Cutting helices on the lathe or not
Most larger lathes have a leadscrew. The leadscrew is driven, via many gears from the headstock spindle. The leadscrew is connected to the saddle so, if everthing is connected together, the saddle can be driven along automatically. This can be used for surfacing cuts or for cutting threads.
fig the gear train from the spindle to the gear box
fig the gear box
Usually the lathe is supplied with a table showing what combination of gears will give what threads.
In the case of the Colchester Bantam the coarsest metric pitch that can be cut is 6mm.
The angle at which cutting occurs
If the cutter is not moving left/right and it engages the workpiece then the force of the lathe is acting directly downwards on the cutter. There are times when is happens and can produce useful results. But much of the cutting that is done is with the cutter moving against the workpiece as the cutter moves left/right or in/out.
In all these cases the force of the workpiece is acting downwards at an angle on the cutter. Usually the angle is small enough that it can be ignored.
When cutting screwthreads the workpiece moves deliberately so that the cut is at an angle. It is easy to see that if the angle becomes very large two things are going to happen firstly the shape cut by the cutter will not be the same as if the cut was vertical, Secondly, the forces acting on the workpiece the vertical force will decrease but, at the same time, more and more force will be needed to move the workpiece as it cuts to the left or right.
Cutting threads with long pitches
This cannot be done with the standard equipment found on a lathe. This is because the gears available cannot drive the saddle fast enough. Furthermore they cannot apply the force required.
Of course, one should always bear in mind, “is there a problem that needs me to be able to do this?”. One problem if rifling a gun barrel.This is normally done by pulling a cutter through the barrel as it slowly turns. There are special lathes made for doing this. In most cases, pulling the cutter is done quite separately from anything else. The rotation of the barrel is done with a mechanism closely related to the tradition taper turning device.