In turning a number of pieces requiring to be of the same diameter, it is to be borne in mind that a great part of the time is consumed in accurately setting the tool for the finishing cut, and that if one piece is finished at a time, this operation will require to be done separately for each piece.
It is more expeditious, therefore, to rough all the pieces out, leaving enough metal for a fine finishing cut to be taken, and then finish these pieces without moving the tool; which may be done, after the tool is once set, by letting the tool stand still at the end of the first finishing cut, and taking the work out of the lathe. The carriage is then traversed back to the dead centre, and another piece of work is put in, and it is obvious that as the cross-feed screw is not operated after the tool is once set, the work will all be turned to the same diameter without any further measuring than that necessary for the first piece.
If the tool is traversed back to the dead centre before the lathe is stopped or before the work is removed from the lathe, one of two results is liable to follow. If the lathe is left running, the tool will probably cut a spiral groove on the work, during its back traverse; or if the lathe be stopped, the tool point will mark a line along the work, and the contact of the tool point with the work will dull the cutting edge of the tool. The reason of this is as follows: When the slide rest and carriage are traversing in one direction, the resistance between the tool and the cut causes all the play in the carriage and rest, and all the spring or deflection of those parts, to be in an opposite direction. Now if the play and spring were precisely equal for both directions, the tool should cut to an equal diameter with the carriage traversed in either direction, but the carriage in feeding is fed by the feed nut or friction feed device, while when being traversed back the traversing handle is used; thus the power is applied to the carriage in the two cases at two different points, hence the spring of the parts, whether from lost motion, or play from wear, or from deflection, is variable. Again, even with the tool fed both to its cut and on the back traverse with the hand feed handle, the play is, from the altered direction of resistance of the cut, reversed in direction, and the depth of cut is therefore altered.
Fig. 1208.
Fig. 1209.
Thus, in [Fig. 1208], let s s represent the cross slide on the carriage and r r the cross slide of the tool rest shown in section, and suppose the tool to be traversing towards the live centre, then to whatever amount there may be play or spring between the slide and the slide way, the slide will from the pressure of the cut twist over, bearing against the slide way at a and b, and being clear of it at g and h. On reversing the direction of traverse of the rest, so as to feed the tool towards the dead centre, the exactly opposite condition will set in, that is, the pressure of the cut will force the slides in the opposite direction, or in other words, the contact will be as in [Fig. 1209], at c, d, and the play at e, f. During the change of location of bearing between the slides and the way, there will have been a certain amount of tool motion altering the distance of the tool point from the line of centres, and therefore the diameter to which it will cut. The angle at which the body of the tool stands will influence the effect: thus, if when traversing towards the live centre the tool stands at an angle pointing towards the live centre, it would recede and cause the tool to clear the cut, if removed on the back traverse without being moved to or from the line of centres. Conversely, if the body of the tool was at an angle, so that it pointed towards the dead centre, and a cut was taken towards the live centre, and the tool was traversed back without being moved in or out, it would take another cut while being moved back.