What have I gotten myself into!: CNC lathe conversion

From debugging hell, to success.

From my earlier experience setting up Mach3 to run with the mill, I knew that some painful debugging would surely lay ahead, and I was not to be disappointed. I will give some details of my troubles here, in case someone else were to follow in my footsteps.

With the latest release Mach3 at present time (R3.043.066) running on Windows 7, a UC-100 USB to parallel controller (update 2.146), and a G540 (ver. #8), I’ve been having an issue with a runaway axis (or two). This is not a new issue, I’ve had this happen to me infrequently in the past, maybe once every few months or so, but now it's happening every single time, and it’s making my machines completely unusable, and somewhat dangerous.

Unable to continue with my conversion work, and quickly running out of ideas, I was able to catch this problem as it happened on video. Unsure as to which was the source of the problem, I sent it to GeckoDrive support, CNCdrive support, and posted it in a new thread on the Mach3 support forum.

Runaway X axis

Of course, there was always a chance that Windows might be to blame, so I updated it, though it ended up not making any difference. Getting pretty desperate at this point, I deleted, then reinstalled Mach3. This finally appeared to fix things, and I regained control of the lathe and the mill… until the problem started again about 1 hour later, as I was setting up the motor tuning options for the lathe, this time causing me to loose control of the Z axis.

I decided to limit the top speed to 40in/min to more precisely position the cutter by hand

Top speed on a lathe is pretty irrelevant

I captured that in a second video, sent it to the same outlets as before.

Runaway Z axis

By this time I had finally gotten a hold of the Gecko tech support, which said exactly what I was expecting, but hoping they would not say, you probably guessed it… their board could not be the one causing the issue, it just reacts to input signals.

I tended to agree though, since the UC-100 screen shows an active "Run" LED every time there's a runaway, so it knows something is going on. On the other end, Mach3 DROs (digital readout) showed no movement whatsoever while the axis was running away, like it had nothing to do with it at all. Perhaps the UC-100 was the party to blame after all.

To compound the lack of actionable troubleshooting information, running the mill and lathe from my laptop produced no runaway axes at all, like the problem resided with the “all in one” computer in the shop instead. I was quickly loosing faith, until someone in the Mach3 forum had the bright idea to suggest I turn off the backlash compensation, and all of a sudden everything was right with the world again.

Apparently backlash compensation is calculated in the UC-100, and not in Mach3, so I passed the information on to the Hungarian support team at CNCdrive, who promised to have their programmers look into it the next Monday.

Meanwhile, I moved on with other setting up tasks.

One thing that really surprised me of the BDtools CNC conversion, was the ability of the anti-backlash nut in the cross-slide (X axis) to completely remove all backlash from the lead-screw, when properly adjusted.

Anti-backlash nut and its adjusting setscrews (top bolts are for attaching it to the cross-slide)

Checking for backlash on the X axis lead-screw

The Z axis did not do as well with 0.0023” (0.0584mm) of backlash, even though it was sporting a very tight ball-screw.

Checking for backlash on the Z axis ball-screw

This meant that I really didn’t need to use Mach3 backlash compensation after all, though I was happy to have played a part in the debugging of the UC-100 unit, of which I am very fond, and would definitely buy again.

Having sorted out these very important issues, I felt confident enough to run the first air-cutting test, a traditional first step.

Cutting tool running under CNC power for the first time

I really felt like things were moving along, until I realized that everything the computer would come up with was in fact happening not only in reverse, but also in the wrong direction and quadrant on the actual lathe.

I’m going to give you the short-short version here... it took me 2 more days of troubleshooting hell to make this right.

I hate setting up Mach3!

The good news now is that I will save my Mach3 turning profile in a safe place, so that I will never run the risk of having to go through this painful experience again.

I will include some pictures of my config pages at the bottom of this page, in order to spare you the trouble (should you be interested). You might still have to tinker slightly with the motor’s steps per unit numbers, since lead-screws and ball-screws are not all created equals, but this should get you really close. Note that I am using the lathe in “diameter mode”, as opposed to “radius mode”, as in normal manual operations. I am also working with X+ toward me, and X- on the further side of the spindle centerline, though backward, it was the only way I could get the code, the screen, and the cutting tool to agree on the same plan of action.

Finally, I started playing with the “turning wizards” included in Mach3, and practiced a bit on a ¾” (19mm) Delrin rod.