with a VFD..  still using steppers as that's what I'm familiar with..

Set the speed manually on the VFD. Having a variable speed will screw up the thread cycle.

These threads are the internal threads on the faceplates to match the spindle nose.

Try cutting an external thread and see what class thread you are achieving. The spindle nose threads are usually
ground and yes you want a matching tolerance. A shop would chase them.

RICH

Cheers Rich and hood,

I figured out the manual speed part..

Internal and external threads are now perfect when cut in machinable wax.
The size of the swarf is now well within the machine's capability

I have to do a bit of fettling on the machine before I cut the final version in steel..
I have a grub screw that keeps wanting to undo itself right at the critical moment!

No point in rushing when I am tired...Fingers crossed for tomorrow !

The accuracy required and difficulty of machining this particular thread is as hard as I want to go.
I'd much rather machine a nice easy fine thread..  but no choice..  I can't get anything ready made with the correct thread.
It seems to be a bit of a hybrid, all metric apart from the thread angle of 55 degrees.

At least i know a lot more about the workings of Mach, and the capabilities of my lathe now !

Well,

to round this thread off.  

success !

I managed to cut a well matched thread to fit the spindle nose.. first attempt in steel.

It can be taken off and replaced with no detectable runout..  yippee !

I broke the threading tool after I had cut the thread to size, attempting to really get a good finish....  my stupidity, I was tired by then
but it was only a few quid and well worth sacrificing to be able to mount my 25Kg 3 jaw chuck.

Now I understand the wizards a little better, the task seems less daunting.. and I will make a couple more..

thanks again for the help

Happy it turned out well.

RICH