Unless you have a larger than average lathe you will have probably encountered problems
cutting thread sizes in the order of M8 and larger using a normal threading die.
By larger than average I mean anything larger than 100mm bed to centre height.
The problems are twofold, first, the torque required to cut the thread using a die
will be more than the three-jaw chuck can withstand and the part will rotate in the
chuck's jaws. A four-jaw though will have more chance as its grip is greater. Of
greater significance is that, with the torque required, turning the tailstock die
holder in the limited confines of the bed/ saddle/ tailstock will be almost impossible,
and at best very uncomfortable.
Therefore, when requiring these larger threads, there are but two options, one to
screw cut the thread on the lathe, or alternatively, to move the part to the vice
and make the thread using the die in a diestock. Perhaps there is a third option
and that is a combination of the two, partially screw cut the thread and finish off
using a die.
If a die of the required size is available this would seem the easy and quicker method
as screw cutting it would be a relatively slow process. This would be particularly
so if the lathe does not have a screw cutting gearbox and changewheels have to be
Accurately starting the thread with the part in the vice can though be a problem
and if the thread is on the long side it is likely to wander becoming worse the longer
the thread. In an attempt to live with the problems of chuck grip and torque required,
I have tried starting the first few threads on the lathe followed by the diestock
and the part held in the vice. Even with this method I have found that the tendency
to wander is still a problem.
If the thread being cut is just a piece of studding for use as a clamping stud on
the milling machine then the wandering thread can probably be tolerated albeit not
visually very pretty. However, it was when attempting to use an M10 thread for a
leadscrew that I realised that it was either a screw cut job or I had to make a guided
die holder. As my lathe is an imperial one screw cutting the thread was ruled out
and I set about making this guided die holder.