In screw cutting the cutting tool requires to be withdrawn from the thread while the carriage traverses back, and it is somewhat difficult to know just how far to move the tool in again in order to put on a proper depth of cut. To facilitate this the device shown in [Fig. 585] (which is taken from the “American Machinist,”) is sometimes employed.
It consists of a ring c inserted between the cross slide d and the handle hub b having journal bearing on and rotating with the latter. When the first cut is put on, the mark on c is coincident with that on d, and the ring is then, while the first cut is traversing, moved (supposing the cross feed screw to have a right-hand thread) to the left, as shown in the figure, to the amount the handle will be required to move to the right to put on the next cut, and when the next cut is put on the handle will be moved the distance it was moved to withdraw the tool for the back traverse, and in addition enough to make the marks coincide, then while the second cut is being taken the ring is again moved to the left, as in the cut, to give the depth of cut for the next traverse, and so on.
If the cross feed screw has a left-hand thread, the mark on the ring would require to be moved to the right instead of to the left of the mark on d. It is obvious that this answers the same purpose, but is more exact than the chalk mark before referred to, and, indeed, that chalk mark could be used in the same way, leaving the chalk mark d and rubbing out that on c while the cut is proceeding and making a new one for the next cut.
Fig. 586.
Another device for use on lathes specially designed for screw-cutting is shown in [Fig. 586], in which a represents the cross feed screw. It is fast to the notched wheel b, and is operated by it in the usual way. c is a short screw which provides journal bearing for the screw a by a plain hole. It is screwed on the outside, and the plate in which it fits acts as its nut. It is fast to the handle d, and is in fact operated by it. The handle or lever is provided with a catch e, pivoted in the enclosed box f, which also contains a means of detaining the catch in the notches of the wheel, or of holding it free from the same when it is placed clear. If, then, the lever d be moved back and forth the feed screw a, and hence the slide rest, will be operated; while, if the catch be placed in one of the notches on the wheel b, both the screws, a and c, will act to operate the rests. When, therefore, the tool is set to touch the diameter of the work, the catch e is lifted and the feed wheel b rotated, putting on the cut until the catch e will fall into the next notch in b, the lever d resting in the meantime on the stud g. When the cut is carried along the work to the required distance the tool is withdrawn by moving d over until it rests upon stud or stop h. While the slide rest is traversing back e is lifted and b rotated so that e will fall into the next notch, and when the tool starts forward again d is moved over from h to g, as shown in the figure, and the tool cut is put on.
When the device is not required to be used e is thrown out, d rests on e, and the feed is operated in the ordinary manner.
