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Thread: X-Axis is Jittery/Skipping

  1. #1
    Join Date
    Oct 2014
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    Utah, USA
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    15

    Question X-Axis is Jittery/Skipping

    I'm not sure how to word this better, so forgive me if the issue isn't very clear. I will try to post a video soon. I am a student in a High School, and we have a gantry style ShopBot CNC in our wood shop, one of the large ones. I think it is a pretty old one. I have attached a picture of it. I am the only one in the school who knows how to operate the CNC, so I handle all the upkeep for it as well.

    The CNC had been working perfectly until one day it started acting up, being jittery, skipping around, and losing its place while carving. The v-bearings on the right side of the x-axis were falling off the rails when the x-axis was moving at high speeds as well. I don't know how it happened, the teacher said that someone may have accidentally gotten something in the way of the x-axis while it was moving, but nobody knows for sure what happened.

    Basically the issue is that when the x-axis is moving at high speeds, (unsure about the exact speed, I will get back on that. Maybe 50 inches per minute?) you can definitely see the x-axis getting stuck or jumping around. It makes a bad grinding sound sometimes too. This causes problems because the computer thinks the CNC is in a different place than it really is, and it screws up the entire carving when this happens. Things end up in the wrong spots, overlapping, etc. At first the v-bearings on the right side of the x-axis would fall off the track/rail they are on, but that part seems to have been pretty much solved. We had to re-square the entire machine multiple times, and the rail that the v-bearings ride on seemed bent, so we used clamps to pull on it while someone tightened the bolts underneath, and that seemed to straighten it out. We haven't had issues with the v-bearings falling off anymore, but it does still skip/jump/jitter/etc a lot. It happens a lot less when I lower the speed.

    Currently we have the CNC moving very slow because that's the only way to get a consistent carve without jittering, but it is a bit annoying because we have a lot of students who want to carve and can only get through a couple per period because of the time it takes to carve.

    Can anyone offer up some suggestions on what might be going on here? I don't know how much detail I need to provide, so if there is something you need, I will be happy to share more details. It may take time to give more details because I can only work on the CNC for a few hours a day since I have other classes. I can record videos, take pictures, etc. I know a lot about computers and electronics, so feel free to ask the more technical questions. I can get you guys any details you need in order to help me fix this. Thanks!
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  2. #2
    Join Date
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    iBILD Solutions - Southern NJ
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    Mitch,
    It sounds like you have a 'tired' motor driver - or even a bad wiring connection somewhere on the lagging X axis. Shut the control box down and inspect the wiring from the control box to the affected motor. Any break in the wire or fraying can cause the driver to fail or burn out...so look carefully.

    Are you running the original control box (white & says ShopBot on the front) or did you upgrade it to newer controls (4G upgrade) ?

    Is this running on DOS (ShopBot v2 control) or Windows (SB3)?

    -B
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  3. #3
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    Oct 2014
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    Utah, USA
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    I believe it is the original control box. I have attached a picture I took of it earlier today. (The uploader decided to rotate the image, sorry about that.) The computer is running Windows 7 and ShopBot v3.

    Thanks for the suggestion on checking the wires! I'll make sure to do that tomorrow and let you know if I find anything.
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  4. #4
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    If your wiring is in good shape, turn the control box off & unplug the cable to the 'tired' X motor. Plug it into the 'A' port on the control board (next to Z). Then in SB3, type in VI and change the value for Driver Channel 4 to say X instead of A. Then you should be back in business.

    I cannot stress how important it is to have tidy, neat & strain relieved wires on those tools. Bad connections or wires that get pulled out while running will kill a motor driver in short order. I'm not sure if SB repairs those boards anymore (some are an easy fix by replacing diodes - others are not), so you may want to consider a 4G upgrade. It's around $1500 or so and will make that tool faster, smoother and the drivers are easily replaced.

    -B
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  5. #5
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    Oct 2014
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    Utah, USA
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    Great, thanks!

