Most lathes have either sent with them, or attached on them, a table of the necessary change wheels to be used when screw-cutting. The change wheels supplied with most lathes are those required for usual standard screw threads, but for odd threads it may easily be that other change wheels will be wanted; the obvious thing to do is to avoid the use of such odd threads wherever possible. The whole operation of screw-cutting may be reduced to the very simple statement that to cut say x threads per inch, it is only necessary to make the slide rest move along one inch, while the work turns round x times. Now the slide rest moves along one inch when the guide screw which controls its movements has revolved a number of times equal to its number of threads per inch, Suppose the guide screw be 1/8 in. pitch, that is 8 threads per inch, then when the slide rest has been moved along one inch, the guide screw has revolved 8 times in order to move the slide rest the one inch along the lathe bed. Now suppose that we wish to cut a screw thread on some work in the lathe, and this thread is to have a pitch of |in., that is 8 thread* per inch, also that the guide screw has 8 threads per inch, then all we have to do as regards the change wheels is to arrange that the lathe mandrel and the guide screw revolve at exactly the same number of revolutions each, so that when the work turns round 8 times, the cutting tool fixed on the slide rest will have moved along one inch. As to the actual wheels used, a simple train only is required, i.e., each wheel has its own axle for itself alone, the number of teeth of the wheels on the mandrel and on the guide screw must be the same, but any wheel or wheels may be used between them to connect them; the number of teeth in these connecting -wheels does not matter in the slightest, provided they mesh properly. One point only has to be observed, that is that when the work or lathe is revolved, for the cutting, the slide rest does progress along the lathe in the right direction for the thread required; if it does not then one wheel must be taken out of the train arranged already, or one more additional one must be put in, all this merely to reverse the direction of rotation of the guide screw.

Suppose now that the thread required to be cut is to have 16 per inch, then this simply means that the mandrel has to turn round 16. times instead of 8 times as in the above example. All that is required is to put a wheel on the guide screw that has double the number of teeth as compared to the one on the lathe mandrel. If the above general principle is kept well in mind, there is no need to get into difficulties with any gear ratio required. It is not always possible to use a simple train as above, but in all cases the same principle holds good as regards the wheels to be used. If however any difficulty be found in quickly finding the best wheels to use in cases used frequently then it is suggested that when the best arrangement is found for each case that a small freehand sketch be made showing just what is used, etc., and this be kept for reference. In good screw-cutting it is very essential that correctly formed tools be used both as regards the actual cutting edges, so as to get a good surface on the work, and also as regards the correct angle of the tool used, so as to get a finished thread that will interchange with other screwed parts. It is not possible to grind these correctly, unless an accurate gauge is used to check the angle against; as real accuracy is required here, in order to turn out good work, it is recommended that a gauge for the purpose be bought from those firms making a speciality of such work, and with the use of such a gauge that the tool be most accurately ground to fit it, as and when required. A chaser to go over the threads will probably be required, unless it be finished with a tap, or a standard nut, etc. Many experienced workmen can chase a screw thread using a chaser only, on brass, etc., and can turn out most excellent work, but this requires long practice, and even then interchange-ability as now understood, to within a few thousandths of an inch is seldom attained.