Feed of Tool and Depth of Cut.—The amount of feed and depth of cut also vary like the cutting speed, for different conditions. When turning soft machine steel the feed under ordinary conditions would vary between 1/32 and 1/16 inch per revolution. For turning soft cast iron the feed might be increased to from 1/16 to 1/8 inch per revolution. These feeds apply to fairly deep roughing cuts. Coarser feeds might be used in many cases especially when turning large rigid parts in a powerful lathe. The depth of a roughing cut in machine steel might vary from 1/8 to 3/8 inch, and in cast iron from 3/16 to 1/2 inch. These figures are intended simply to give the reader a general idea of feeds and cuts that are feasible under average conditions.
Ordinarily coarser feeds and a greater depth of cut can be used for cast iron than for soft steel, because cast iron offers less resistance to turning, but in any case, with a given depth of cut, metal can be removed more quickly by using a coarse feed and the necessary slower speed, than by using a fine feed and the higher speed which is possible when the feed is reduced. When the turning operation is simply to remove metal, the feed should be coarse, and the cut as deep as practicable. Sometimes the cut must be comparatively light, either because the work is too fragile and springy to withstand the strain of a heavy cut, or the lathe has not sufficient pulling power. The difficulty with light slender work is that a heavy cut may cause the part being turned to bend under the strain, thus causing the tool to gouge in, which would probably result in spoiling the work. Steadyrests can often be used to prevent flexible parts from springing, as previously explained, but there are many kinds of light work to which the steadyrest cannot be applied to advantage.
Fig. 15. Roughing Cut—Light Finishing Cut and Coarse Feed
The amount of feed to use for a finishing cut might, properly, be either fine or coarse. Ordinarily, fine feeds are used for finishing steel, especially if the work is at all flexible, whereas finishing cuts in cast iron are often accompanied by a coarse feed. [Fig. 15] illustrates the feeds that are often used when turning cast iron. The view to the left shows a deep roughing cut and the one to the right, a finishing cut. By using a broad flat cutting edge set parallel to the tool's travel, and a coarse feed for finishing, a smooth cut can be taken in a comparatively short time. Castings which are close to the finished size in the rough can often be finished to advantage by taking a single cut with a broad tool, provided the work is sufficiently rigid. It is not always practicable to use these broad tools and coarse feeds, as they sometimes cause chattering, and when used on steel, a broad tool tends to gouge or “dig in” unless the part being turned is rigid. Heavy steel parts, however, are sometimes finished in this way. The modern method of finishing many steel parts is to simply rough them out in a lathe to within, say, 1/32 inch of the required diameter and take the finishing cut in a cylindrical grinding machine.
Effect of Lubricant on Cutting Speed.—When turning iron or steel a higher cutting speed can be used, if a stream of soda water or other cooling lubricant falls upon the chip at the point where it is being removed by the tool. In fact, experiments have shown that the cutting speed, when using a large stream of cooling water and a high-speed steel tool, can be about 40 percent higher than when turning dry or without a cooling lubricant. For ordinary carbon steel tools, the gain was about 25 per cent. The most satisfactory results were obtained from a stream falling at a rather slow velocity but in large volume. The gain in cutting speed, by the use of soda water or other suitable fluids, was found to be practically the same for all qualities of steel from the softest to the hardest.
Cast iron is usually turned dry or without a cutting lubricant. Experiments, however, made to determine the effect of applying a heavy stream of cooling water to a tool turning cast iron, showed the following results: Cutting speed without water, 47 feet per minute; cutting speed with a heavy stream of water, nearly 54 feet per minute; increase in speed, 15 per cent. The dirt caused by mixing the fine cast-iron turnings with a cutting lubricant is an objectionable feature which, in the opinion of many, more than offsets the increase in cutting speed that might be obtained.