Fig. 1915.
When the cutter is required to cut on the sides of the work as well as on its upper face it is termed a face cutter, and its side faces are provided with teeth, as shown in [Fig. 1914]; and when these cutters are arranged in pairs as in [Fig. 1915], so as to cut in the side faces only of the work d, they are termed twin or straddle mills, both being of the same diameter.
Fig. 1916.
In mills or cutters used in this way the cutting duty is excessive on the outer corners of the teeth, which, therefore, rapidly dull; hence it is usual to provide teeth on both sides of the cutter, as in [Fig. 1916], so that after having been used in the position shown in the engraving until the teeth are dull the positions of cutters may be changed, bringing the unused cutting edges into use.
Twin or heading cutters are right and left hand, a right-hand one being that in which the teeth at the top of the wheel revolves towards the right, while a left-hand one revolves (at the top) towards the left.