[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]

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.

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]

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]

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

[One of Many]

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

Manual threading will come with time and experience, as will accuracy. Get yourself some thread measuring pin sets and/or a set of thread pitch triangles that go onto the mic anvils. Relying on a nut to size things just won't do in all work. It is a matter of knowing what is left to remove before it is too late.

Break down the movement of the hand cranks into smaller sequences.

With the compound set at 29.5 degrees, I use the compound as my "per pass infeed". This puts only the leading flank of the bit doing the majority of the cutting.

The X axis or cross slide is set to the dial zero and I only use this for pull out at the end of the thread. With practice, you can rapidly crank out the cross slide one full revolution and disconnect the half nuts in one smooth but synchronized motion.

I do a couple practice runs with the cutter far off my part to get a feel for the spindle speed and synchronize my movements accordingly. If not comfortable, I can slow things down with a speed change to the spindle.

So, basically I start the thread according to the appropriate threading dial number with the compound cutting maybe .003-.005 for a first pass, then only move the compound in .002 each pass after that. Harder materials may need a spring pass in between, but the chip load is an indicator here.

OK spindle on, gear pitch selection set, waiting for the thread dial target.......go

Engage the half nuts.

Let it cut to my marked line or short of a shoulder, whatever.

Pull out the X one rev and disconnect the half nuts. 1, 2 done

Move back to the beginning, return the cross slide to zero. Should be one revolution in.

Feed in on the compound as needed, but conservatively. and go again, same fire drill waiting for the thread dial target.



Keep in mind, threading on lathes with an English pitch leadscrew and cutting Metric thread pitches or a Metric leadscrew cutting English thread pitches, that you cannot disengage the half nuts. You will lose gear sync with the threading gage, so you must leave the half nuts engaged and reverse the lathe carriage back to the beginning of your thread. Rather intimidating if the spindle coasts a while when you expect it to stop instantly where your thread ends. Practice.......or reverse the process to cut outward in reverse and an upside down cutter. More practice?

DC

[alexccmeister]

Hi DC,

Thanks for the info. Much appreciated. I should really be getting some tools for measuring the thread while I cut and I agree completely with your statement about not using the nut as a measuring tool.

Interestingly though I did not use a threading dial in my first venture into manual threading. I have a lathe that has metric leadscrew. Basically everything was set up in metric. I just use the reverse and forward knob to control the leadscrew rotating. When I make the first pass, I let the tool cut a small bit of the work. After it has reached the end of the thread, put the knob to neutral and turn the cross slide out. Reverse to the start, turn the compound slide a bit to cut a bit more and engage the leadscrew forward. I keep doing this until I get the thread I want. Its very interesting.

Alex

[One of Many]

Alex,

As long as you are cutting metric threads with a metric leadscrew you do not need to reverse the spindle to the beginning of your cut. This really only applies to the unit conversion since the lathe approximates the sync through a mutiplier gear. Learning how to use the thread dial will reduce the wear and tear on the half nuts. Once they become sloppy, what is refered to as a drunken thread can be the result. Loose half nuts allow the carriage some backlash, so the pitch of the screw being cut could lead and lag within that area.

The nice feature of a manual lathe is for recutting/reworking a thread for whatever reason. With some experience, you can re-align the thread to the spindle centerline, then re-sync the gearing by moving the cutter tip into position with the compound and cross slide, once the half nuts are engaged and the spindle stopped mid thread. Using a version of the method I decribed in my previous post, you can then fine tune the cutter into position while the lathe is in motion and finish cutting the thread in the normal process.

This feature is, for the most part lost to electronic gearing, once the cnc conversion is completed.

DC

[alexccmeister]

Hi DC,

Thanks for the info. However I am still abit confused with your first line though. How do I do the recut again without reversing the spindle? I am assuming you meant that I disengage the half nut for this operation and just wind the tool post back. If thats the case, then I definitely need a thread dial.

