Screw Cutting on the lathe 3

Harold Hall


Method three

Many lathes do not have a slow enough speed for screw cutting and owners of these lathes use a mandrel handle to turn the workpiece, in which case stopping exactly at the required point is very easy. Even so, some workshop owners who have a lathe with a slow speed still prefer to use the method as it needs less concentration, so it is not restricted to those who do not have a slow speed available.


Recess, yes of no?

In my case, having a clutch, I rarely provide a recess at the end of the thread. However, providing one, say one pitch wide, will make the process easier, especially if using method two.


This is because, if the user is a little late in stopping the lathe and it runs on a little further, then it will avoid the cutter suddenly being called upon to cut a full depth thread. For my part, I tend to stop the lathe just a very little earlier as the thread deepens. Whilst I do this it is not something I recommend unless a recess has to be avoided. See Photograph 2 showing that a recess has been made.


Setting the chaser

The chaser should be set such that it only cuts on the first tooth. To do this, bring the chaser up to the workpiece so that it touches it at its leading tooth but just clears at the final tooth, say 0.05mm to 0.01mm. The resulting angle is so small that it will have negligible effect on the thread being cut.  


Being used in this way, only the first tooth is used with the remainder having no purpose, other than to ease setting as above. Because of this the teeth can be ground away making it possible to use the chaser in the same way as is normally used for screw cutting. That is, producing a recess for the cutter to run into and using the lathe's half nut to stop the cutter traversing but with the lathe still running.


For me, I prefer to leave the chaser as made, as should the first tooth become damaged it can be ground away still leaving teeth available that can be used.


Workshop Projects

Using a metric chaser on an imperial lathe, or visa versa.

The viewer will no doubt understand that in this case a perfect pitch is rarely achieved and a very small error has to be accepted. Because of this, the pitch being cut will not match perfectly with that of the chaser. Just how much a problem this will cause will depend on the degree of the error as some pitches result in greater errors than others. My recommendation is, though I have never had the need to use it, is to grind away the trailing teeth as suggested above


Using Tipped threading tools

Do also view my pages on using indexable tipped tools for cutting threads for more essential information.



Screw cutting on the lathe



When used in a Coventry die head, chasers are diameter specific, typically, M8 x 1.25. However, when used singularly the die could be used for any diameter that requires a 1.25mm pitch thread.