You need to measure how much torque is needed to turn the leadscrew.  Wrap some string round the l/s handwheel and pull with a spring balance while the half nuts are engaged.  You’ll need to apply a multiplier of 3 or 4 for acceleration and cutting forces.  Divide by any reduction gearing you are apply.  Then select a suitable modern 2 phase stepper motor from Stepper Online.  I’d guess that one of the higher torque NEMA23 types would be suitable.  What driver do you have?

FWIW I used one of these Closed Loop Steppers for my installation of Clough42’s ELS system.

I installed onto a Chinese 125mm centre height lathe and have never had any stalling issues, even when roughing down lumps of steel with 5-6mm doc.

Looks like that particular one is no longer available, but the picture shows the main details of NEMA 23 with 3Nm and a matching driver unit, purchased back in September 2020 so probably yet cheaper now.

Mine was installed in the void under the headstock and drives via a toothed belt and a down ratio of (I think) 1:2.
Best of luck,

Simon

Buy from Stepper Online, not the absolute cheapest but a good supplier for both motors and drivers.  I once bought a NEMA08 motor from an anonymous vendor on eBay for what seemed like a very good price.  It would have been great if not for the bits of ferrous swarf left in the motor.  Buy cheap buy twice!

If I understand the kit correctly you need an encoder on the lathe spindle, not the stepper, so has to be separate.  Depending on the steps/rev you need you could make your own. I’m using Mach3 which only supports a single pulse per rev encoder, just uses a slotted disc on the back of the spindle and a slotted opto.  I guess the Clough kit must have both a 1ppr encoder for “indexing” (i.e. knowing where the thread starts) and another to track spindle rotation.

On my Super 7 CNC conversion the X axis is driven by a NEMA23 stepper through a 2:1 reduction.  The drivers (recently upgraded) are DM542T.  That could also drive a NEMA34 stepper, it can support up to 4.5A, 50V.  You only need an encoder on the stepper if it is a closed loop type in which case it needs a specific driver.  Frankly I think closed loop steppers or servos on a hobby lathe are a waste of time unless you are also fitting ballscrews and doing a full CNC conversion.

On 2 May 2024 at 21:52 John Haine Said:

If I understand the kit correctly you need an encoder on the lathe spindle, not the stepper, so has to be separate.  Depending on the steps/rev you need you could make your own. I’m using Mach3 which only supports a single pulse per rev encoder, just uses a slotted disc on the back of the spindle and a slotted opto.  I guess the Clough kit must have both a 1ppr encoder for “indexing” (i.e. knowing where the thread starts) and another to track spindle rotation.

 

John, you are correct in that you need a separate encoder on the spindle. The Clough42 solution needs to have a high resolution encoder to operate successfully – I believe that I used a quadrature output, 200 PPr, thus 400 detection edges per revolution. The chosen encoder resolution is set in the ELS configuration file along with any stepper to leadscrew and other ratios (due to belt drive ratios, etc).

As far as I recall, there is no ‘index’ function, the ELS monitors the status of the spindle constantly.

Also, for the OP, the point of having an encoder on the spindle is to detect the spindle state. Putting an encoder on the stepper will not achieve that aim. You may have been confused by steppers with inbuilt encoders for use as closed loop steppers. Unlike John, I like the use of closed loop steppers (with their dedicated control box) since this more or less eliminates the risk of step loss – or at least alerts when this has happened. Unlikely to be an issue when screwcutting – unless you do so at elevated speeds – but important if you are significantly reducing a diameter with autofeed.

Simon

On 3 May 2024 at 10:40 simondavies3 Said:

…..

As far as I recall, there is no ‘index’ function, the ELS monitors the status of the spindle constantly.

 

Hmm…not sure that’s a good idea!  I’m fairly sure that Mach4, LinuxCNC and UCCNC all use both an encoder with say 400ppr plus an index which generates 1 pulse per rev.  The high ppr is used so the controller can continuously monitor spindle speed and compensate if cutting forces slow it down.  The index tells it what the spindle angle is once per rev so it knows where to start the acceleration to take a cut.  In principle you could just use the ppr encoder but these pulses will be say 400 times faster so the controller is more likely to miss one.

A lot of talk of “NEMA” frame sizes, but little information about the actual rated torque of the motors used.

NEMA 17, 23, 34 etc is just a motor frame size designation – basically sets the mounting flange dimensions. A NEMA 23 frame motor can be had in several different torque ratings, so just saying “NEMA 23” isn’t that helpful.

Clough42 did a video here in which he did a lot of tests on the performance of various steppers & drives for his ELS project. Worth a look.

Nigel B.