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[4]

Replace change gears with a stepper?

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ttabbal

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#1
To be clear, I do NOT want to go full CNC. I wonder about losing the gears and using a stepper to drive the main gearbox, which looks like it has 1:1 as an option. Maybe even using a pair of gears for reducing the required torque. There's already an RPM pick up so I could use that for timing or add another hall effect sensor. For bonus points, add a timing system for metric threading and auto stops at the ends of travel via limit switches or input from a DRO scale.

All the controls would still be manual, but I could easily switch turning speeds and thread pitches. Sort of like an electronic QCGB.

The idea seems simple enough, but I don't see it done. It would also make a nice indexer if I had it drive the spindle instead.

Random musings from what passes for a brain in my head... :)
 

silverhawk

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#2
To be clear, I do NOT want to go full CNC. I wonder about losing the gears and using a stepper to drive the main gearbox, which looks like it has 1:1 as an option. Maybe even using a pair of gears for reducing the required torque. There's already an RPM pick up so I could use that for timing or add another hall effect sensor. For bonus points, add a timing system for metric threading and auto stops at the ends of travel via limit switches or input from a DRO scale.

All the controls would still be manual, but I could easily switch turning speeds and thread pitches. Sort of like an electronic QCGB.

The idea seems simple enough, but I don't see it done. It would also make a nice indexer if I had it drive the spindle instead.

Random musings from what passes for a brain in my head... :)
It sounds like you will do threading. It will work to go stepper, but you absolutely have to keep the lead screw in sync with the spindle if you do any threading, or the threading will fail miserably.
 

JimDawson

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#3
No reason it wouldn't work. As long as you have a way to electronically gear the stepper to the spindle at the proper ratio it would work fine. This really implies some kind of computer or at the very least some kind of adjustable counter system to set the gear ratio. A QCGB is just a mechanical computer.
 

ttabbal

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#4
Thanks for the info. I was worried I'd missed something about it. The plan would be to use a microcontroller to control the stepper and watch the spindle speed to maintain sync. I am far more familiar with electronics than machining. :) I'll probably have more trouble adapting the stepper shaft to the lathe.

Any ideas on how much torque one would need for something like this on a PM1127?
 

JimDawson

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#5
Any ideas on how much torque one would need for something like this on a PM1127?
That is a very good question. And the answer is: I have no idea. It would seem that threading would put the greatest load on the system. So the question is how do you read the torque on the lead screw with a threading setup? I'm thinking a couple of spring scales, one pulling on a string wrapped around the gear on the lead screw to get the torque, but I'm not sure what to do with the other one. :)

Maybe just one spring scale would work as above, but set up for a heavy cut and just feed the lead screw with the spring scale and the string. Anything to get a rough idea of the max torque required.
 

mksj

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#6
You would be looking at a system similar to the Babin Hardinge HLV-H toolroom lathe with servo electronic digital threading, so would need more precise spindle position and probably some form of feedback with an encoder. The encoder could also give positional control if the half-nut stayed engaged, otherwise you would need some form of electronic scale or carriage drive for positional control. Also see Electronic Lead Screw (ELS) below, they also have a Yahoo forum. At the end of the day, you may have more cost tied up in the threading system then the cost of the lathe. It is doable, but may be a bit more complicated then anticipated. I have been toying with a similar idea to cut tapers with an electron feed on the cross slide coupled to some form of carriage positional control, use a PLC or small computer to set the angle and the stepping motor increment.
http://www.babinmachine.com/index.php?HLVELECLEAD
http://www.autoartisans.com/ELS/
 

Bi11Hudson

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#7
A retired electrical engineer myself. With (sorta) machine shop capability for modeling. There is no way I would trust a stepper motor to handle threading. A stepper has a fixed rotation, so many jerks per revolution. At best you would need an analogue servo. But that would still need to be geared into the system. And I personally still wouldn't trust it.

Looked at end on, a thread is a smoothly tapering spiral around the shaft. It has a small but measurable step for each "Nth" degree of rotation. If you could figure out some way to make a stepper with 720 steps per revolution, it might conceivably work. Maybe...... Then imagine the number of steps to do a really coarse thread over say a 3/4-10.

Now, I work with smaller threads, more like 0-80. 1/2-20 is huge to me. In any case, I would want the threading train directly coupled to the spindle so a bobble in speed would show up as the same bobble in the thread.

Just an opinion, of coarse, but quite staunch.
 

