Hobbing steels are in a highly specialized class of materials. Many China mold makers prefer to use a practically pure iron for hobbing mold cavities. It is the softest and easiest to work of all of the hobbing steels, although it must not be subjected to high crushing loads because the soft center core metal below the case hard-high speed steels require a considerable amount of heating time. In such cases, the advantage of minimum time in the furnace is not always clear cut.
Some bumper mold making tooling applications require the use of tool steels because carburizing steels are not applicable. Hobs, for example, must be extremely hard at the surface, and the hardness should extend rather deeply into the center in order to make the hob resistant to the severe crushing which it must stand, and to have high tensile strength, as well. Hobs may be made of air hardening, water hardening, or oil hardening tool steels. Oil hardening is usually preferred, with water hardening steelthe least used, and then generally only for small, simple pieces not critically subject to cracking in heat treatment, nor subject to heavy hobbing pressures.
Tool steels are furnished normalized, annealed, or hardened to the limit of machinability. Often, it is satisfactory to have a mold part made of a pre-hardened steel and omit heat treating it after machining. This is a practice which must be pursued carefully, however, and the steelmaker’s recommendation should be sought before proceeding. Usually not much depth of machining should be made on a pre-hardened block of steel if it is thick, because of possible non uniformity of the steel structure due to the thickness. Pre-hardened steels are useful in small plastic mold company which do not have proper hardening facilities, and they are useful when mold parts must be made quickly, without risking possible spoilage in heat treating. If long life and severe service conditions are factors in the use of a mold, pre-hardened steels are usually not satisfactory, because they cannot be hardened enough to stand the service conditions without becoming non machinable.
Hot rolled tool steels are available in rounds, flats, and squares, in sizes from Vk” to 8” diameter, from x to x 10″ or 12”，and from V4n to 6″ square. Larger sizes may be obtained as forgings. Specially finished tool steels are also available. Drill rod is accurately polished to fractional, decimal, number, and letter drill sizes. It is obtainable as plain carbon, water hardening steel, or
as high speed steel. Ground flat tool steel, plain carbon, is available. This material is used for gages, shims, templates, retainers, parallels, gibs, and for many other applications which may conveniently use a stock which is finished all over.