Henry Jarvis

The chasing of inside and outside threads in the lathe is a task essayed by a great many amateurs, but their work is often attended with very poor results, the cause usually being either that they do not go the right way to work, or that they have not sufficient perseverance to survive the first failure. Now there is no reason whatever why one and all should not be successful in screw chasing, the necessary appliance being few and cheap and the experience required being very little.

First as to tools required. These consist of one or more pairs of chasers (one for the inside and one for the outside screw thread) which must be of such a pitch as to suit the work in hand. A good size for a beginner is sixteen or eighteen teeth to the inch, which will cut a thread suitable for small boxes. It is well not to commence with either very fine or very coarse work.

The shape of the inside chaser is shown in Fig. 1, and a front view of the same in Fig. 2, while Fig. 3 shows the outside tool. The teeth of both the inside and outside tools are made at an angle, as in Fig. 2, to suit the pitch of the screw they will cut ; they are also made at a slight angle with the side of the tools, so as to give the necessary clearance.

For the sake of illustration we will suppose that a small box has to be provided with a screw-on lid and that it is already turned up to the proper sizes, ready for chasing. This is commenced as in Fig. 4. The box must be held in the lathe in a chuck, or if too long for this, a mandril can be fitted into the opening, to enable it to be mounted between the centers. The outside chaser is held flat on the rest and fed up to the box, so that the first tooth will just catch in the wood ; and if the lathe is turned very slowly, the beveled teeth of the chaser will draw themselves evenly and truly, until the side of the tool touches the shoulder of the box, as in Fig. 5.

If this has been done and no slipping has occurred, the wood will now be as in Fig. 6, the threads of the screw being represented by slight cuts only, scarcely more than scratches. The same process must be repeated, deepening the scratches slightly, as in Fig. 5, and gradually going over them again and again, until the finished result is as Fig. 7.

The tool will be found to work best if the cutting part is on an exact level with the lathe centers, as in Fig 8, where A represents the body of the box, B the reduced part on which the screw thread has to be chased, C the chasing tool and D the rest.

Now for the inside chasing, which will be found the most difficult to do. The box lid must be mounted truly in a chuck of some description, and the rest fixed close up to it, across the end as in Fig. 9. The opening should be chamfered slightly at E to enable the tool to start easily, and the tool is then placed so that it will just bite at the commencement, when it should draw itself along, making a slight thread only, but the same depth throughout, until it reaches the end of the opening, as in Fig. 10. The thread must be gradually deepened in the same way as before until it is cut to the full depth as in Fig. 11, or until the first part will easily screw into it. As it approaches the finish it is necessary to frequently try this, or the lid may be spoilt through making it too large.

All this looks very easy on paper, and it looks nearly as simple to watch an experienced person doing it, but for the novice there are many pitfalls which it will be well to describe so that they may be avoided. The first risk lies in taking too deep a cut, this having the same effect as in ordinary fine turning, tearing the work about and throwing it out of the center and causing a general upset. The remedy is obvious. Use the tool carefully. Another fault is running the lathe at too high speed ; this causes all command over the tools to be lost, and when the shoulder is reached, so that the tool can go no further, it cuts off the threads already made. The remedy is to run the lathe as slowly as possible, even to dispense with the treadle and turn the mandril by hand; or to borrow the ser-services of an obliging friend to do so, at least during the first stages of experimenting with the tools.

Another very frequent fault with beginners is allowing the tool to draw itself in or out, after starting at the proper depth. This is caused through not holding the tools parallel or at right angles with the work at the commencement. It is easily rectified in the outside tool, but the fault is not apparent in the inside one until the mischief is done, hence care is required in this respect.

Another mistake often made, especially by novices, is to turn the two parts so that they fit one in the other before the threads are chased, so that when the latter is done the lid is of course too large for the other part. For threads of the size mentioned above, about an eighth of an inch should be allowed ; it need not be done to any exact size, as it is easy to take an extra cut if the parts fit too tightly.

It is always best to chase the outside thread first, as this can then be fitted into the other as required, without disturbing the center. And it is as well to chase the inside thread while the wood is mounted on the original chuck, cutting the lid off and finishing after the thread is cut. When a very short screw is wanted, it should be cut off to the length after chasing, as it is much easier to make a screw of say an inch long, than it is one of three or four threads only ; in fact, it is as well to always allow for cutting off a short length at the end of a thread, as it then makes a clean finish, or rather beginning, which is not always the case when the turning is finished to length before chasing.

When using either the inside or outside tool, it must be held quite straight with the work and also perfectly horizontal, as shown in the drawing (Fig. 8) the latter especially being very important. Do not attempt o chase screws on very soft wood ; a close, hardgrained wood is the best, especially for trial.

If all the points here mentioned are attended to. there should be no difficulty in turning out good screws, but if any one is neglected, failure will certainly result. - Hobbies, London.