An Electronic Leadscrew Controller using a Pi Pico

[AlanB]

That's more like it! Looks like what I had on my desk when I started. Wires and whatnot everywhere. I consolidated my driver receiver boards to a single board just to get some space back.

Just a thought, either a disable on the stepper or just set the numerator to zero? Been out in the sun a bit, perhaps that won't work, but just throwing out the thought.

In my case my spindle position is always updated, but I can control the accumulation. No accumulation, then the stepper never moves. Or just a simple if statement in the stepper routine. I like the latter best because it is obvious what is blocking stepping. Other ways are less obvious. In the midst of some bizarre bug, you want things to be very apparent. Subtlety is often lost when thrashing through the problem. But that's just my preference.

That's three separate test setups, one for Pi with the big breakout Tee and several displays, one for Pico/display/gps, and one for ESP32/display. Plus the watch repair. Surfaces around here tend to get filled up with 3D printers, radios, and other projects.

Stepper driver disable or zero numerator should work fine. Probably will use both, at different times, for different purposes.

 

[AlanB]

Made one measurement on the PM-1228 feed and it was right on, so maybe those numbers in the manual and on the label are accurate, that would be nice. Pretty easy to do with turns counter and DRO, set the gearbox, engage the feed, take the slack out with the magnet right under the pickup, zero the DRO, run it 99 turns, do the last one by hand and end up with the magnet under the pickup, read the DRO. Repeat for 15 gear settings, leadscrew, feed and crossfeed. Only 45 measurements to take. 44 to go.

Actually it won't take that many, if the relationship between the Z and cross feeds and leadscrew are handled separately from the gearbox. It should only take 17 measurements that way.

 

[AlanB]

Now I remember what the deal was. Z feeds are correct. X cross feeds are wrong. The book says X cross feed is half of Z feed. I measure a 0.660 ratio between them. The plate on the machine shows 0.0048 and 0.0025. Another reason to have an ELS, then the wrong numbers won't be used.

 

[AlanB]

I measured the imperial thread motion for 24 tpi. The distance moved makes 24.27 tpi. I wondered if the DRO was out of calibration. I checked it against a Grizzly dial on a magnet. It showed close agreement with the DRO. So the lathe is more than 1% off over one inch in threading mode. Didn't expect that. Is that typical? So another feature of the ELS will be to correct for errors in the leadscrew?

 

[WobblyHand]

I measured the imperial thread motion for 24 tpi. The distance moved makes 24.27 tpi. I wondered if the DRO was out of calibration. I checked it against a Grizzly dial on a magnet. It showed close agreement with the DRO. So the lathe is more than 1% off over one inch in threading mode. Didn't expect that. Is that typical? So another feature of the ELS will be to correct for errors in the leadscrew?

That sounds like a lot of error. I know that Little Machine Shop publishes the gearing errors for their lathes and I don't recall errors that large. (Maybe the error is shown in their online gearing calculator.) The error you measured is slightly over 1% pitch error.

One can drive the cumulative pitch error to zero with an ELS, but there's still some quantization error that is relatively significant for some fine pitches. Guess we could speculate about machine rotational inertia and its smoothing effects, but the error is finite. We can drive it towards zero at the expense of stepper torque.

 

[WobblyHand]

Made one measurement on the PM-1228 feed and it was right on, so maybe those numbers in the manual and on the label are accurate, that would be nice. Pretty easy to do with turns counter and DRO, set the gearbox, engage the feed, take the slack out with the magnet right under the pickup, zero the DRO, run it 99 turns, do the last one by hand and end up with the magnet under the pickup, read the DRO. Repeat for 15 gear settings, leadscrew, feed and crossfeed. Only 45 measurements to take. 44 to go.

Actually it won't take that many, if the relationship between the Z and cross feeds and leadscrew are handled separately from the gearbox. It should only take 17 measurements that way.

This measurement is what I'd like to avoid, but I know I will have to do it at some point. I have a simpler lathe with only 9 feeds. Haven't even thought about mounting my hardware yet, but it is time for me to get to thinking about it.

 

[AlanB]

If you have a DRO it is not very hard to measure. A spindle turns counter helps, but isn't necessary, just something to line up the spindle to. I'll repeat the measurement today but I'm seeing about a quarter turn error in 24 turns of the spindle. Checking the DRO against the Grizzly gauge was within a thousandth, this error is about 10x that. So the DRO seems fine. The feeds are ok so this points to the leadscrew. A 25.0mm instead of 25.4mm leadscrew error would be 1.6%, this error is not quite that much. Easy enough to measure even with just 10 spindle turns. Probably within tolerance on actual threads, but unexpected. I just wanted some rough numbers, it can be fine tuned later. Perhaps I can measure the leadscrew directly with Mitutoyo digital calipers.

 

[AlanB]

I verified the measurement for the 24 TPI setting on the gearbox (B4). 24 precise turns produces 0.989 inches of travel, after removing the slack. I sent PM an email asking if this is normal but it is very late in their day so it may not be responded to today. I have looked at a lot of specifications for threads but most talk about diametric rather than pitch tolerances.

 

[AlanB]

PM did respond and said to check the leadscrew drive gear tooth count, they are sometimes mismarked. Should be 90 but they have seen some 91 tooth gears mismarked as 90. I count 91 on this one. That would about do it. 0.989 times 91/90 equals 1.0000. Excellent.

 

[WobblyHand]

Glad you found that. Is PM going to supply you with a 90T gear? Probably should get one if available.