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Electronic Leadscrew for the New Lathe

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vtsteam:
Will I truly appreciate your organizational bent, but the requirements were set out early on:

1.) Get away from conventional change gears  :lol:

And by golly Will, if you can cruise to a thread gear setup from a feed on my Craftsman 12 x 36 in 5 minutes time my hat is definitely off to you!

I'm no better than Martin Cleeve with my own lathe, who says changing, and then threading a half inch long part, and then changing back killed an evening of his, and why he designed his gearbox.

Maybe different lathes vary this way. Certainly people do!  :beer:

Manxmodder:
Steve, the quest for a reliable electronic screw drive is,I think, very much worth the headache of figuring out a workable scheme. Maybe you'll go that route right now,or perhaps the idea needs time to evolve a bit more and you can upgrade to a full e system later.

  Earlier on I think you touched on the possibility of toothed belt and pulley drives. These are worthy of further consideration as they are relatively cheap to purchase,light to handle,clean due to no grease  and fairly straight forward to make your own pulleys from casting and milling.

I certainly like the idea of belts from the practicality viewpoint.

Agree also that not all lathe brands are as easy to change back gears on,and can understand why it makes some reluctant to get involved in the process when it takes up so much time.....OZ.

vtsteam:
Is it at all possible to skip software entirely with a stepper motor?

Put an encoder disk on the spindle that directly generates the signals needed by the stepper motor driver -- step and direction?

By varying the disks, couldn't you select different "gear ratios"?

Wouldn't that also deal with the problem of a stalled spindle?

A straight stepper would have 200 steps per revolution -- a belted reduction, a multiple of that. Would it be fine enough to do it?

vtsteam:
Okay so thinking about this hypothetically:

1.) Say you have a 200 line encoder on the spindle and a 200 step stepper on the lead screw.

2.) The encoder connects (through whatever buffer necessary) to  the STEP pin of the stepper driver.

3.) For the experiment, you connect the Direction pin to a Direction switch., and bring low or high as required

4.) The leadscrew is a 10 tpi pitch.

You rotate the spindle one revolution by hand.
This should rotate the stepper by one revolution.
And the carriage would move 1/10 inch.
If you were cutting a screw you'd get a 10 TPI screw thread.

Now you change out the disk in  the encoder for a 100 line disk.
One revolution of the spindle turns the stepper one half revolution.
The carriage moves 1/20 inch
If you were cutting a screw you'd get a 5 tpi screw thread

And a 400 line encoder would generate a 20 tpi screw.
etc.

But what about threads not integer divisible into 200 steps/rev for the stepper?

If the stepper was reduced 6 to 1, you could get quite a few more factors. Also the fineness of even an aproximation would increase. One step would equal 1/12,000 inch of carriage movement. For a prime pitch -- take 13 tpi, for instance, you'd need a 923 line encoder, and the thread would be accurate to about .00008" in 1"

Another possibility -- increase the encoder speed.

How about putting the encoder on the spindle motor (if DC variable drive type) and you used a timing belt from the spindle motor to the spindle, they would be in sync. The spindle motor might run 6 times the spindle speed. So the encoder would run at that speed. For our 13 TPI example we then need a 154 line encoder instead of 923. Accuracy is a little lower (154 x 6 = 924), but still good enough.

Or you could use the same belt, and spindle pulley, but add another stub spindle for an encoder pulley set to whatever ratio was convenient. You could mix and match encoder pulleys and encoder disks where necessary. This should still be a lot easier (and cleaner) than change gear trains, since it involves only one pulley and no oil.

Encoder disks could also contain multiple line grates, similar to a dividing plate, so several ratios could be on just one encoder disk. You move the sensor out to to the appropriate band to select the ratio.

vtsteam:
It's possible that encoder disks could be printed out on a printer, if the number of lines was reasonably low, and the disk reasonably large.

An optimum set of pulley ratios for the motor, spindle and ledscrew, with the encoder on the motor might work out very well for homemade encoder disks.

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