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Electricuting the lead screw

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eskoilola:
First of all - I am fully capable to program a microcontroller (C/ASM) when and if that is needed. I have done several project with Atmel (AtMega-series) and PIC (16F25K-series). So if this needs one then I am not stranded with that one.

I have a 250 x 700 mm chinese lathe which is quite rigid with geared spindle and change gears for feed and threading.
See description here.

I have installed a new 1Kw 3-phase motor with VFD and a quick change toolpost. In addition to these some other gear like 4-jaw chuck etc ...

I am very satisfied with the rigidity and most of the work quality in this lathe. That being said there are a few flaws which I dislike:
- saddle feed cannot be reversed. Only right hand threads and only feeding towards the chuck (with normal spin direction)
- saddle has a half nut. Yes really, there is just the other half of the nut the other half being a hardened cylinder
- the spindle gearbox is quite noisy
- changing between feed and threading is a major PITA

My first question is: What is the minimum torque for a stepper motor assuming it is directly connected to the lead screw (3mm/rev) ? When is this torque an overkill ?
I have a heavy duty stepper motor Superior electric Slo Syn SS250B which has a maximum load inertia of 3lb/in or 8.8 kg/cm and a running torque 250Oz/in or 177n/cm. I do not have the faintest idea what those figures mean. Is that much and is that enough for my application ?

Data sheets for these motors are here and here.

I also have the big brother of that stepper. A Slo Syn SS400B which has 4.5lb/in or 13.2kg/cm max load inertia and running torque 400Oz/in or 282n/cm. It is a larege(ish) motor.

Both motors have 200 steps/rev resolution and 120V operating voltage (0.7/1.4A). There are only 3 wires coming out of these which means a quite strange drive setup with true AC feed. However - that is not a problem. This motor type can be driven with a sinusoidal drive which makes the "steps" a lot smoother.

My second question is about the accuracy of the lead screw. Is 0.015 mm/step accurate enough?
If the stepper is connected directly to the lead screw then this results to 0.015 mm/step saddle movement. To me this seems like good enough. Is there any reason why that should be more accurate? One can make this more accurate by applying some sort of a transmission (timing belt or such) but that will also impact the maximum speed of the saddle which limits the speed range I can use for threading.

The last question: What is a suitable resolution for the rotary enconder for the spindle?
I assume that the limit here is the speed of the microcontroller and the availability / easiness to make the needed slot wheel. I also understand that there should be an index so the uC knows the whereabouts of the spindle.

Hmmmm.....

PK:
When using ball screws, belts, or other low friction drives, you can calculate the required torque. Lead screws are a different beast because of the wide range of friction losses you may see between different lathes.

I've done a bit of work with this http://www.caswa.com/cncathome/lathe.html and http://www.caswa.com/cncathome/els.html

I recommend setting up a lump of steel in the chuck, adding a bar to one end of the lead screw and using a spring scale to get a feel for the required torque.

Your main consideration with this job will probably not be cutting torque, but rather top speed. Lead screws tend to be fairly fine pitched, so the slower top speed of the stepper means a max rapid speed of 1-2m/min. If a servomotor is out of your budget, then plan to run the stepper from > 50VDC

As per encoders on spindles. You need one pulse per revolution.

Hope that helps.

PK

sparky961:
If you're dreaming of rigid tapping in the future, a spindle encoder with a few thousand counts per revolution would be in order.

JHovel:
Have a read of this page and all relevant linked pages: http://www.autoartisans.com/ELS/
I've built this ELS and it works very well. My encoder is, in fact, a small tab screwed to the main spindle inside the headstock and passes through the gap in an HP IR optical sensor. That's enough.
There is now a complete kit available and John is amazingly helpful with all questions relating to driving leadscrew and cross-feed screw on lathes and other machines.

eskoilola:
As PK in his project page describes, a lathe electronic control can be (and should be) brought that tiny step further making it a partially capable CNC lathe. This in turn involves precise control/sensing  over spindle and both axes.

I am really tempted to make this just that. A lil' bit more capable as it actually does not seem to be awfully more stuff to do.

What I am going to have fun or struggle with (which ever way one wants to put it) are the mechanical things. Electronics and uC programming are easy for me. And as it seems most of this modification is actually just that.

I have also thought about using a servo motor on this one. It is my assumption that a servo motor controller needs some sort of PID controller to work as intended. In addition it needs either a rotary encoder or better yet a linear encoder  to tell the position of the saddle. I'll check what the stepper can actually do speedwise. The stepper is actually a nominally 120 volt device dc voltage for nominal current being around 30 volts. No problem as then the currents are smaller.

Of course this would be quite a bit more than just a electronic lead screw (ELS) but on the other hand - making the lathe to understand G-code (or whatever it is named) would make it compatible with GPL software which in turn has a temptation on me. Being a programmer makes me want to do everything in a computer.

I have an old Gerber photo plotter in my posession. This unit has one meter long ball lead screw with two nuts and another 70 cm long equally with two nuts. The steppers are too weak and slow for this application but because the unit has to work very precisely (being a foto plotter) the lead screws are quite massive (about 2 cm in diameter). As my lathe has a lead screw of about 90 cm length this could quite easily be replaced with that ball thing.

Using a ball lead screw has it's implications. There is no more a half nut to disconnect. This means that the apron handwheel has to be electricuted as well. Maybe a torsion sensor for manual control making it feel exactly as sloppy as it is now. Another torsion sensor to the end of the leadscrew handwheel and that thing could be operated exactly as it is operated now in a manual mode. The current half-nut lever and the auto-feed clutch would then be simple switches to disable/enable the manual handwheels. All this would of course mostly spoil the feel and touch of the manual operations.

On the other hand the ball lead screw is very precise and the lead screw backslash is mostly eliminated by using two nuts. Backslash originating from the leadscrew ends is a bit more difficult to eliminate - not much though.

I have to sleep on this one - this may run out of hands.....

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