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  1. #1
    I have a Chinese 6040 router that increases the z-axis height with every pass. I set it to zero, start the program (Mach 3) and with every pass, it lowers a little less.

    I am working on a long carve and put the machine on pause for the night. I went back to the shop this morning to continue and everything started fine. In a few minutes, I noticed that it was cutting in the air. It was not going down to the wood.

    Any idea what the problem could be?

  2. Are you confident that you're only experience inaccuracy on Z? Does this happen on air-cutting or only when cutting material?

    If air-cutting:-

    Write some g-code to test each axis - X, Y , Z independently, measure Z height at start of each axis. Run each axis for 10 minutes independently and re-measure Z.

    I expect that rapid slews on X/Y will not impact Z height at all.

    If you lose Z height on the Z-axis test only then I'd be concerned that the Z stepper is possibly stalling, or otherwise missing steps. Missing steps is perhaps unlikely if your other axis are okay and the control system is the same for all, but it could possibly be one stepper driver (Z) is more on-the-edge than others. Check Mach's stepper driver pulse widths are sensible for the stepper driver (or just change them to 5us anyway).

    If this only happens when cutting material:-

    Is this possibly the stepper stalling when trying to plunge into the material - check the Z stepper driver current setting.

  3. #3
    If the stepper were stalling on plunge it would be taking shallower and shallower cuts until it only just skims the surface.

    I would check that you have solid electrical connections to your Z motor. An intermittent connection to a phase can produce some weird motion.

  4. Quote Originally Posted by cropwell View Post
    If the stepper were stalling on plunge it would be taking shallower and shallower cuts until it only just skims the surface.
    ...resulting in the Z0 position effectively rising, and subsequent plunges being positioned higher than expected.

    EDIT: But the advice on checking wiring is worth following through.

  5. #5
    Doddy, if the stall is caused by the load required to cut into the workpiece, it would only cause a shallower DOC than expected, this would not cause the tool to eventually spin in free air, because each cut layer adds a new DOC in. Once it has stalled on plunge enough times to equal the DOC, there will be no load to cause it to stall.

    I say check the wiring, because it happened to me (and it was a bugger to find!).

  6. Cropwell - I'm not explaining myself well and I don't particularly want to drag out this discussion as it's a distraction to the real OP post. However, if you'll bear with me for a contrived example.

    Workpiece surface = Z0, clearance place = Z3.

    Pocket #1 to Z-10. Controller moves to Z0, then proceeds to pocket to Z-10, but either motor stalls or - something not yet discussed (but something I have experienced) - tool slips in collet - controller believes tool cuts to Z-10, but actually loses e.g. 2mm due to either stall or slip. Tool position is, in this case Z-8, controller believes tool is at Z-10. Controller then retracts 10mm plus clearance- to a total of 13mm. Controller believes tool is at Z3, but having started the retract at tool position Z-8 the 13mm retract results in a tool Z5, controller Z3. Essentially the controller has lost the true reference to the tool Z.

    Pocket #2 is depth Z-1; controller moves by clearance height to Z0 (tool Z2), then cuts to what it believes is Z-1, but is actually Z1 according to the tool cutting face. And on it goes. Contrived example perhaps, but if the OP is 2.5d carving then feasible. Of course this is just one example of one possible problem.

    OP - this considers another possibility of tooling slipping in the collet. All things discussed here are possibilities to investigate.

  7. #7
    Doddy, you are right in your first example, but if you are cutting a 10mm pocket in 1 pass you will cut an 8mm pocket and retract to Z5 and stop. but if you work the example of a 10mm pocket with 2mm DOC and 1mm stall slippage you will get a 5mm pocket and retract to Z8 at the end, but never cut air, just a shallower DOC.

    Anyway. I would be interested to know if the OP saw the cutter moving around way above the original material surface, or just above the cut level. It still points to a stepper / driver wiring issue. Or maybe condensation messing up the control signals.

    My experience of tool slippage is that it works its way downwards out of the collet before it snaps due to excessive forces. This has happened to me twice, and one of those times it went through the spoil board to the bed.
    Last edited by cropwell; 08-12-2019 at 09:24 PM.

  8. #8
    If the machine has always done this (is it fairly new?) then it could be as simple as wrong polarity dir signals to the stepper. In effect, the direction change signal is happening at the wrong point of the step pulse and you gradually get a drift in direction on that axis. Not an uncommon problem when a machine is first set up. There is quite a number of threads talking about this but it's late and my brain is a bit too frazzled to give chapter and verse here.

    On normal profile cutting, you generally don't see the problem as there are fewer direction changes but if you are doing 3D carving and, in effect, lose one step in Z every direction change, then it builds up. That's how I found exactly the same problem on my own machine, except that in my case, the drift was downwards and potentially more damaging...

  9. #9
    Quote Originally Posted by Doddy View Post
    Are you confident that you're only experience inaccuracy on Z? Does this happen on air-cutting or only when cutting material?

    If air-cutting:-

    Write some g-code to test each axis - X, Y , Z independently, measure Z height at start of each axis. Run each axis for 10 minutes independently and re-measure Z.

    I expect that rapid slews on X/Y will not impact Z height at all.

    If you lose Z height on the Z-axis test only then I'd be concerned that the Z stepper is possibly stalling, or otherwise missing steps. Missing steps is perhaps unlikely if your other axis are okay and the control system is the same for all, but it could possibly be one stepper driver (Z) is more on-the-edge than others. Check Mach's stepper driver pulse widths are sensible for the stepper driver (or just change them to 5us anyway).

    If this only happens when cutting material:-

    Is this possibly the stepper stalling when trying to plunge into the material - check the Z stepper driver current setting.
    Thanks for your response. It turns out that the problem was cold weather. My shop is not heated, and as soon as it warmed up everything worked fine.

  10. #10
    Quote Originally Posted by pcoovert View Post
    Thanks for your response. It turns out that the problem was cold weather. My shop is not heated, and as soon as it warmed up everything worked fine.
    I'm not sure I'd be happy with that answer if this were my machine. Might imply an underlying issue, like an underrated PSU?

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