MikeInOr

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#8
You might take a look at a used Parker Compumotor SX6 stepper controller (very cheap on ebay). They are an all in one stepper driver, micro controller and powersupply. A SX6 will give you a lot more capabilities than a simple stepper indexer (at least the simple indexers I have worked with)... like calculating load from the current the stepper is using and allowing you dynamically adjust based on load. From the work I have done with them I think they would have enough intelligence to do what you need. They have several programmable I/O's and their own movement control programming language that wasn't too hard to master. I specifically remember a trace function that you can use to set the movement of the motor being controlled based off the movement of another motor... helping with the bobble issue... if the control frequency slows then slow the indexing proportionally.

Toothed belts with a good amount of reduction should give you the torque and resolution you need with almost no backlash.

It has been a while snce I have programmed one so I can't guarantee they will do what you need but I believe they will.
 
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markba633csi

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#9
Oh it would work just fine Bill
until lightning struck nearby while you are threading that 5000$ NASA part
Mark
 

ttabbal

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It's true that steppers have downsides. One I had overlooked a little is missed steps. Not a huge issue if you have sufficient torque, or position feedback, but a possible issue anyway.

I also might be underestimating the timing requirements. For feeding, I think it would be fine. Threading could well be another issue. I need to consider the timing and accuracy requirements more as it's obvious from some of the responses that there may be more precision required than I'm considering.

Resolution is an issue. 200 steps / rev is common. 400 is available reasonably. And you can gear them, but that introduces speed limits and potential errors. Large servos are a possibility, but those get expensive fast. I don't see myself doing 80 tpi anytime soon, but it's important to know what the limitations would be.
 

Asm109

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#11
If you a looking for a project to fool around with, go for it. Its a hobby, Have fun.
If you are thinking of the time saved from changing gears fahgeddaboutit. You will sink way more time in your project than you will ever spend changing gears.
 

RJSakowski

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#12
A stepper motor drive for the lead screw could work but only if you have a means of knowing the angular position of the spindle. A stepper driven CNC is capable of complex coordinated movements in x, y, and and a if you have an RT 4th axis. Addition of a rotary encoder to the spindle and some means of knowing how many revolutions were made from a "home" position would presumably do the trick.Rotary encoders are capable of decoding thousand of positions per revolution.

The Tormach Slant Bed Lathe is capable of single point threading.
 

British Steel

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#13
As has been said, you would need an encoder on the spindle - I haven't put it to full use yet, but put a pair of Hall-effect sensors* to read tooth position from one of the 70-tooth** gears on my lathe spindle in quadrature (e.g. one lined up with the centre of a tooth, one with an edge) which allows electronics to get 4 x the tooth-count and also the direction of rotation. I was putting one in for the spindle tacho, thought "why not, while I'm in here? Future development..." This will give me 280 positions / rev, I hope that'll be enough if I ever obtain the Tuits...

The stepper / servo should have an encoder, the risk of missed steps from a stepper is either a bad thread or a broken tool-tip... A third sensor on each encoder would help, used as an index mark to get absolute rather than relative position (e.g. the stepper would rotate faster/slower until the angular position matches the spindle BEFORE commencing a threading pass) so they always start with the same angular relationship, as you'd get with a single-tooth clutch.
It would be good to micro-step the leadscrew (intermediate positions between the e.g. 200 steps), it's possible to get e.g. 16 intermediate positions for much smoother motion, at the loss of some torque, by incrementally varying the winding currents (even the Chinese 10-Local-Currency-Unit stepper drivers do this).

There's an open-source "electronic leadscrew" you can probably google up, which uses common logic ICs and vintage-looking thumb-wheel switches, and I'm pretty sure there are Arduino versions, the maths to make one work are pretty simple! It might be worth searching YouTube for "Holbrook CNC", Andy Pugh (a fellow on the Holbrook group) did a full conversion and it may give you a good start on stepper / servo requirements?

EDIT: I had a quick look at Mach, LinuxCNC and how they do it - Mach only uses the "index" pulse once/revolution when threading, so an encoder-based solution would beat that... LinuxCNC allows any count/rev, and uses it properly :)

Hope this helps, rather than confuses!

Dave H. (the other one)

* Allegro ATS667 - tiny, built-in magnet, give a TTL-compatible output. 3 wires: ground, +v supply, output plus one to leave disconnected (the "test" pin)
** Had to hack the tacho board by swapping the timing crystal for one 7/6ths the frequency because of the unusual tooth-count - if it had been a 60-tooth I could have stayed with the original and read frequency from the sensor, read out RPM when set to measure frequency...
 
