Fig. 1853. Fig. 1854.
The position of the dies in the head and with relation to the work is, in bolt cutting machines, a matter of great importance, and in all cases the dies should be held in the same position when being hobbed (that is, having their teeth cut by the hob or master tap) as they will stand in when put to work, and the diameter of the hob must be governed by the position of the dies in the head. If they are placed as in [Fig. 1852] the diameter of the hob must be 1⁄32 inch larger than the diameter of bolt the dies are intended to thread, so that the point or cutting edge may meet the work first and the heel may have clearance, it being borne in mind that the clearance is less at the tops than it is at the bottoms of the teeth, because of their difference in curvature. In this position the teeth are keen and yet retain their strength, acting somewhat as a chaser. If placed in the position shown in [Fig. 1853] the hob or master tap must be 1⁄32 inch smaller than the diameter of bolt they are to thread, so as to give the teeth clearance. In this case the dies are somewhat harder to feed into their cut and do not cut quite so freely, but on the other hand they work more steadily as the bolt is better guided, while left-hand dies may be used in the same head. If placed as in [Fig. 1854] they must be cut with a hob 1⁄32 inch larger in diameter than the bolt they are to thread, so that the teeth will have less curvature than the work, and will, therefore, have clearance. In this position the dies do not cut so freely as in [Fig. 1852].
The dies should be broad enough to contain at least as many teeth as there are in a length of bolt equal to its diameter, and should be thick enough to withstand the pressure of the cut without perceptible spring or deflection.
Fig. 1855.
The cutting edges of dies may be brought in their best cutting position and the dies placed in radial slots in the head by forming the dies as in [Fig. 1855]. Face x is at an angle of 18° to the leading or front face of the die steel, and the heel is filed off at an angle of 45° and extends to the centre line of the die. This gives a strong and a keen die, and by using a hob 1⁄32 inch smaller than the diameter of bolt to be cut, the clearance is sufficiently maintained.