    The school hasn't taken very good care of the CNC, and from what I've seen the wires aren't very tidy. Now that I think of it, with the way that the wires have been treated over the years, I wouldn't be surprised if one of the wires got broken. It would probably be smart to run new wires to all the motors, but for now I'll just inspect the ones that are already there, check the connections, etc. Saw dust could have gotten into the connections, too. I know the control box has stopped responding multiple times due to saw dust, but a good blow with the air duster generally fixes it. (The computers aren't very protected...) Eventually I will have the entire thing fixed up so that when I leave school in two years they'll still be able to use it without much help.

    I'll let you know tomorrow if it is fixed! (And if it isn't, I'll see what I can do about recording a video) Thanks for the help

  6. #6
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    I inspected all of the wires running to every motor today and there were no breaks in the wires. I inspected the motor connectors plugged into the back of the control box, and one of the x-axis motor connectors is a bit broken, but it was definitely still connected properly. I tried swapping the x-axis motor I suspected had a "tired" driver with A, but after replacing A with X in the software, only the motor still plugged into the other X driver would move.

    Playing with the machine, I noticed that the jittery behaviour never occurs when jogging, only when running a command like JX -6, or running a ShopBot file exported from ArtCAM. I made a video of the behaviour which I hope will help you understand my issue more, and I attached a couple pictures of the current speed and ramp settings. This behaviour leads me to believe it is a software issue rather than a hardware issue. I could be wrong, of course.

    Any ideas? Thanks for all your help!
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  7. #7
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    Your VR values look way too aggressive for your tool. It looks like the VR settings from DOS were transferred over to SB3, which handles ramping completely differently. I would reset them back to default - there should be a link to Reset this at the bottom left of the fill-in sheet.

    It could be that one of the motors is stalling, but I still think that one of the motor drivers is either flaked out or cooked. A 4' PRT can drag the weaker motor along for the ride at lower speeds without you really noticing. I'd still recommend upgrading to a 4G setup.

    Since your X car is now racked from the tired motor...some squaring is in order once you get that other motor working.

    -B
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  8. #8
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    Thanks for the tips! I'll talk to my teacher about upgrading to a 4G setup. Thanks again.

  9. #9
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    Jun 2004
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    Springfield Mo
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    809

    Default Harmonic vibration

    If there is a certain speed that is bad, what happens if you double it or half that speed ? If so, it could be some harmonics which is common to many moving things.

    Can you swap the motor side to side, or just see how each motor acts alone ?

    Brady probably has it figured. The 4G setup is much better.

    Do a 4G fundraiser with some local CNC firms to help out.
    The decimal point seems to be the most important on the z axis... x & y not so much....
    ShopBot... Where even the scraps and things you mess up and throw away are cool....

  10. #10
    Join Date
    Apr 2001
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    South Elgin, IL
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    Are you using Shopbot software version 3.4.27 ?

    Your machine is exhibiting similar characteristics mine had when the pinions on both X motors were not square to the rack. In other words, one X motor pinion was engaged in the rack one (or more) tooth closer or farther from X=0. I think this can cause the stalling or jerking you are having.

    You can also check each X motor individually by first shutting off the control box, then unplugging one X motor, turning on the control box and give your jog commands.

    I would do this with the pinion gear of the connected motor engaged in the rack, and then again with it not engaged in the rack and this could give you a better idea of what's happening.

    You mentioned at some point it had jumped off the rails.
    Did the turnbuckle tensions get changed? Check to make sure both X motor turnbuckles are at similar tension. If I remember right, it should be about 1 1/2 turns tighter after you first start to see the spring becoming tight as you turn it.

    Check that the allen screw on each pinion gear is tight and that the gears line up square with the rack - not too far in or out.

    It's difficult to see in the video if your machine has any hard stops installed on the X rails. If it does, jog or move the gantry until it's almost at the stops on one end, then shut it off and disconnect the X motors. Then manually pull the gantry against the stops and you will be able to tell if it has been put back on the rails in an out of square position after it had jumped the rails. If it's not on squarely you may be able to get it close just by lowering one X motor pinion away from the rack, pulling it square to the stops, and while keeping it against the stops raise the motor pinion so that it engages the rack again and then re-tighten the turnbuckle properly.

    Then reconnect the motors electrical connections, turn on the control box and test again.

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