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IndustryArena Forum > WoodWorking Machines > Commercial CNC Wood Routers > how to adjust for bumpy rough parts
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  1. #1
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    how to adjust for bumpy rough parts

    I am having trouble getting nice clean parts on my CNC, I am machining solid wood and every time the edges come out very bumpy, I tried a variety or cutters, two flute and three flute, I tried changing feeds and speeds and I never really seem to solve the issue. In a different post of mine the subject came up and someone suggested it could be the gantry that isn't stiff enough. This seems like a probable cause, now that I examine it closer it does seem to be pretty shaky. It is a rack and pinion system, I had some problems with the rack along the bed of the machine not holding the gantry tight enough, I adjusted that but the problem persists.

    This is the machine we have at the shop https://lwmcnc.com/cnc-mills/maverick-4x8-2020/
    Ours is a bit older so not quite the same specs but a very similar build.
    I also attached a photo of a part, it was actually sanded a bit already so it doesn't look as bad as fresh of the CNC but it can give an idea of what I'm talking about.

    I am not very well versed in the mechanics of these machines so I'm not sure what to look for in terms of getting this thing tighter, or if it is even possible.

  2. #2
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    Re: how to adjust for bumpy rough parts

    How deep are you trying to cut at a time? There's a limit to how much wood a tool can cut, which depends on lots of things, like the rigidity of your machine, the feedrate and RPMs, the length of the tool, etc. I'd suggest trying multiple passes of the tool instead of trying to cut the whole thing in one go; cutting no deeper than the diameter of your tool.
    Andrew Werby
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  3. #3
    Community Moderator ger21's Avatar
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    Re: how to adjust for bumpy rough parts

    Try leaving about .02" of material, and do a finish pass with a 1/2" or larger bit at a slow feedrate.
    Gerry

    UCCNC 2017 Screenset
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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  4. #4
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    Re: how to adjust for bumpy rough parts

    so I do go in steps no deeper than the width of the tool and I usually do a finishing pass with 0.02" left of material, the piece in the image was done this way actually. Though I'm not sure what speed I should run a finishing pass with 1/2" diameter tool.

  5. #5
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    Re: how to adjust for bumpy rough parts

    ...looks like could be mechanical problem. Are the under cut marks equally spaced? Chips stuck on the gear tooth or belt teeth of axis drive probably. Operator's with an air hose will cause it most of the time.

  6. #6
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    Re: how to adjust for bumpy rough parts

    What speeds and feeds were you using?Climb cut or conventional?Chip load might be another factor.I would hazard a guess that it is a resonance issue from the regularity of the rippling shown.The source might be easier to track down if we knew which way the specimen was facing when machined-those gantries look very substantial but with the Z axis running near the table there is a lot of leverage and torsional flexing might be a factor.

  7. #7
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    Re: how to adjust for bumpy rough parts

    So I'm conventional cutting with a 1/2" end mill with a chipload of 0.0057" at 18000 rpm and a feedrate of 307.8 in/min. I'm doing 0.4" step down using 30% of the cutter width I've been playing around with leaving different amounts of stock for my finishing pass, I usually do 0.02" on my last batch I went up to 0.08", either way it still creates the ripple. I'm not really slowing down the tool for the finishing though.

    I was also previously doing these using a 2 flute end mill with the same chipload and rpm but a feedrate of 200ipm and I had the same problem, I also previously tried climb cutting instead and tried 25% of cutter width.

    I also even notice this when I use a 1/4" mill, it just creates a tighter ripple.

    The marks on the wood look pretty well evenly spaced, it's sort of variable whether they are present or not though. I'm doing six at a time on the machine bed, I attached an image of two from the last batch, on one the ripples are really deep on the other they just show up a little bit on one end.

    My parts are setup on the CNC so that the length of the piece, (the edge you see the ripples on) is parallel with the x axis of the machine, the x axis runs along the 96" length of the machine.

    What I find surprising is when I do a finishing pass with 0.02" left of material I wouldn't expect there to be much strain on the tool, like its barely cutting anything so torsional flex seems surprising, but maybe possible.

    I would hazard a guess that it is a resonance issue from the regularity of the rippling shown
    what does this mean exactly? or at least, what do you think causes a resonance issue?

    @machinehop5 not sure what you mean by

    Operator's with an air hose will cause it most of the time.
    like if an operator is hosing down the machine and bits get pushed into the gears?

  8. #8
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    Re: how to adjust for bumpy rough parts

    The cutting data doesn't look bad,even though the cut quality is not that good.Have you thoroughly cleaned the collet and toolholder?If it isn't play in the Z or Y axis I'm suspecting the gantry itself is flexing a tiny bit and the natural frequency of the beam and the rate of the flutes striking the job is agitating the movement.I take it you have tried lowering the Z axis to the sort of height it is doing the cutting at and pushing the spindle sideways against a dial gauge?That should give an idea of the amount of force needed to deflect things.All you have to do then is track down the part that is moving.Something obviously is.

