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  1. #1
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    Aug 2009
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    Jerky motion with Davinci

    I picked up an old Stepper Davinci at an auction a while ago to use for pearl inlay work on banjos, finally getting to trying it out. I'm finding that running shapes with curves, the machine "stutters", a jerky sort of motion. As if it's running path segments individually rather than as a smooth curve. I'm working with Illustrator and Fusion360, WindowsXP on IBM Thinkpad with native serial port set at 9600.

    I tried a circle - it ran smoothly. Then an ellipse - got stuttering. Adding a few extra anchor points to the circle made it stutter also.

    So is this just the way those Davinci controllers work? I know everyone recommends upgrading with a Gecko or such, but I'm not looking for speed, was hoping to be able to use it stock But I'm thinking .020 cutter would never survive that kind of jerky motion.

    If I do upgrade to a Gecko, can I use the Techno stepper motors? They seem very substantial, as does everything on the machine, which is why I bought it.

    I'm a complete newbie to cnc, enjoying clawing my way up the learning curve. The software was pretty daunting, but once I got the new vocabulary and concepts it became fun. Hope its the same for actually using the machine. I'm a semi-retired professional furniture maker, just making banjos now.

    Thanks, Richard

  2. #2
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    Apr 2004
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    4877

    Re: Jerky motion with Davinci

    You might have it running in "Exact Stop" mode (G61), rather than "Constant Velocity" (G64). A circle is typically done with a G2 or G3 command, which doesn't have any intermediate stopping places, while an elipse is made up of short sections. Try inserting a G64 early in the program, and see if that helps.

    If the Techno motors don't need more than 3.5 amps or 50v, the G-540 should work fine. But your controller's probably okay; you just need to get used to the way it works.
    Andrew Werby
    Website

  3. #3
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    Re: Jerky motion with Davinci

    Thanks for the suggestion Andrew. I looked at the G code and there weren't any G61's. I inserted a G64, but it didn't change anything.

  4. #4
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    Re: Jerky motion with Davinci

    You might check in the General Logic Configuration panel, to see if Constant Velocity is chosen, and adjust the look-ahead, under CV control. There's some info on setting it up in this PDF: ftp://ftp.machsupport.com/Docs/Mach3_CVSettings_v2.pdf
    Andrew Werby
    Website

  5. #5
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    Re: Jerky motion with Davinci

    I think you're right about it being an issue with CV and look-ahead, but my machine doesn't use Mach3, it still has the proprietary Techno controller and G-code interface. I don't think it has those features at all, at least I haven't found them yet.

  6. #6

    Cool Re: Jerky motion with Davinci

    My gcode controller has a feature called continuous motion. That really smooths things out. It also has a feature called acceleration and I find that even if I'm not cutting in continuous motion, if I dial back the acceleration rate it smooths things out quite a bit. Not sure how it is with steppers though. I have servos. They work great. There's just a few tweaks I need to make with the post and it should be pretty good for me. I think I should probably post a rant about that because the official cnc software post isn't even known to exist. The post I use is 2or3 versions older than the software. What the heck? Still this vintage machine runs just like my vintage german made vehicles. Shweet!

  7. #7
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    Re: Jerky motion with Davinci

    I wound up replacing the Techno drivers and controller with a Gecko 540 and Mach 3 - what a difference! Should have gone right to that as so many suggested. I kept the original steppers and power supply.

    AMG - what model machine do you have? I'm very impressed with the build quality and accuracy of the Davinci, but could really use a larger machine.

  8. #8

    Re: Jerky motion with Davinci

    Mine is a gantry 3 model 130 with servos. It's the largest table from that line. About 4sq ft.
    Question.
    The original g code interface for stepper that you have doesn't have a continuous motion function?
    BTW if you are cutting pearl you have to go real slow. I cut it at about 5ipm with acceleration set at 3 using a 0.022" endmill. 21000 spindle rpm and 50% stepdown. But those tiny bits are just too small. I'll bet you can go much faster with a 1/16" bit. The problem is holding the small pearl parts. I've seen people use blue tape crazy glued together on large aluminum pieces. I'll have to try that. I have used double sided tape but it's not always reliable.
    This is a job for a small spindle, not a big 3 1/4 hp Porter Cable! Ha, ha! But it works. I'd like to put a small spindle on the Z just for pearl and other small stuff. Any suggestions? I'm thinking something like a little dremel but the bearings on those ain't that great.

  9. #9
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    Re: Jerky motion with Davinci

    It's been a year since I ditched the OEM controller and driver, don't remember if it had continuous motion function. But the Gecko and Mach3 combo has been great.

    I've had no problems cutting and inlay pearl using mills as small as .015". My machine has an old Kress router, probably 24,000 rpm. It had too much runout, around .003 -.004, so I reground the taper to get around .0001" TIR. Those tiny mills would never have survived!

    I glue the pearl down to mdf with Titebond, then soak it off in water afterwards. I've also glued it on with CA glue or Duco, and then used acetone to release it, but I prefer water to solvents.

    I think a small auxiliary spindle makes a lot of sense.. Maybe a pneumatic pencil grinder, thats what I used to use for inlaying into ebony. They're inexpensive, weigh nothing, and go up to 60,000 rpm. Would be easy to make a little bracket to piggyback on your router mount. Or a 1/4" die grinder if you want more balls.. Just check the runout and return them til you get an acceptable one. The dremels are way too flexible, you can wiggle the spindles with your fingers, the bearings are held in plastic, not rigid at all.

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