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Ray C

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#14
Lathes like this have been around for a long time and they do digitally controlled threading. No interest in having one but would love to see what they're all about. I believe they are Hardinge work-alikes.

south_bend_sb1009.jpg
download (2).jpg

Ray
 

GinStC

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#15
You have several choices for existing solutions:
There is a group on Yahoo for Electronic Leadscrew. The product web site is http://autoartisans.com/ELS/
There is a modernized version from Germany: https://www.youtube.com/channel/UCMvLkegD66QvfeOzFVr1lYg, product site is https://www.rocketronics.de/els/?v=3e8d115eb4b3
(this is the one that Stefan Gotteswinter uses on his 9x20 lathe)

On YouTube there are also some videos of a Russian version using Arduino's. Source code is available but the comments are in Russian.

Using LinuxCNC is a bit overkill but it works.

The favourite encoder for this type of work seems to be the AM102/AM103 since the number of pulses/rev is selectable over a considerable range. And it is affordable. For an example of its versatility, https://www.youtube.com/channel/UCF91fwCPdAZfGbnRfu8_dXg/videos
 
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gzoerner

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#16
My project to use a stepper to drive the leadscrew is just about complete. The software is complete and I'll finish the cases to hold the electronics this week. I don't intend it for threading. Synchronizing the spindle to the leadscrew appears to be pretty difficult (though not impossible).

My lathe is a Grizzly G4000 (9x20). The system uses an Arduino Nano for the controller, a DM556 stepper controller and a Nema 23 3 N-m stepper. The display shows the spindle RPM, the feed rate and an animation showing the feed direction. The controller measures the spindle speed then calculates the proper stepper speed for the desired the feed rate. I can adjust the feed rate from 0.001"/Turn through 0.010"/Turn, forward and reverse. There is an optical sensor that stops the stepper when the (adjustable) limit is reached. When the stepper stops (almost instantly), it reverses 1/2 turn to release the pressure on the feed lever.

Driving a stepper under load requires proper acceleration profiles. This entailed quite a bit of research. I found a paper by David Austin that described how the acceleration profiles can be calculated in real time. His work was quite good. My adaptation of his equations worked perfectly.

My intent is to publish my work on this forum. I'll include the schematics and software for those of you who are interested.

Glen Zoerner
Spicewood, TX
 

gradient

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#18
You might take a look at Clearpath-Teknic, https://www.teknic.com/products/clearpath-brushless-dc-servo-motors/ They offer a wide range of servo motors with all controls built in. A little pricy but everything is included, just hook up to a micro-controller or even switches, etc. No missing steps and very precise positioning and rotation. Nice videos at
by the NY CNC guy.
 

Beerfan

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#19
It is interesting to watch CNC lathes thread at low spindle RPM, they will rapid the tool to the start position then pause until the spindle reaches the correct radial position then feed through the thread. They move fast enough that at higher spindle speeds it is not noticeable.

One of the advantages of such a system is that one may produce mating threaded parts of any thread lead you desire, one may easily produce an assembly that is threaded 3/8-16 1/2 TPI, this will cause considerable head scratching for someone working on the equipment in the future.

It is fun however.
 

markba633csi

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#20
Also make proprietary parts so they have to buy from you :p
 

markba633csi

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Oh they will, cause I'm that red shiny fruit company
 

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#23
The Electronic Lead Screw is what you are thinking about. Not full CNC but does tapers, threads, etc

I have that same Wabeco D6000 lathe (Mine is a D6000e) and tried to buy a kit but it is no longer made. I would buy a kit is something used or new old stock turned up somewhere..

 
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wrmiller

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The Electronic Lead Screw is what you are thinking about. Not full CNC but does tapers, threads, etc

I was seriously considering doing this to my little South Bend 8x18 when I had it. I was to the point where I had the parts list identified, a friend who was going to do the circuit board layout, and a borrowed in-circuit emulator for debugging my code.

That's as far as I got with it.

Now the little SB is gone and it's replacement has a Norton gearbox on it. It was a good idea while it lasted though. :)
 

Cadillac STS

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#25
Could just put on a simple stepper motor with a driver and a knob for speed control to conveniently adjust the speed of the axis. Pull off one gear to disengage the gear drive but also be able to replace the gear and do the gear change for threading. The stepper setup would free wheel when turned off so the lathe could work as normal too.

