Mach3 Lathe Threading Struggles

Welcome and thanks for reading my post.
So basically I'm trying to cut some threads on my Boxford Model B that I converted to CNC and am using Mach3 to control it.
The spindle is driven by a VFD, controlled by a breakout board 0-10v signal.
I have a spindle feedback system installed and it's fairly steady (the fluctuation is about 3rpm).
If I have the Closed Loop Spindle Control ticked, the threading program runs fine but it seems like every pass is slightly out in relation to the previous one and the final thread is all messed up.
If I have the Closed Loop Spindle Control unchecked, the program starts to run but pauses just before it is about to cut the thread. Is it waiting for the RPM to stabilise?
If I have Use Spindle Feedback In Sync Mode checked, the same thing happens as above.
Please have a look at the attached pictures, I hope they will help you guys diagnose the problem.
Any help would be much appreciated.

Mach 3 cannot do succesful threading without a motion controller.
It's single pulse effort never worked unless it had a very high inertia motor to keep it at exactly the same speed along the thread. The box ford hasn't got this!

Thank you for your reply, I should have said that I'm now using a 1hp 3 phase motor to drive the spindle, it doesn't seem to slow down while cutting.
Anyway, what is bugging me is that the threading program won't start if the "Use Spindle Feedback In Sync Mode" is checked. And it needs to be checked according to some sources.
Also, some people report that they have good results in threading using the parallel port :
https://www.machsupport.com/forum/in...p?topic=9861.0
For this reason I'm thinking it's a matter of getting the settings right... But you guys would know better.
Does anyone have other ideas?

i tried for ages to get this working on my boxford- just kept breaking tools!
buy uccnc or pokeys- or one of the other threading controllers!

A friend of mine struggled to get his lathe threading again with Mach3 after someone swapped his failed inductive pickup for an optical one, I widened the slot in his rotor to around 5 degrees which seemed to sort it out.

There are plenty people who've successfully threaded with Mach and a parallel port. IIRC there was an issue with the parallel port on it not being able to handle the spindle speeding up during a threading move, but Art fixed it once somebody could consistently reproduce the problem.

Have you ran through the Mach threading guide? It's buried somewhere on the Mach forum.
It's a long time since I've used Mach3 on the lathe, so I'd say you're far more likely to get help on the Mach forum, but make sure you mention you're using a parallel port, and post your configuration XML so people can check your settings.

the problem with mach threading is its handled in the pc. the pc can be interupted by anything else - so can never be real time.
i tried every combination to get succesful threading- but it wasnt repeatable enough. - ie if 1 pass out of 100 fails to sync properly- the tool is broken and the thread ruined.
( i even updated my boxford with mitsubishi servos- so it could respond much faster than a stepper.
the only way ive found to get a thread right is to follow an encoder. - direct into a motion controller.
heres an example of it in its simplest form. running via a uc300 eth motion controller. - encoder is 2 optos reading the pulse wheel.
https://www.youtube.com/watch?v=GnDHt4IQdpo

If the PC can handle the step generation, then it should handle threading, as IIRC threading is handled in the low level parallel port driver code.

I used to cut threads using my old Conect with it's DC spindle motor, although it was running a Smoothstepper, but it was still only using a single slot sensor. I'd guess the toothed belt drive from the motor probably helped by eliminating belt slip and reducing how much the spindle speed changed under load, as the KBIC done a good job at maintaining speed under load.

It does seem to be something that works perfectly well, or gives endless headaches.

personally- i wouldnt waste my time now with the old parallel port- it was done as a hack by art to get mach 3 going- (and worked darned well for most things- router and mill based)
if you want to do threading properly- add a cheap(ish ) motion controller- where the motion is calculated real time in hardware- not software. hit and miss/ will it wont it scenario isnt much good when your making a part- especially if the material and tooling is expensive.
ken crouch was one of the first to do real encoder threading using a galil controller. ive tested the code here for him at the time.
the results are superb- even being able to stop and restart a thread half way along. - skip to 2.3 of this video to see how easy it is.
https://www.youtube.com/watch?v=JfnQxDPWoEQ

Thank you all for your input. Out of curiosity, what would this cheap-ish controller cost? Can someone point me to a website? I'm trying to do things on a budget as it's only a hobby lathe for a hobby "machinist" 😊