Hi everyone. I've been using mach3 for about 18 months now and so far have been able to find all the answers to my questions in the manual or on the forum. I've run into an issue I can't figure out though. On my lathe I can't seem to get my spindle synch working with threading. I've got a Hall effect sensor hooked up and my rpm displays correctly. When I run the threading wizard and make a part the thread cuts and each pass matches the previous one but, as best as I can tell, Mach is using the rpm set in the wizard rpm field rather than the actual rpm. It must be taking notice of the spindle pulse because it's giving me clean threads so it's starting each pass at the correct time. For example: I program a 1.5mm pitch thread and set the rpm field at 500. My lathe doesn't have a gear for 500 rpm, it's nearest is 420. I start the spindle in Mach and then switch the motor on (no relay at this point) the rpm dro reads 420. I hit run, it cuts, I get a nice clean thread that's nothing like 1.5mm pitch. Now, if I set the rpm to 420 in the wizard I get a thread that's very nearly perfect.

I've checked everything I can think of so i figure I'll throw it over to the crowd.

Cheers in advance

A bit more info, gecko g540, 1.8 degree steppers coupled directly to a 2005 ballscrew, 400 steps per rev on the z axis. X is a 1605 geared 3:1 at 1200 steps per, just FYI.
Mach is set to mm native units.

 

Mach is using the rpm set in the wizard rpm field rather than the actual rpm

Initially it will try to use whatever rpm you have noted in the wizard, but, the actual  rpm is  read and used during threading.
Adjustments are made accordingly. Mach would see the actual rpm  as a slow down in rpm and try and compensate.

 Now why you would put the wrong info into the wizard in the first place I do not understand.

You should note what the actual rpm for each gearing your lathe has and use those values as the rpm input .

Have a look at the Threading on the Lathe write-up which can be found in Members Docs. Also make sure you are
using the recommended Mach version.

Do a scribe test to confirm all is well.

RICH
 

I suggest the answer is simple. The wizard writes G-code for the speed you entered. If you look at the G-code there is likely to be an S word for the spindle speed that your lathe does not respond to. So the index pulse does nothing more than set the starting point consistently as you saw.  If you had a VFD on the spindle the threads would have been the correct pitch.

If you had a VFD on the spindle the threads would have been the correct pitch.

Yes because the set rpm matches the input rpm.
BUT,unless there was a change to threading, one should set the VFD manually since  the VFD may fight the threading compensation.

RICH
 

Ok, simple as that. Sounds obvious when I say it out loud. I can certainly work with that. I'm still confused though because when I first set the lathe up (on different electronics) I'm almost certain that I was getting correct threads regardless of the rpm I set in the wizard, I'd never paid any attention to it. In fact the wizard defaults to 1000 rpm, at least it was today, and there's no way I'd have tried to cut a thread at 1000 RPM. I should have ended up with a pitch of about 20mm but I didn't. The first piece of stock I threaded was a perfect 40x1.5mm. I think I'll see if I can dig up the original xml file from the old pc.

It is a long time since I used the parallel port on a lathe but as far as I recall I did not have to input the correct RPM for it to work well, I always had it close but as the lathe had fixed RPMs via an auto gearbox I probably just input a speed close to what I wanted.
 That makes me surprised that yours seems to only thread correctly if you command the correct RPM especially as you seem to have the Use Spindle Feedback in Sync option chosen. To my mind that should make Mach look at the true RPM and  adjust the Z axis velocity accordingly.

Suppose as long as it works for you now then no big deal but I wonder what would happen if the spindle speed altered during threading due to load, sounds to me like it would not produce a good thread.

Hood

Hood,

If the rpm varies due to load you get what you get for that current pass as it will try to save the threading operation. Been a long time since  testing was done, but at one time I was able to almost bring the spindle to a stop and could still finish the operation. You can't do that on a big lathe. Mach will try to change the Z feed rate to maintain the pitch based on rpm. The Z can only go as fast as configured in tuning.

When using the wizard one should use the calc button in the wizard to confirm that rpm and max velocity / accel are appropriate for your inputs.

I don't recall ( it is in the write up ) but spindle slow is  good down to about 75% of speed.  Once a thread is cut badly, well. face it,
the thread is poop! Small rpm variations you could get away with.

Now if you have a nice solid rpm then one can actually adjust the pitch some if there was a slight pitch problem found.

RICH

Thanks for the input guys. I dug out the old pc that I first setup for the lathe and copied the xml file over. Sure enough it works as it should. Rpm from the sensor is used to determine the z travel speed. Rpm set in the wizard is irrelevant. If I get a chance I'll try comparing the two files to see what the culprit is.