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IndustryArena Forum > WoodWorking Machines > CNC Machining Centers > Contour cuts have chatter, rough change in direction, and random misalignments
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  1. #1
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    Exclamation Contour cuts have chatter, rough change in direction, and random misalignments

    Hello!
    First may I put a disclaimer here that I'm new here today after having visited a number of times in the past for various issues. I think this is the right sub-forum to post this (holy jesus there are so many), but if not please let me know and I can relocate. I've tried searching for an existing thread with the issue I'm having without luck so I wanted to post a fresh topic with some specific details. I'm reposting this here from the Milling forum because it seems slightly more active here and also seems to fit the overall topic.

    To start out, we use an older Techno CNC LC Series 4896 machine, paired with MasterCAM 2021 Industrial version. Our jobs are mainly 3/4" plywood contour cuts and pockets of various designs, a handful of small jobs throughout the year. I have a basic understanding of the machine and MasterCAM, but nothing too technical.

    On to the problem.

    Over the past couple years, I've noticed a slow but noticeable degradation in contour cut quality. Sinuous lines or circles became less precise and no longer match the input geometry as closely as designed. See cnc1.jpg for a recent example of this, taken between tool changes. The circle budges at the transition of the Y to X axis movement (moving counterclockwise). There's also a slight indent inwards where the bit begins/ends a pass. This occurred for the full set of 9 circles cut. I'm aware of the tuning of feed and speed to reduce chatter and have had positive results in the past, but this job was turned down to half the typical speed we'd run (in an effort to avoid these issues) and it still produced these abnormalities.

    I'm not sure if it's caused from vibrations or axis problems or a combination of both - or something else entirely.

    A second, more strange issue is a misalignment in passes that occurs along the Y axis only. It appears that very first pass of a contour is shifted just over 1/32" towards the Y origin. The second pass (same toolpath)then corrects itself back down. See cnc2.jpg. This has affected overall dimensional precision of pieces only along straight Y axis cuts.

    One last problem that occurs in tandem to this, and I'm unable to explain as well, is a sudden jump back at the last 1-2 passes, resulting in a tiny shelf that' we've had to shave down. See cnc3.jpg. Occurs only along straight Y axis cuts as well. Thinking it might be linked to the problem above?

    Apologies for the long form of this post and for any glaring omissions/stupidities on my end. I'm happy to provide more info too.

    Thanks!
    Attached Thumbnails Attached Thumbnails cnc1.jpg   cnc2.jpg   cnc3.JPG  

  2. #2
    Community Moderator Jim Dawson's Avatar
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    Re: Contour cuts have chatter, rough change in direction, and random misalignments

    This really sounds like a mechanical problem. Something loose in the drive for the Y axis.
    Jim Dawson
    Sandy, Oregon, USA

  3. #3
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    Re: Contour cuts have chatter, rough change in direction, and random misalignments

    Thanks for the insight. We are quite behind on cleaning maintenance for that drive shaft. I'll try giving it a full wipe down and re-greasing to see if that has any effect.
    All the screws seemed to be pretty tight.

  4. #4
    Community Moderator Jim Dawson's Avatar
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    Re: Contour cuts have chatter, rough change in direction, and random misalignments

    Could be a loose coupling, sprocket, or belt if any of those apply.
    Jim Dawson
    Sandy, Oregon, USA

  5. #5
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    Re: Contour cuts have chatter, rough change in direction, and random misalignments

    Hui Bnnrm - sounds like the machine needs some TLC. Needs some down time and a mechanical check over. Peter

  6. #6
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    Re: Contour cuts have chatter, rough change in direction, and random misalignments

    Okay, thanks again to you all for the tips. I just finished a deep clean and lubrication of both axis. It definitely sounds a lot smoother. But in terms of actually running a job... the circle problem is still there...

    Ran the same job with the same results - weird uneven circle profile.

    There is a weird rattle noise when I move the machine up the Y axis (has sounded this way for awhile, didn't notice it much). Starting and stopping a motion looks like it produces some weird slack and vibrations in the ball nut?

    I took a video of it the best I could here: https://drive.google.com/file/d/186P...ew?usp=sharing

    Skip to around 0:20 mark. The nut wiggles back and forth for a second when the machine stops.
    Could there be some loose item/screw to tighten? Belt has lost slack?

  7. #7

    Re: Contour cuts have chatter, rough change in direction, and random misalignments

    Those machines had two different versions when built, high powered and low powered. The low powered models used pretty complicated belt assemblies underneath that table with a direct coupling assembly that drives a pulley. The pulleys use set screws to hold it to the shaft, and the couplings uses allen screws to clamp them to the motor shaft and the pulley shaft. There is also a rubber spider coupling that adjoins the motor to the rest of the assembly. These rubber couplings do deteriorate over time which would leave a big gap between the motor and drive assembly. Quick way to determine that is to either remove the motor and check it out, or with the power on, try to push the y axis. If there is a little bit of play when you try to move it by hand, chances are that coupling needs to be replaced.

  8. #8
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    Re: Contour cuts have chatter, rough change in direction, and random misalignments

    Quote Originally Posted by Bob_Valentine View Post
    Those machines had two different versions when built, high powered and low powered. The low powered models used pretty complicated belt assemblies underneath that table with a direct coupling assembly that drives a pulley. The pulleys use set screws to hold it to the shaft, and the couplings uses allen screws to clamp them to the motor shaft and the pulley shaft. There is also a rubber spider coupling that adjoins the motor to the rest of the assembly. These rubber couplings do deteriorate over time which would leave a big gap between the motor and drive assembly. Quick way to determine that is to either remove the motor and check it out, or with the power on, try to push the y axis. If there is a little bit of play when you try to move it by hand, chances are that coupling needs to be replaced.
    This was it!
    I actually just came back and saw your reply. I've been consulting with a local Techno technician and he came to the same suspicion as you. The rubber coupling had hardened and fallen apart inside the housing. I believe that clacking noise when moving the axis was the two couplings hitting each other when the motor started up each time.

    Replacing the part was a bit of a challenge. Trial and error trying to figure out the order of assembly and best access. Eventually I got tired of working under the table and just unplugged and removed the whole assembly (motor and drive shaft). The housing around the couplings made it very difficult to align things without getting a clear view through an access port on the side, and a firm grasp on both pieces. Unscrewing the coupling from the motor side, interlocking it with the rubber and other coupling of the drive shaft, and reinserting it to the motor shaft ended up being the way to go. The access port lines up to allow you to get an allen wrench to tighten the screw on that coupling (couldn't reach the reverse one).

    Then just put the belt back on, lined it up, and remounted it in place.

    Cuts are way cleaner and correctly follow geometry now. Thanks again for the help here!

    Adding some photos to give some visual help to anyone else that might encounter this problem down the line.

    1. Discovering the rubber gasket had disintegrated (broken up and black pieces laying inside the housing)
    2. The piece itself reassembled to see its original shape
    3. Do NOT try to reassemble the couplings while they're both on their respective axis, inside the housing. This was the biggest frustration of it all. The fit is simply too tight and visibility too poor (under the table) to get them to align and lock in place. This is when I decided to take the whole thing up-board and re-assemble it on the table where it was MUCH easier to align and retighten screws.
    Attached Thumbnails Attached Thumbnails IMG_1997.jpg   IMG_2003.jpg   IMG_2045.jpg  

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