Variable speed and backlash and ballscrews.
Variable Speed:
I've never seen a VFD for a single phase motor, and I can certainly understand why.
With a single phase motor, there needs to be some way to start the motor in a particular direction, which is usually an extra winding or a cap. The VFD electronics would have all kinds of problems dealing with this added reactance and so would be VERY expensive as a result.
Your choices (for variable speed) are a 3 phase motor with VFD or a DC motor with suitable controller. I use both in my little home shop, and my experience is that the DC motors give more power at lower RPMs, which is in fact to be expected.
The DC controllers themselves are also considerably less expensive than a VFD, but this is offset by the fact that a GOOD DC motor is considerably more money than a 3 phase motor.
Note that there are all kinds of DC motors, from good to crappy. For example, my lathe uses a Pacific Scientific motor that probably weighs 25 pounds. The 2 HP "treadmill" motor I _was_ going to use (until I saw it) weighs about 5 pounds.
The only 3 phase motors you can buy are good ones, because they're only used industrially.
The good news is that sometimes 3P motors can be had cheap and sometimes VFDs can be had cheap too. I bought a 2 HP 3P Baldor off eBay for $35.00 and a 3HP Mitsubishi VFD for about the same.
I use this pair on my little drill press, in place of the 1/2 HP Chinese motor that it came with. Sounds like overkill, but not really. I set the VFD to run at 120 HZ, which spins the motor at double speed. Now, at 1/4 the nominal speed, I get 1/4 the power, or back to the 1/2HP it came with stock. So I end up with an 8:1 speed range and never less that 1/2 HP.
Two little gotchas in this.
1. The theory and the manufacturers both say not all 3P motors will run with a VFD. I haven't found any that won't, but that's what they say.
2. Most motors rely on the fan at the end of them to keep them cool, and a 3P motor with VFD under load at low speed can certainly heat up. On my mill with a 3P motor (no bucks for the DC motor), I just mounted a couple of big computer fans aimed at the case, one on either side. Seems to work.
Backlash:
You're correct, backlash could be an issue. On my Sherline, I use timing belt pulleys and a timing belt; no more lash than anywhere else in the system. I would expect to have to do this here as well.
There are a few encoder "strategies" for lathes. The first is used by Mach3, and uses a single encoder. This essentially provides speed information, as well as a starting point (rotationally) and by all accounts, seems to work to create nice threads (the only reason for requiring a spindle encoder).
EMC purports to use whatever encoder you have, from 1 CPR to many CPR. Not sure about this, though, as I'm not an EMC user.
And I believe that DeskCNC will thread, using a multi count encoder mounted on the spindle.
Finally, there's the Electronic Leadscrew project that a guy might want to look at.
For my Sherline, I cheat. I still use CNCPro for DOS, that knows nothing about lathes or spindles. I call the spindle the Y axis and report in the steps PER INCH config the steps for 1 ROTATION.
The (simplified) GCode to make a 1" long 20 TPI thread then becomes
X1 Y20
A 1" long 1mm metric pitch thread (which I admittedly have not yet cut) should be
X1 Y25.4
Ballscrews:
Yeah, no problem making a disconnect, but you have to admit, a system that does not require it would be pretty neat.
Alan
Are you still happy with the CT089?
Hi,
Its been a couple of years now since you posted this and I was wondering how the lathe is holding up for you?
They have them on sale this week and I'm considering getting one to replace my 3-in-1 that is doing only lathe duty since I made my cnc mill.
Any major issues since you last posted?
Did you ever cnc it?
Thanks,
Mike