electronic lead screw

[RJSakowski]

The two questions I still have are: 1. What is the maximum torque that the lead screw will encounter during turning or threading and 2. what does the torque speed curve look like for any given choice of motor.

The first is dependent upon the depth of cut, the feed rate, the material being cut, and the geometry of the cutting tool. The relationship between the input torque on the lead screw and the delivered force, neglecting friction, is torque x rotational angle = force x distance. My lead screw has 12 tpi and therefore moves .08333/ revolution and ine revolution is equal to 2π radians so the force delivered = torque x 2π/.0833 =75.4 x torque (torque in lb.-in. and force in lb.). If my input to lead screw is 200 oz.-in, or 12.5 lb.-in., I could deliver 940 lbs. of force to the carriage drive, less frictional losses. My carriage crank has a mechanical advantage of 11:1 so I would have exert around 85 lbs of force at the crank to create the same 940 lbs.

The second question is more difficult to answer. When I dealt with NEMA 11 and NEMA 17 steppers in the past, I had published torque speed curves to guide my selection. Unfortunately, the motors that I have seen for this application don't seem to have that information available. Additionally, looking at a variety of motors on line, there doesn't seem to be a set relationship between holding torque and and available torque at speed. I am considering two motors at the present; a stepper with 425 oz.-in. of torque and a hybrid stepper with 602 oz.-in of torque. My gut is telling me to go with the more expensive hybrid but I really would like to see some torque curves for it. The hybrid can run at higher rpm so torque multiplication via gears is an option. Unfortunately, conventional steppers don't enjoy the same advantage due to the serious decline of available torque at increases rpm.

As to actually measuring torque, it should really be done at the desired operating speed. A dynamometer would be useful. A simple one could be constructed using a brush type motor. A cordless drill/driver comes to mind. The speed control would for the drill would have to be disconnected and the two power leads connected to a variable resistance. The voltage across the resistance and the current through the resistance would be monitored as the resistance was slowly decreased until the stepper lost steps or the hybrid errored out. The product of the voltage times current would be the output power in watts which could be converted to horsepower at 748 watts/hp. Horsepower - torque x rpm/5252 so, knowing the rpm, you can calculate the delivered torque. Torque in lb.-ft. can be converted to oz.-in. by multiplying by 12 x 16.

 

[GreatOldOne]

 

[RJSakowski]

All parts are ordered. I decided to go with 600 oz.-in. hybrid stepper, https://www.ebay.com/itm/1-Axis-Clo...928169?hash=item3654528529:g:yDwAAOSwVMFdfyOU and a 1:1 gear ratio. I am a little concerned as James used a 3:1 gearing for an output torque of 900 oz.-in. but I can always fall back on the 2:1 gearing in my gear box if necessary.

I did some measurements of torque required to turn the engaged lead screw and was somewhat surpised to see that it required 370 oz.-in. of torque to turn the unloaded lead screw. My Tormach 770 uses a 500 oz.-in. stepper to drive a 5mm ball screw and I have not had problems with lost steps with it. A little investigation lead to the discovery of overly tight half nuts. With the half nuts disengaged, the torque required to turn the gear box gears and lead screw was 80 oz.-in. With the half nut lever backed off ever so slightly, the torque was 150 - 190 oz.-in. Backlash increased from .001" to .002". It doesn't appear that the half nut mechanism is adjustable but I may be able to use a shim to limit the amount of closure.

 

[alan856]

There's a nice ELS from Germany that they call an Electronic Lead Screw - Stefan Gotteseinter did a review of it on his youtube channel. They will have a complete English manual available early this year. It leaves manual operation untouched and is DIY for the home machinist.

Greets! I am a proud owner of an ELS3 from Louis Schreyer (Rocketronics, Germany). It has been installed on an LMS-3536 (Sieg SC4) lathe. I have some photos of the installation if any body is interested. Also I'd like to start a thread (where?) where we can discuss ELS vs CNC.

Currently Stefan's company (Rocketronics.de) has no English manuals. Even though his site indicates such - they are still in German. You can get the PDF and use Google Translate to get a reasonable version. I also think he is currently not selling to the US - I guess there is too much red tape!

