Again, in the case of work having a taper part to be fitted, it is very difficult to obtain the exact proper fit and entrance of taper to an exact distance, hence it is best to leave the work a little too long, with its collars too thick, and to then fit the taper properly and adjust all other end measurements to suit the taper after it is fitted.
Before any one part of a piece of work turned between the lathe centres is finished to diameter, all the parts to be turned should be roughed out, and for the following reasons, which apply with additional force to work chucked instead of being turned between the lathe centres.
It is found, that all iron work changes its form if the surface metal be removed from it. Thus, though the lathe centres be true, and a piece of work be turned for half its length in the lathe, after it has been turned end for end in the lathe to turn the other half of its length, the part already turned will run out of true after the second half is turned up. This occurs from the tension and unequal internal strains which exist in the metal from its being forged or rolled at a constantly diminishing temperature, and from the fact that the surface of the metal receives the greatest amount of compression during the forging.
In castings it is caused by the unequal and internal strains set up by the unequal cooling of the casting in the mould, because of one part being thicker than another.
When the whole of the work surfaces have been cut down to nearly the finished size, this alteration will have taken place, and the finishing may be proceeded with, leaving the work as true as possible. In chucked work, or the most of it rather, it is impracticable (from being too troublesome) to rough out all over before finishing; hence at each chucking all the work to be done at that chucking is finished.
The roughing cuts on a piece of work should always be taken with as coarse a feed as possible, because the object is to remove the mass of the metal to be cut away rather than to produce a finish, and this may be most quickly done by a deep cut and coarse feed. Theoretically also the finishing should be done with a coarse feed, since the coarser the feed, the less the length of time the cutting edge is in action. But the length of cutting edge in action, with a given tool and under a given depth of cut, increases as that edge is made longer to carry the coarse feed, and the long cutting edge produces a strain that tends to spring or bend the work, and that causes the tool to dip into seams or soft spots, or into spongy or other places, where the cutting strain is reduced, and also to spring away from hard spots or seams, where the cutting strain is increased. The most desirable rate of feed, therefore, is that which is as coarse as can be used without springing either the work or the tool, and this will depend upon the rigidity of the work of the lathe, and of the cutting tool. Short or slight work may be turned very true by a light cut fine feed and quick cutting speed, but the speed must obviously be slower in proportion as the length of the work increases, because the finishing cut should be taken without taking the tool out to resharpen it, since it is very difficult to set the tool to the exact proper depth a second time.
Since the cutting edge will, at any given rate of cutting speed, retain its keenness better for a given surface of work in proportion as the time it is under duty is diminished, it follows, therefore, that the coarser the feed the better (so long as both the work and the tool are sufficiently rigid to withstand the rate of feed without springing).
Under conditions of rigidity that are sufficiently favorable a tool, such as in [Fig. 948], may be used on wrought or cast iron, at a feed of 1⁄2 or even 3⁄4 inch of traverse per lathe revolution, producing true and smooth work, providing that the tool be given a very slight degree of clearance, that its cutting edge is ground quite straight, that it is set parallel to the line of feed, or what is the same thing, to the work axis, and that the length of cutting edge is greater than the amount of tool traverse per lathe revolution, as is shown in the figure, the amount of tool traverse per lathe revolution obviously being from a to b. It may also be observed that the leading corner of the tool may with advantage be very slightly rounded as shown, so that there shall be no pointed corner to dull rapidly.
In proportion as the work is light and the pressure of the cut may spring it, the feed must be lessened, so that on very slender work a feed of 100 lathe revolutions per inch of tool travel may be used. On cast-iron work the feeds may be coarser than for wrought-iron, the other conditions being equal, because cast iron cuts easier and therefore springs the work less for a given depth of cut. But since cast iron is apt to break out, exposing the pores of the metal, and thus leaving small holes plainly visible on the work surface, the finishing cut should be of very small depth, indeed a mere scrape; and if the surface is to be polished, a fine feed and a quick speed will leave a cleaner cut surface, and one that will require the least polishing operations to produce a clean and spotless surface. Brass work also is best finished with a fine feed and a quick speed.
It is obvious that the top face of the tool should be given more rake for wrought iron than for cast iron or steel, and that in the case of the very fine feeds, the form of tool shown in figure is the best for finishing these metals.