Software & Electronics for Mini-Lathe?

[SteveHGraham]

I got the lathe going, but the accuracy was disappointing. I haven't messed with it in a while.

I am told the lead screw I bought is not going to get the job done, and I am a little reluctant to blow a huge pile of cash on a good one for a lathe this small. Maybe I'll get back on it.

I am using a Kmotion/Kflop combination.

[WJH]

I got the lathe going, but the accuracy was disappointing. I haven't messed with it in a while.

I am told the lead screw I bought is not going to get the job done, and I am a little reluctant to blow a huge pile of cash on a good one for a lathe this small. Maybe I'll get back on it.

I am using a Kmotion/Kflop combination.

Mini-lathe 7x are junk despite their loyal following. 8x HF or Lathemaster is a far nicer lathe. That Kflop is a nice controller, make a closed loop system with it using linear glass scales

[SteveHGraham]

The lathe wasn't really the problem. I'm not promoting little Chinese lathes as precise, but my big problems were with the things I added.

It would have been nice if the guy who drew the plans had said, "By the way, this lathe probably won't be all that accurate."

I want to do CNC milling, and I looked at the mills people put together, but a friend of mine who knows a great deal about this stuff said you can basically buy a real CNC mill or expect poor accuracy. The last thing I need in my garage is a another multi-ton machine.

[Bill Shields]

your buddy is absolutely correct.

[ctwo]

The pro's even cringed when I told them I got a Bridgeport BOSS CNC (the knucklehead version).

[daGrouch]

I guess I'll add my experiences to the mix. I'm just a tinker and your mileage will vary.

The story thus far: I have an old Smithy 3-in-1 and I went the cheap (kinda) route. Chinese Nema 23 and 34 stepper kits with printer port controllers. I run 2 printer port BOB's as I needed more inputs. I have a single pulse spindle index signal and 3 simple rotary encoders to move the axis' under power. The Z axis is belt driven and the other two have a flexible rubber hose as a direct drive coupler (I keep snapping spiral couplers). I am using the stock square tooth lead screws and it's all run through Mach3. Shielding is poor and the whole thing is a lash-up work in progress.

Threading: This is what started it all. I didn't get any change gears with the machine and it has no half-nut so I went CNC mostly for threading. Contrary to some previous posts I saw the index works just fine and all my threads have been spot on as far as pitch goes. Since I run at realistic speeds for my heavy table, pull out has not been an issue, either. My spindle is manual controlled so I have to match the speed with what I program in the thread code. With all its slop I have threaded 2 mm fine pitch screws. In short, steppers and an index pulse can work just fine here.

The shortcomings many and in different areas:
Mach3 - while it does work, the wizards are frustrating to me. I like using Diameter mode but some bits of Mach3 (like Depth Of Cut) uses radius sometimes. The Windows mode printer driver, while a marvel of code from Art, is still a bitch because of Windows and pulses do get lost. I would like to try EMC2 but I am not Linux savvy and cannot compile my machine into the software.

Backlash: This is a killer for me as steppers are open loop. Even with backlash compensation, unless it's dialed in PERFECTLY and often, errors will propagate and multiply. You would need to convert to ballscrews for both the accuracy and to keep stepper motor speed low. I kept the stock screws as I genuinely like machining and work manually most of the time and ball screws are lousy for manual work. I am REALLY interested in the earlier comment about Kflop and closed loop operation.

Software: Unless you plan to become a G-code expert you need some CAD/CAM stuff to make parts. I have yet to find a package that fits a hobby budget and mindset. One app might do tool paths okay but have a hideous CAD side (Bobcad) or might be okay drawing parts but be so geared to full blown CNC shops that the machine setup is like building one from scratch (Mastercam). All of them are geared for carbide insert tooling so home ground HSS is a bugger unless you know your tip radius ect. for tip compensation to work. For simpler 2D stuff like lathe work and simple milling a draw app like Corel or Illustrator are so much easier to do a layout only to find the CAM software won't import it correctly and you can spend all day working the bugs out of a 20 minute manual operation. I've made some pretty parts in AutoCAD Inventor only to have to do it all again in the CAD package.
THEN you need to learn how to edit a machine post interpreter of whatever CAD/CAM you have to get the G-code into a format your machine will like. You could use Mach3 wizards but they are poorly documented AFAIK and don't tell you if they are using Diameter, Radius, or readings from sheep entrails. There are packages from Kentech called Kipware that are wizards on steroids but they are pricing themselves out of the hobby market.

My setup works - kinda. I can let it run to do some roughing but I can't trust it so I do the finish work manually. A pure conversion with ball screws and lapped gibs will fare a lot better.

My conclusions:
Open loop steppers can work on a tight smooth lathe.
Chinese stepper kits are mismatched. My first was a Nema 23 kit with 460 oz motors and a 36 volt power supply. They seemed a bit weak so I got a bigger Nema 34 kit with 48 volt supplies. When I ran the small motors on the larger supplies they ran great on the higher voltage and I only swapped one of the motors (Z axis / heavy table).
Run stepper motors as slow as you can without losing resolution. My Z axis was geared down 2:1 as I thought it would help the torque - WRONG. When I swapped the pulleys it worked better as stepper torque is all in the low speed range.
Start with a better motion controller than printer port if you can like USB or Ethernet. More inputs/outputs and you can run a better faster computer ie. future proof and avoid the whole Windows background interrupts and lag mess. It took me days to get 2 P-port bobs working and I think I am still losing pulses somewhere.
Convert to CNC all the way. Running a half-in-half has many compromises.
If you use Mach3 stay with either Imperial or Metric. I use both as metric is easier on me math wise but I can only estimate and visualize in Imperial. I have ruined many parts by mixing the two.

I guess what I am saying is you can spend some money and a lot of time to get your mini lathe working okay or a LOT to get it working a bit better but is it really worth thousands of dollars to make $4 parts as a hobby? I not still not sure but sometimes it's nice to have the machine do the grunt work.

[DICKEYBIRD]

Great post Ed! I can definitely tell you have "been there & done that."

Like you, I'd love to use LinuxCNC on my lathe but I certainly don't have the computer savvy to switch over. I loaded up the standalone CD & tried to run with it but could see very quickly that the learning curve was just too darn steep for me.

KFLOP is unfortunately a similar situation for me since C programming skills are required. Only thing I know about "C" is that it comes after B.

[SteveHGraham]

I'm thinking I've overlooked a truly worthy purpose for the CNC lathe: woodworking. I should keep trying to improve the accuracy for metal, but it's stupid to neglect the potential for wood, which is not nearly as demanding.