On of the immediate "PROs" of having the lathe controlled by cpu/steppers is that the feed-rate is ALWAYS constant and you can stop at the SAME place every time! What amazing finishes you get... And of course the nearly infinite threading capability is yet another story... plus you can do curves and tapers and balls...
Personally I have no interest in CNC on a lathe - MAYBE on the mill. A good ELS gives you the flexibility of manual turning with the precision and control of a "computer" driven machine.

 

[Dabbler]

Currently Stefan's company (Rocketronics.de) has no English manuals.

That is no longer true. The English manuals for all versions can be downloaded from the Rocketronics site.

I was unaware that Stefan was associated with Rocketronics, but rather I thought he liked their unit and used it on his previous lathe...

Personally I have no interest in CNC on a lathe - MAYBE on the mill. A good ELS gives you the flexibility of manual turning with the precision and control of a "computer" driven machine.

I agree. The need to learn such a depth of tools makes CNC less attractive than a purpose-built ELS for me anyway.

 

[Qtron]

clipped..

The second question is more difficult to answer. When I dealt with NEMA 11 and NEMA 17 steppers in the past, I had published torque speed curves to guide my selection. Unfortunately, the motors that I have seen for this application don't seem to have that information available. Additionally, looking at a variety of motors on line, there doesn't seem to be a set relationship between holding torque and and available torque at speed. I am considering two motors at the present; a stepper with 425 oz.-in. of torque and a hybrid stepper with 602 oz.-in of torque. My gut is telling me to go with the more expensive hybrid but I really would like to see some torque curves for it. The hybrid can run at higher rpm so torque multiplication via gears is an option. Unfortunately, conventional steppers don't enjoy the same advantage due to the serious decline of available torque at increases rpm.

As to actually measuring torque, it should really be done at the desired operating speed. A dynamometer would be useful. A simple one could be constructed using a brush type motor. A cordless drill/driver comes to mind. The speed control would for the drill would have to be disconnected and the two power leads connected to a variable resistance. The voltage across the resistance and the current through the resistance would be monitored as the resistance was slowly decreased until the stepper lost steps or the hybrid errored out. The product of the voltage times current would be the output power in watts which could be converted to horsepower at 748 watts/hp. Horsepower - torque x rpm/5252 so, knowing the rpm, you can calculate the delivered torque. Torque in lb.-ft. can be converted to oz.-in. by multiplying by 12 x 16.

Ha U hitting nail on the head here, THANKS for your entry!
I had a 'wrestling match' with a 'Leadshine' supplier, SteppersOnline-OMC, an otherwise good chinese communicator, who delivered a couple of nice grunty Nema 34's to me. But, on this matter, & best they could manage from their Eng. dept., was a rule of thumb for Holding vs running torque. pfft.
Not worth quoting here, too vague, & too long ago too.
If U dig hard enough, its findable, (generalised graphs), but maybe not for your specific motor.
- i am hoping Briney eye will clarify, soon, (Jon's) Torque wrench test is of course a holding torque test only.
BTW, it would have to be a hell of a drill! esp for Nema 34, thru a reduction pulley setup. AND, you would need to know the efficiency of the motor/generator, to get close to the mark, yes?-> i do appreciate the approach tho, & have used the method on a home-made, low powered VSD once. [I used a digital audio power amp, 300W, to drive a single phase cap run motor- no, don't try it - a real time sink!!]
Qtron

 

[Qtron]

DIY dyno = 24V 200 watt brush motor, on a 1:1 pulley to a single ph. ( motor run cap) 340 watt motor.
(

cast casing unrelated)

 

[Qtron]

clipped
.. I am considering two motors at the present; a stepper with 425 oz.-in. of torque and a hybrid stepper with 602 oz.-in of torque. My gut is telling me to go with the more expensive hybrid but I really would like to see some torque curves for it. The hybrid can run at higher rpm so torque multiplication via gears is an option. Unfortunately, conventional steppers don't enjoy the same advantage due to the serious decline of available torque at increases rpm.

