Hobbing steels are in a highly specialized class of materials. Many pallet mould maker 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 hardened surface has no alloying elements to respond to heat treatments.
Those who prefer this type of steel would rather have the advantage of greater safety in nobbing than the help of alloying elements to withstand more severe service conditions. The alloy hobbing steels must be made with the same care which is employed in the "pure" iron steels.
Both types, because of their specialized application, are neither available nor required in such a wide range of mill sizes and shapes as are the other mold steels. They can be furnished quickly in popular sizes, however.
Hobbed mold cavities are usually made from a stock size steel blank somewhat larger than the finished size of the cavity block. For example, a 2" outside diameter by 1%" long finished hobbed cavity block might be made of a blank 3" to 5" diameter by 2%" to 3%" long. A four inch diameter hobbing blank might be used to make a cavity block 1%" wide by 3" long.
Tool room stocks should not be extensive for this type of material, unless a large amount of hobbing work is done. Generally it is convenient to carry hobbing blanks in stock diameters of 3", 4", 6", 8", to suit available hobbing chase rings, and to carry rectangular blocks of suitable sizes to fit rectangular chase rings which have been found to be convenient for the size of work generally encountered.