Fig. 749.
Fig. 750.
The methods of overcoming these two sources of error are as follows: Instead of the end of the dog being bent around to pass within the slot in the face plate, as in [Fig. 745], the leverage a in that figure may be avoided by the means shown in [Fig. 748], in which a driver having straight ends is used, and a pin p is fastened to the face plate to drive the carrier. But this does not remove the tendency (shown in [Fig. 746]) acting to spring the work from the pressure of the cut; hence, to obviate this latter tendency, two driving-pins p p, in [Fig. 749], are sometimes used with the idea of driving the work from both sides, and thus equalizing the strain. But this is effective only when each pin is in working contact with the dog. This condition is difficult to secure for several reasons. First, suppose the two ends of the carrier to be of equal thickness, and the driving-pins to be of equal diameter, while the work receiving hole of the carrier is quite central to these two ends, then the work also must be true, in order to cause the pins to act equally on the ends of the carrier. Hence, this method is only applicable, even if all the above conditions be fulfilled, to the finishing cuts, and these would have to be taken on work that had been sprung in the roughing cuts, so that it would be difficult to obtain accurate results. A nearer approach to correctness is therefore sought by various means. Thus, [Fig. 750] represents a face plate provided with an annular T-groove, having a cut at h to admit two nuts into which the pins p are screwed. These pins may be tightened lightly, so that they will slip under the pressure of the roughing cut, and thus come to an equal bearing upon the carrier or work, as in case of the arms of a pulley where a carrier is not used. When the pins have adjusted themselves to have as near as may be an equal driving bearing, they may be tightened up. By this means the pins are compelled to act at an equal leverage upon the carrier or work, but there is no assurance of an equal degree of pressure of the pins p.
Fig. 751.