Will not disengaging the half nut cause it to wear done faster? Thanks.

Alex

[One of Many]

Hi DC,

Thanks for the info. However I am still abit confused with your first line though. How do I do the recut again without reversing the spindle? I am assuming you meant that I disengage the half nut for this operation and just wind the tool post back. If thats the case, then I definitely need a thread dial.

Will not disengaging the half nut cause it to wear done faster? Thanks.

Alex

Alex,

Sorry, I made a presumption that you had a threading dial on the leadscrew. This is used as a reference marker in the gearing from the leadscrew to the spindle, so the half nuts start or restart the thread in the same groove each pass. So, yes, you are stuck with keeping the half nuts closed. Since this uses both flanks of the half nuts threads to move the carriage with each thrust force direction, the wear is doubled. Keep the screw clean and well lubed at least before a threading operation will reduce grit buildup that will eventually compromise the life of the assembly.

A couple more links to clarify the process.

Threading link

Thread dial example

DC

[alexccmeister]

Hi DC,

Thanks for the links, will check them out later.

Anyway today I tried a couple more threading operation. In all cases I got the thread nicely cut, but none of the nut will fit. I don't have a thread measuring device so abit difficult to say for sure if the thread pitch was correct. But I changed the gears according to Sieg's Lathe manual so it should give me the right pitch. Again, in all cases I had to use the thread die to rework the thread so the nut will fit.

I even try to cut the diameter of the stock rod smaller than needed and cut the thread the same way.Measured the max diameter and its abit smaller than the nut but it just wont fit.

It could be the depth of the pitch or the pitch itself may be out slightly. Any idea why?

I suppose I can just cut the thread as near as I can get to the size of the nut and to finish it off use the thread die to rework the thread so the nut will fit. But then would prefer to just do that once with the lathe. My apology if this thread has gone out of topic somewhat.

Thanks.

Alex

[alexccmeister]

By the way, can i use TCMM carbide insert to cur thread? Or should I be using a threading tool specific for threading? Thanks.

A;ex

[One of Many]

Hi DC,

Thanks for the links, will check them out later.

Anyway today I tried a couple more threading operation. In all cases I got the thread nicely cut, but none of the nut will fit. I don't have a thread measuring device so abit difficult to say for sure if the thread pitch was correct. But I changed the gears according to Sieg's Lathe manual so it should give me the right pitch. Again, in all cases I had to use the thread die to rework the thread so the nut will fit.

I even try to cut the diameter of the stock rod smaller than needed and cut the thread the same way.Measured the max diameter and its abit smaller than the nut but it just wont fit.

It could be the depth of the pitch or the pitch itself may be out slightly. Any idea why?

I suppose I can just cut the thread as near as I can get to the size of the nut and to finish it off use the thread die to rework the thread so the nut will fit. But then would prefer to just do that once with the lathe. My apology if this thread has gone out of topic somewhat.

Thanks.

Alex

Hello Alex,

The TCMM cutter would work, but the nose radius needs to be small. This nose radius also plays a part in where the actual pitch diameter and the true point of the cutter angles would be. This can set you up to keep cutting deeper with the larger nose radius, only to find when the nut does finally go on, it is VERY LOOSE. Next would be the tool holder and tool post that orients the insert, so as not to effect the rake angles much.

Check out page 286 of the first link pdf I posted. It shows a thread centering gage. This cheap little arrow helps set the 60deg V on your cutter perpendicular to the centerline of the spindle. If the cutter tilts in any direction, the angle of the thread will be cut likewise... leaning, narrow or wide. If you have a bolt with the same thread pitch, use it to check the pitch you just cut for a comparison. Placing a brightly lit, white sheet of paper behind them and look between the 2 screw threads to see how they match up form and pitch wise. Crude tooling still does the job.

Be aware of spring in your part or the lathe itself, if while being cut, things flex. It is not always convenient to use a tailstock and a live center, but trying to get the taper out of a flimzy threaded rod is a pain.

DC