You would need to sense the position of the spindle for threading and could add that on with another control board later.
 

ttabbal

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#26
Could just put on a simple stepper motor with a driver and a knob for speed control to conveniently adjust the speed of the axis. Pull off one gear to disengage the gear drive but also be able to replace the gear and do the gear change for threading. The stepper setup would free wheel when turned off so the lathe could work as normal too.

You would need to sense the position of the spindle for threading and could add that on with another control board later.

I think that's the route I would start with. It's a good proof of concept that just needs finer timing control to make threads later. Well, "just" might be over simplifying a fair bit.. :)

I was looking at the existing sensor on the spindle last night and noticed it's got 2 magnets, offset about 90 degrees, so I should be able to sense direction, speed, and orientation with some accuracy. It's an interesting experiment, but I would want to make sure I can use the machine as designed without too much hassle whatever way I go with it. It makes mounting motors and such potentially more complicated, but it makes it more flexible.
 

whitmore

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#27
The plan would be to use a microcontroller to control the stepper and watch the spindle speed to maintain sync.
Oh, the synchronization of speeds is only part of it. The spindle motor (AC motor) has known speed, but only
on the average. Any variation of the torque applied by the cutting tool becomes a phase error in the motor position,
and the stepper won't follow that kind of variation unless you monitor the rotation with some kind of spindle
rotary position sensor with very fine-grained resolution. Otherwise, every other
motor in the shop will telegraph to your workpiece over the AC power lines.

Oddly, threading of a uniform shaft is one application where it MIGHT work, because the cut is never interrupted
and occurs on a well-centered workpiece. The problems, though, would include a washboard-road kind of
resonance, where a cut pass makes minor corrugations due to steps by the stepper motor, and each subsequent
pass makes the corrugations larger... so, it's hard to predict success. Steppers move in steps; that's
going to encourage chatter, and not the social kind.

Servomotor spindle drive and servomotor carriage drive might work, OR you could use your regular AC
motor to drive the spindle, and gear a light set of gears (a clockwork-sized gearbox) into a synchro
generator, and drive the carriage with a synchromotor. The carriage screw is relatively slow moving,
and coarse, but a modified carriage screw (finer pitch) could be fitted to improve the
accuracy of such a scheme, and the efficiency of the motor.
 

GinStC

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#28
Whitmore, it doesn't require servos although if you have the cash then go for it. This threading 'problem' has been solved many times already, including at least 2-3 commercial units. They all work with steppers, half-step microstepping. Encoders outputting 400-1600 pulses per rev are inexpensive as well.
The German controller that I linked to above is 159 euros for a lot of functionality, including what the original poster is looking to do. It will work with just the Z axis or both Z and X. It requires minimum of 400ppr encoder on the spindle, AMT-103's are less than US$30. The recommended stepper drivers can do 200-51200 steps/rev although anything above 800 is overkill and also reduces your movement speed.

Hacking your own software might be fun, http://www.airspayce.com/mikem/arduino/AccelStepper/supports acceleration and deceleration . Personally I am no longer interested in programming so am saving up for the ELSII to put on my KC1022 (King Canada's G0602 version).
 

gzoerner

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#29
Cadillac STS,

That's what I'm doing with my project. I've taken it a step further in that I measure the spindle speed and calculate the proper stepper speed so the feed rate is calibrated in thous/Turn (between .001/T and .010/T).

While it's certain that others have solved the threading problem, as Whitmore points out, it's not simple. That's why you pay the big bucks. Steppers can't change speeds instantaneously. They must accelerate or decelerate under software control. Achieving synchronization with the spindle itself is fairly straightforward, but maintaining sync when factoring in backlash and acceleration is a challenge.

For myself, I'll be happy with not having to listen to the gears whining and wearing out for normal turning. I don't thread very often and when I do I use a hand wheel on the spindle.
 

magicniner

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#30
Cadillac STS,

That's what I'm doing with my project. I've taken it a step further in that I measure the spindle speed and calculate the proper stepper speed so the feed rate is calibrated in thous/Turn (between .001/T and .010/T).
The sensor and drive system should be independent of speed and directly link rotational position of the spindle to that of the lead screw, that way the stepper speed is always correct and variations in speed, including hand winding the spindle, will not result in any errors.
 
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