Now suppose that the tool-carrying slide a, [Fig. 1496], is traversing forward and the tool will be moving across the work on the cutting stroke, as denoted by the arrow k in [Fig. 1502], the line of tool motion for that stroke being as denoted by the line c a. At a is the point where the tool will begin its return stroke, and if the work is moved by the feeding mechanism in the direction of arrow e, then the line of motion during the return stroke will be in the direction of the dotted line a b, and as a result the tool will rub against the side of the cut.
Fig. 1503.
It is to obviate the friction this would cause to the tool edge, and the dulling thereto that would ensue, that the pivot pin l for the apron is employed as shown in [Fig. 1497], this pin permitting the apron to lift and causing the tool to bear against the cut with only such force as the weight of the apron and of the tool may cause. Now suppose that in [Fig. 1503] we have a piece of work whose edge a a stands parallel to the line of forward tool motion, there being no feed either to the tool or the work, and if the tool be set to the corner f its line of motion during a stroke will be represented by the line f g. Suppose that on the next stroke the feed motion is put into action and that feeding takes place during the forward stroke, and the amount of the feed per stroke being the distance from g to h, then the dotted line from f to h represents the line of cut. On the return stroke the line of tool motion will be from h along the dotted line h k, and the tool will rest against the cut as before. Suppose again that the feed is put on during the return stroke, and that c c′ represents the line of tool motion during a cutting stroke, and the return stroke will then be along the line from c′ to b, from c to b representing the amount of feed per stroke; hence, it is made apparent that the tool will rub against the cut whether the feed is put on during the cutting or during the return stroke. Obviously then it would be preferable to feed the work between the period that occurs after the tool has left the work surface on the return stroke and before it meets it again on the next cutting stroke. It is to be observed, however, that by placing the pin actuating the rod e, [Fig. 1501], on the other side of the centre of the slot g in f, the motion of e will be reversed with relation to the motion j of the slide; hence, with the work feeding in either direction, the feed may be made to occur during either the cutting or return stroke at will by locating the driving pin on the requisite side of the centre of g.
Fig. 1504.
An arrangement by Professor Sweet, whereby the feed may be actuated during the cutting or return stroke (as may be determined in designing the machine), no matter in which direction the work table is being fed, is shown in [Fig. 1504]. Here there are two gears a and d, and the pawl or catch c may be moved on its pivoted end so as to engage either with a or d to feed in the required direction.
Suppose the slide to be on its return stroke in the direction of l, and f be rotated as denoted by the arrow, then the pawl c will be actuating wheel a as denoted by its arrow, but if c be moved over so as to engage d as denoted by the dotted outline, then with the slide moving in the same direction, c will pull d in the direction of arrow k′, and wheel a will be actuated in the opposite direction, thus reversing the direction of the feed while still causing it to actuate on the return stroke.