How does CNC lathe works to cut thread?

[alexccmeister]

Hi all,

I am learning to cut thread on my C3 lathe and managed to cut an 8mm dia. x 1.25mm pitch thread on a brass rod. It didn't work out as well as I wanted but good enough for a start.

I am doing this on a manual lathe (without a thread dial), i.e. I had to change the gear in order to set the right pitch for the thread.

I am also planning to cnc my C3 lathe in the near future. I know I have to fix a stepper motor onto the bottom leadscrew. This would mean I have to disengage the leadscrew from the gears in the headstock. Without the gears controlling the speed of the rotation of the leadscrew in relation to the rotating chuck, how do I make sure the pitch of the thread will be correct? Does Mach3 have a setting to control the speed of the lead screw for the correct pitch? Thanks in advance.

Alex

[kawazuki]

CNC lathes have an encoder on the spindle, this determines the speed of rotation and this is input into Mach - it does the working out of how fast to traverse the tool to cut the thread. Don't ask me to explain how - I haven't the foggiest!

[alexccmeister]

Hi Kawazuki,

Thanks for the explanation. However, I am still confused as how it actually works. The relationship between the headstock chuck turning speed and the leadscrew speed is controlled by the gears which I need to change. The faster the chuck turns the faster the leadscrew turns as well hence keeping the pitch constant. But on a cnc lathe, that would mean that the chuck turning is independent of the leadscrew as the leadscrew is controlled by the stepper motor. So I need to set the rotating chuck speed in advance in order to make sure the leadscrew will match the pace to produce the right pitch. I think I understand the concept but how to actually make it work in real life?

Anyone have a link to a good DIY thread on a cnc lathe that explains this? Thanks.

Alex

[kawazuki]

When CNC'd the leadscrew is independant of the spindle - not connected - no reason to.
The stepper drives the leadscrew so progressing the tool at whatever rate Mach determines. Once you've sussed out how to set the tooling the cutting programme is quite easy.....I think.......not an expert in Mach yet!

[alexccmeister]

I was doing some reading and found some info on CNC lathe. I think I need to sync the spindle (chuck) speed of rotation with that of the leadscrew via a index pulse card from CNC4PC.com. The sensor will than send the rotating speed of the spindle to Mach and then the software will adjust the voltage and hence the speed of rotation of the stepper motor hence adjusting the pitch accordingly. I may be wrong, but this is what I could gather with my limited knowledge of cnc lathe.

If any seasoned machinists or axpert happen to read this, could you confirm what I just wrote? Thanks.

Alex

[Al_The_Man]

Mach uses the one pulse/turn on the spindle to synchronize the Z axis with the spindle.
High end controllers that close the loop in the control, use the spindle encoder marker pulse as a sync point, then gear the Z axis servo to the spindle encoder for the rest of the cut, rather than just sync 1 pulse/turn.
Al.

[acondit]

EMC2 uses an encoder with index to sync the Z-axis to the spindle rotation. A minimum of something like a 100 mark encoder is recommended.

Alan

[alexccmeister]

Originally Posted by Al_The_Man Mach uses the one pulse/turn on the spindle to synchronize the Z axis with the spindle. High end controllers that close the loop in the control, use the spindle encoder marker pulse as a sync point, then gear the Z axis servo to the spindle encoder for the rest of the cut, rather than just sync 1 pulse/turn. Al.

Hi Al,

Which one shall I go for then? Is the Index Pulse card from CNC4PC good enough to work with Mach or I need a high end controller as you have suggested. Which high end controllers would that be? Any suggestion. I currently am using mach and Xylotex driver setup for my X2 mill.

Originally Posted by acondit EMC2 uses an encoder with index to sync the Z-axis to the spindle rotation. A minimum of something like a 100 mark encoder is recommended. Alan

Hi Alan,

Thanks for the comments but again I am lost as to the '100 mark encoder is recommended' phrase. Any suggestion as to which encoder I should look for? I am intending to use either mach or turbocnc for doing thread cutting.

Thanks.

Alex

[Al_The_Man]

If you are already using Mach, you can try their method, apparently most are getting some acceptable results, I have not used Mach so I cannot comment how accurate it is.
It sounds like EMC will use the geared axis method.
Al.

[One of Many]

Originally Posted by alexccmeister Hi Al, Which one shall I go for then? Is the Index Pulse card from CNC4PC good enough to work with Mach or I need a high end controller as you have suggested. Which high end controllers would that be? Any suggestion. I currently am using mach and Xylotex driver setup for my X2 mill. Hi Alan, Thanks for the comments but again I am lost as to the '100 mark encoder is recommended' phrase. Any suggestion as to which encoder I should look for? I am intending to use either mach or turbocnc for doing thread cutting. Thanks. Alex

Alex,

For high end hardware motion controls this is more commonly known as electronic gearing. Where in, the axis are synchronised by the encoders pulses via a dedicated CPU. The higher resolution of the encoder pulses used, along with the processing speed of the controllers I/O contributes to its accuracy and ability to maintain sync under various loads.

The PC based systems often using one pulse per revolution of the spindle, is tracked via software through the parallel port and "part time CPU", while driving a stepper without feedback encoder pulses. Which is a compromise on accuracy based on a step pulse timing ratio to achieve anywhere from x:1 to xxxxx:1 or Z step pulses: 1 Spindle Revolution. The limiations can be in the max frequency of the step pulse output to drive the Z motor steps per revolution, based on spindle rpm. Most threading is done in the lower RPM ranges, but this limitation is an example of price verses performance I suppose.

Weak Z axis motors may lose steps without a closed loop feedback encoder to keep things timed properly under heavy cutting forces like those seen in large pitch threading.

I am leaning toward the EMC2 system with high hopes to emulate an EZ Path lathe if I were to CNC one of mine, but I am in no hurry. Maybe a perfect application for the CNC Brain if you have followed that thread. Neat stuff!

DC

[acondit]

Originally Posted by alexccmeister Hi Alan, Thanks for the comments but again I am lost as to the '100 mark encoder is recommended' phrase. Any suggestion as to which encoder I should look for? I am intending to use either mach or turbocnc for doing thread cutting. Thanks. Alex

Alex,

I am currently building an encoder for my lathe.

You can buy an encoder from US Digital and drive it off the spindle. Some people use a belt off the spindle, some others were talking to US Digital about getting a custom encoder wheel made that would fit the spindle.

Alan

[alexccmeister]

Hi DC,

I am getting some of what you said bearing in mind I am at this point in time a total newbie with cnc lathe.

I have tried out one threading operation with the lathe and it worked out very well for me although the thread was too big for the nut. I think this is due to the fact I didn't get the max diameter of the bolt right. So as far as manual lathe threading is concern, I love it. But thats on brass, I want to do threading on ballscrews which is hardened steel. A cnc lathe will help me cut tiny bit off without damaging the cutting tool and the lathe itself. I just can't do that with my hand operating the wheels. Anyway, thats a bit of my background knowledge. I am getting there. Thanks for the info though. I am very sure I will understand all that you have said in the very near future.

Hi Alan,

Will check out US digital. Thanks for the info.

Alex