Electronics > Mechanical & Automation Engineering
Daydreaming about an Electronic Leadscrew
jpanhalt:
The leadscrews on my lathes are imperial thread. I would like to cut metric threads. I know there are conversion gears for some lathes, like older Hardinge, for which one of the pair is 127 teeth and the other is 125 teeth (or similar). The little lathe I want to convert is a Prazi SD300 (attached). It's a bit unusual in not having half-nuts or threading dial, but can still be used for threading with various change gears. It's still a PITA to do. The leadscrew is 20 tpi (0.05" per turn). My first thought was to make an electronic half-nut and threading dial, which should be relatively easy. Here's the daydream part: If going to that trouble, why not design for real metric too?
My rough plan is to use cog belts and a stepper motor to drive the leadscrew with a rotary encoder coupled to the spindle. I will probably have an encoder on the leadscrew as well, either for feedback or simply for doing what a threading dial does.
I haven't found any cogged/timing pulley that has 127 teeth. Can anyone suggest another simple way that will give a very close approximate to metric? I haven't worked out the allowable tolerance for various metric threads, but read that 1% error per thread (i.e., 0.01mm per 1mm pitch, or 0.05mm off at each end for 10 threads) is OK for a home workshop.
tpowell1830:
There are several YTers that developed an electronic lead screw but I liked this guys approach.
Andy Watson:
I've been watching the Electronic Lead Screw project , perhaps one day I will get around to doing something about it. In the meantime, I have both 21 and 63 tooth gears in my set of change wheels. If you only require occasional use, could you 3D print a 127 gear?
jpanhalt:
Thanks for the speedy replies. I am not too worried about the coding. I did some of that several years ago to coordinate a stepper motor to a rotary encoder. I used an optical one (lost in memory) and a magnetic one (AMS5048).
My main concern was mechanical for getting as close to 1.27 to 1 (or whatever). Of course, doing metric threads on imperial lathes is not new, and I found a table some generous person posted years ago (attachment1). W= gear on spindle (Werk) that drives Z1; Z1 and Z2 on a keyed shaft (white gears) and turn together; L = leadscrew (attachment2). The gear combination shown for 1mm pitch came up to 1.000606, which is less error than I suspect for the leadscrew when cutting a 20 tpi thread. ;) Hope my math is right.
I need to start playing with a spreadsheet. What are good hobbyist sources for cogged pulleys? I am not totally adverse to gears, but belt drives are quiet.
james_s:
--- Quote from: jpanhalt on March 28, 2023, 04:49:29 pm ---I need to start playing with a spreadsheet. What are good hobbyist sources for cogged pulleys? I am not totally adverse to gears, but belt drives are quiet.
--- End quote ---
I would try McMaster-Carr, they're my go-to for mechanical bits and pieces.
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