Gallery, Projects and General > Project Logs
Electronic Leadscrew for the New Lathe
vtsteam:
Making encoders and quadrature theory:
http://www.societyofrobots.com/sensors_encoder.shtml
http://www.dgkelectronics.com/inkscape-extension-for-creating-optical-rotary-encoder-discs/
https://github.com/Hyvok/Inkscape-rotary-encoder-disk-generator
http://tutorial.cytron.com.my/2012/01/17/quadrature-encoder/
vtsteam:
Going to bed. Seems like this leadscrew drive would all be easy to test out with equipment i already have -- maybe even on the Gingery as a temporary kludge, except for the sensor for the encoder.... there might even be one of those at the local Radio Shack....
awemawson:
Steve, gearing up the encoder is definitely workable. On my Traub the main spindle encoder is 1024 slots, but the same encoder is used for 'C axis positioning' geared up to 90,000 points on the circle !
Fairly obviously you need to avoid any backlash, but the Traub just uses a toothed belt under tension and seems to work !
RussellT:
Steve
I think that's a brilliant idea. :clap: I wish I'd thought of it.
I've been thinking about it. Direct drive to the leadscrew at 10tpi would give steps of 0.0005" which I think should be fine for most purposes. If you need more torque I might think of using the epicyclic gears from an old cordless drill as that would give a compact drive.
A 12tpi leadscrew would give you better solutions for threads like 6,12,24 tpi as well as slightly smaller steps.
As far as your encoder goes the diameter limits the number of steps. For convenience you will presumably be using an ordinary sized printer which would give a maximum diameter of about 8 inches and a circumference of about 25 inches. I'd guess that using plain paper and a reflective sensor you could easily use marks about 1/16 apart giving 8 per inch, or 200 per revolution.
I've taken printers apart and they have finer resolutions than this but using a clear disc with a transmitted light sensor. From the point of view of changing discs it makes sense to have all the hardware on 1 side.
At 200 pulses per spindle revolution the coarsest thread you could cut would be the same as your leadscrew pitch. That's very close to working without gearing on either spindle or leadscrew. If you could make your encoder read finer markings then you're almost there.
You could also have multiple sensors across the radius of your disc so you could use switches to select from a number of different threads and fine feeds on a single disc. Old CDs seem an obvious candidate for discs - diameter is reduced but you could use the centre from a CD case to make changing quick. You need to get the printed markings down to about 0.01" wide to get a thread of 6tpi from a 12 tpi leadscrew.
I also like the potential for having an electronic disengage when thread cutting.
The difficulty is as always picking up the thread.
I don't think a single point clutch on the spindle would work because if you wobbled the encoder disc engaging the clutch you could generate a lot of pulses of unknown direction and lose position, and you couldn't disengage the half nuts for a quick return. If you used a switch to disengage the feed you would lose position too. I think the simplest way might be to mark a starting position on the encoder disc (if you used gearing to drive it you'd need to mark the spindle too) and return the carriage to a fixed starting point. Starting with the lathe stationery also takes care of accelerating the leadscrew as pulses are accelerated naturally as the lathe spindle accelerates and reduces the possibility of missing steps.
Russell
lordedmond:
If you go down the CD route they use to make printers that printed directly on to CD that had a coating on them , don't know if they are stil made though.
Some had a laser in them and burn the info into the disc others used the ink .
Note I do not mean the ones that you print onto a sticky backed paper and put that on the disc that would not be true enough
Stuart
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