-automatic or automatic turning machine, especially designed for repetition work.
=Turning with Front and Rear Tools.=--In ordinary engine lathe practice, one tool is used at a time, but some lathes are equipped with tool-holders at the front and rear of the carriage so that two tools can be used simultaneously. Fig. 31 shows a detail view of a lathe in which front and rear tools are being used. These tools are of the inserted cutter type and the one at the rear is inverted, as the rotary movement of the work is, of course, upward on the rear side. This particular lathe was designed for taking heavy roughing cuts and has considerable driving power.
[Illustration: Fig. 31. Front and Rear Tools used for Roughing]
The part shown in this illustration is a chrome-nickel steel bar which is being roughed out to form a milling machine spindle. It is necessary to reduce the diameter of the bar from 5-7/16 inches to 3-3/4 inches for a length of 27 inches, because of a collar on one end. This reduction is made in one passage of the two tools, with a feed of 1/32 inch per revolution and a speed of 60 revolutions per minute. The use of two tools for such heavy roughing cuts is desirable, especially when the parts are required in large quantities, because the thrust of the cut on one side, which tends to deflect the work, is counteracted by the thrust on the opposite side.
[Illustration: Fig. 32. Lo-swing Lathe for Multiple Turning]
Sometimes special tool-holders are made for the lathe, so that more than one tool can be used for turning different surfaces or diameters at the same time, the tools being set in the proper relation to each other. The advantage of this method has resulted in the design of a special lathe for multiple-tool turning.
=A Multiple-tool Lathe.=--The lathe shown in Fig. 32 (which is built by the Fitchburg Machine Works and is known as the Lo-swing) is designed especially for turning shafts, pins and forgings not exceeding 3-1/2 inches in diameter. It has two carriages _A_ and _B_ which, in conjunction with special tool-holders, make it possible to turn several different diameters simultaneously. At the front of this lathe there is an automatic stop-rod _C_ for disengaging the feed when the tools have turned a surface to the required length. This stop-rod carries adjustable stops _D_ which are set to correspond with shoulders, etc., on the work. The rod itself is also adjustable axially, so that the tools, which are usually arranged in groups of two or more (depending upon the nature of the work), can be disengaged at a point nearer or farther from the headstock as may be required, owing to a variation in the depth of center holes. For example, if it were necessary to feed a group of tools farther toward the headstock after they had been automatically disengaged, the entire rod with its stops would be adjusted the required amount in that direction.