A
I Meant to say, I am surprised that non-hybrid steppers are still avail, given the Chinese output of fairly good to real nice hybrids these days..

 

[Qtron]

The equation appears in slightly different forms. Of course it's a simplification. The terms are:

T = Torque (in lb)
K = Coefficient of Friction (dimensionless, ranges typically between 0.1 - 0.2)
D = Major diameter of the screw (inches)
P = Force (lbs)

If you rearrange the terms to solve for Force, input 88.5 in-lbs of torque (10Nm), and assume the friction coefficient for a 3/4" screw is the worst-case 0.2, you get:
P = 88.5 / (0.2 * 0.75) = 590 pounds of force applied by the screw.

Caveat: I'm not an engineer, I just spent a lot of years working for them.

Yes, it's a closed-loop microstepper. In the absence of steps it tries to stay where it is. What you're describing is precisely the behavior of an open-loop stepper.

I drive the encoder from the stud gear for simple expedience. It was the easiest and closest. As long as things are in motion backlash can be ignored. When you change direction the backlash is what it is. But that's the normal state of affairs anyway. CNC machines go to an enormous amount of trouble to minimize the backlash because they're changing direction all the time and the less they have to compensate for it the better.

Thanks again for your reply, the math, - for some reason i didn't get email notification so missed your response, till now. (was waiting with bated breath!)
so to clarify, for me, & those who have just dropped in on this thread, are you saying that 590 lbf or 2,624N is the lateral force applied to the carriage via the 1/2nuts, with your 3:1 stepper arrangement, driving the leadscrew? (and not via the gearbox).
And is that enough to do more gentle, but not 'toy-ish', depth of cuts for an 8 TPI thread?

 

[Qtron]

I've been following James for some time now. Originally, I was planning on waiting until all the dust settled before diving in but given the popularity of this project, I decided that I had best get going on it. Over the weekend, I ordered all the electronics save the stepper/servo driver. Everything except the I/O interface has been shipped.

I am still waiting to see the final verdict on the choice of motors and pulley size. I have found several candidates for motors, a conventional stepper, and a hybrid stepper/ servo. I expect that the maximum lead screw RPM will be in the neighborhood of 600. At 600 RPM, that would be moving the carriage at 50 ipm which is crazy fast. If I were cutting a 4 tpi thread for some unknown reason, threading at the 150 RPM spindle speed would mean the lead screw would be turning at 450 RPM The torque/speed curves for steppers vary quite a bit from motor to motor but it looks reasonable to expect a 50% drop in torque at 600 RPM.

I modified the opening at the bottom of the lathe bed to accommodate either a NEMA 23 or NEMA 24 frame motor.

i went back & looked at old emails from Stepperonline, ha, i am wrong, as it was a specific answer, not a rule of thumb,
they quoted 0.75Nm driving torque, at 200 RPM, 2.8 Amps, Full step, with a hybrid stepper having a 1.26Nm (178.4oz.in), 2.8A, Holding Torque.
Of course that all becomes wrong at microstepping rates, sinusoidal driving, and who knows what it would be at 1000 RPM.
My understanding is max torque is achieved at full steps only, slow speeds only.. (and no mechanical resonance happening, which shouldn't below mid band resonance freq's). BTW, my assumption here, a leadscrew driven via a "rubber band" toothed pulley may not have sufficient direct coupling to the stepper to dampen this resonance.
Wikipedia:
When the motor moves a single step it overshoots the final resting point and oscillates round this point as it comes to rest. This undesirable ringing is experienced as motor vibration and is more pronounced in unloaded motors. An unloaded or under loaded motor may, and often will, stall if the vibration experienced is enough to cause loss of synchronisation.
Stepper motors have a natural frequency of operation. When the excitation frequency matches this resonance the ringing is more pronounced, steps may be missed, and stalling is more likely. Motor resonance frequency can be calculated from the formula:

f = 100/2pi x sq.root of 2pMh/Jr
Mh Holding torque
N·mp Number of pole pairs
Jr Rotor inertia kg·m²