Precaution When Centering Tool Steel.—Ordinarily centers are so located that the stock runs approximately true before being turned, but when centering tool steel to be used in making tools, such as reamers, mills, etc., which need to be hardened, particular care should be taken to have the rough surface run fairly true. This is not merely to insure that the piece will “true-up,” as there is a more important consideration, the disregard of which often affects the quality of the finished tool. As is well known, the degree of hardness of a piece of tool steel that has been heated and then suddenly cooled depends upon the amount of carbon that it contains, steel that is high in carbon becoming much harder than that which contains less carbon. Furthermore, the amount of carbon found at the surface, and to some little depth below the surface of a bar of steel, is less than the carbon content in the rest of the bar. This is illustrated diagrammatically in [Fig. 30] by the shaded area in the view to the left. (This decarbonization is probably due to the action of the oxygen of the air on the bar during the process of manufacture.) If stock for a reamer is so centered that the tool removes the decarbonized surface only on one side, as illustrated to the right, evidently when the reamer is finished and hardened the teeth on the side A will be harder than those on the opposite side, which would not have been the case if the rough bar had been centered true. To avoid any trouble of this kind, stock that is to be used for hardened tools should be enough larger than the finished diameter and so centered that this decarbonized surface will be entirely removed in turning.
Fig. 31. Three Methods of Facing the Ends Square
Facing the Ends of Centered Stock.—As a piece of work is not properly centered until the ends are faced square, we will consider this operation in connection with centering. Some machinists prefer lathe centers that are cut away as shown at A, [Fig. 31], so that the point of the side tool can be fed in far enough to face the end right up to the center hole. Others, instead of using a special center, simply loosen the regular one slightly and then, with the tool in a position as at B, face the projecting teat by feeding both tool and center inward as shown by the arrow. Whenever this method is employed, care should be taken to remove any chips from the center hole which may have entered. A method which makes it unnecessary to loosen the regular center, or to use a special one, is to provide clearance for the tool-point by grinding it to an angle of approximately forty-five degrees, as shown at C. If the tool is not set too high, it can then be fed right up to the lathe center and the end squared without difficulty. As for the special center A, the use of special tools and appliances should always be avoided unless they effect a saving in time or their use makes it possible to accomplish the same end with less work.
Truing Lathe Centers.—The lathe centers should receive careful attention especially when accurate work must be turned. If the headstock center does not run true as it revolves with the work, a round surface may be turned, but if the position of the driving dog with reference to the faceplate is changed, the turned surface will not run true because the turned surface is not true with the work centers. Furthermore, if it is necessary to reverse the work for finishing the dogged or driving end, the last part turned will be eccentric to the first. Therefore, the lathe centers should be kept true in order to produce turned surfaces that are true or concentric with the centered ends, as it is often necessary to change the part being turned “end for end” for finishing, and any eccentricity between the different surfaces would, in many cases, spoil the work.
