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Electronic Leadscrew for the New Lathe
PekkaNF:
I asked my friends about the quadrature encoder chips and they are using this one:
http://www.lsicsi.com/pdfs/Data_Sheets/LS7183_LS7184.pdf
There is one that is a bit more involved, but it looks like it could offload some processor load (maybe...some pros have pretty nifty counters today.
http://www.lsicsi.com/pdfs/Data_Sheets/LS7166.pdf
There are quite a few different versions:
http://www.lsicsi.com/encoders.htm
At glance max. freg. was not clear, but I think most of them are good for 1Mhz least. Also it was hard to determine how much error they generate at most infavorable case.
I started thinking.... Maybe this error is not that bad here...because you can use index-pulse to reset/initialize the situation AFTER you have both axes rotating but prior engaging the feed. Therefore spurious pulses on start/stationary/stop modes does not matter.
LS7184 has Clk-output and when spindle is at steady speed and "feed" is inching you know the direction.
Bigger issue I see on stepper drive, under load at very slow speed they are close to nominal step error and that is at their best. Microsteping gives smoothness, but does not give that much linear resolution.
I'm pretty confident that VT will pull this off. He has plenty of experience.
I have plenty of experience over/under dimensioning and hit the goldilocks zone after many iterations. :lol: No matter how hard I calculate and plan at first!
Pekka
spuddevans:
The PIC 18F2331 (or 4331) has a Quadrature Encoder peripheral module built in. http://ww1.microchip.com/downloads/en/devicedoc/39616b.pdf
There are also a number of more powerful PIC microcontrollers that have the Q.E. modules built in, some have 2 of them. But for me the one linked above is sufficient.
I have ordered a couple of those pic's and also a Quad encoder off ebay, and a few other bits and bobs as well. This thread has sparked off a little :proj: :D
I plan on using 3 BCD thumbwheel switches as both the means of setting the required Pitch/TPI, and the means of displaying the set Pitch/TPI.
I have a PIC development board I bought ages ago, so I will prototype on that, I've already cobbled together a program, just waiting for the hardware to arrive in order to test it out.
I reckon the total cost should be £30-40 max, especially as I plan on using the 4th axis stepper driver of my CNC mill along with the stepper motor on my RoTab (perhaps with a little modding, without even removing it from the RoTab!!)
Should be fun
Tim
spuddevans:
--- Quote from: PekkaNF on April 09, 2015, 06:15:18 AM ---Bigger issue I see on stepper drive, under load at very slow speed they are close to nominal step error and that is at their best. Microsteping gives smoothness, but does not give that much linear resolution.
--- End quote ---
I was worried about this too, and perhaps with a non-leadscrew equiped lathe it would be an issue, but if the lathe has a leadscrew already, it will also have an arrangement of gearing attached to the leadscrew. You could use a geared reduction from the stepper to the leadscrew. In the case of my mini lathe with a metric leadscrew of 1.5mm pitch and a 2:1 geared reduction, that would result in a resolution of 0.00375mm.
Tim
awemawson:
I'd be tempted to replace the gear train with tensioned toothed belts and toothed pulleys to minimise backlash.
John Stevenson:
--- Quote from: awemawson on April 09, 2015, 12:41:59 PM ---I'd be tempted to replace the gear train with tensioned toothed belts and toothed pulleys to minimise backlash.
--- End quote ---
Yup, gears are so yesterday.
I have replaced the 'standard' ratio on all my lathes with a toothed belt drive to the gearbox.
The gears stacked on the holders are just for specials that are out of reach of the normal ratio's in the gearbox. Means I can run at higher speeds with less noise.
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