  9. #9
    Community Moderator ger21's Avatar
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    Re: how to adjust for bumpy rough parts

    That's about the worst case of that I've ever seen.
    You could try doing the finishing pass at half the speed. But the ripples look deeper than the amount being removed from the finishing pass, so you may need to leave more for the finish.

    Regardless, this is a machine/rigidity issue. Whether its coming from a loose rack and pinion, or just a lot of flex in the machine, I can't tell you.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  10. #10
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    Re: how to adjust for bumpy rough parts

    Those bumps look suspiciously regular for something being cut in multiple layers. Resonance doesn't look like that; it's more random. Just for fun, measure the spacing from bump to bump and compare it to the spacing on your rack. My guess is that they're the same. I think what might be happening is that the preload into the rack has failed, and that the whole spindle assembly is moving sideways each time it crosses a rack tooth.
    Andrew Werby
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  11. #11
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    Re: how to adjust for bumpy rough parts

    Quote Originally Posted by awerby View Post
    Those bumps look suspiciously regular for something being cut in multiple layers. Resonance doesn't look like that; it's more random. Just for fun, measure the spacing from bump to bump and compare it to the spacing on your rack. My guess is that they're the same. I think what might be happening is that the preload into the rack has failed, and that the whole spindle assembly is moving sideways each time it crosses a rack tooth.
    awerby you are right on the money check this image...

    So yes they definitely line up with the teeth. I'm not entirely sure what you mean by "preload in the rack has failed", I'm thinking maybe the rack is pulled too tight against the pinion, would that be possible?

  12. #12
    Community Moderator ger21's Avatar
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    Re: how to adjust for bumpy rough parts

    Pulling the pinion into the rack is not the "proper" method to use, but it's a simple way that usually works well enough for inexpensive CNC machines.

    The problem might be that you need new pinions. Pinions wear out, and need to be replaced periodically.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  13. #13
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    Re: how to adjust for bumpy rough parts

    Quote Originally Posted by ger21 View Post
    Pulling the pinion into the rack is not the "proper" method to use, but it's a simple way that usually works well enough for inexpensive CNC machines.

    The problem might be that you need new pinions. Pinions wear out, and need to be replaced periodically.
    A little while ago I was experiencing what my machine manufacture was calling chatter, essentially the rack wasn't pulled tight enough into the pinion so the gantry was shaky. As per the manufacturers instructions I loosened the screws holding the rack and adjusted bit by bit by pulling further into the pinion until there was no chatter sound anymore.

    I am almost certain that I had this ripple problem to a degree before making this adjustment, but that said I'm not sure if it was as bad as it is now. So that's why I wonder if maybe I pulled it too tight.

    Furthermore the machine is fairly new we got it this winter, I would be a bit surprised if parts were already worn out and needing changed. Then again I'm not sure how long a pinion lasts.

    So anyway to elaborate more on what I'm seeing happen, I'm noticing now that the problem seems to be happening only in specific areas along the table (might need further testing to be sure).

    In one of my photos posted above I was showing one part where the problem was relatively mild and other where it was quite severe. This seems to be repeating itself based on where the part is positioned on the table. So this leads me to think that the problem would be with the rack and not the pinion. I'm just not sure what exactly would be wrong with it though.

  14. #14
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    Re: how to adjust for bumpy rough parts

    It seems that cutting forces are either pushing your pinion (and everything attached to it) away from the rack, or into it. If the problem is worse at certain areas of the machine, that may be places where it's especially loose, or where more leverage is applied. But I think we can safely conclude that it's looseness of some sort that's causing this issue. Usually a pinion is pre-loaded into the rack with a certain amount of tension; this allows it to cut smoothly rather than be pushed around every time it encounters a tooth. My assumption was that this preload is way out of adjustment, which is what caused the problem. But slop or lack of rigidity in other parts of the machine could also be responsible. For instance, if the lead screw isn't holding the axis assembly in place securely, perhaps due to faulty mounts, the whole stage could be deflected as it crosses each tooth of the rack. You need to go over the whole machine and check each mechanical connection until you figure out what's too loose, then tighten it.
    Andrew Werby
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  15. #15
    Community Moderator ger21's Avatar
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    Re: how to adjust for bumpy rough parts

    You might want to call Legacy and ask them what to do. I'm sure they've seen this issue before.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  16. #16
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    Re: how to adjust for bumpy rough parts

    ...If, this is a new machine I would have who ever sold it to you fix it. But, they will probably blame you for trying to adjust the rack....this may help understand the MESH problem.

    https://www.machinedesign.com/motors...s-from-reality
    Myth: Rack and pinions have too much backlash and the only way to reduce or remove it is by pushing the pinion into the rack.

    The reality is that rack and pinions have a theoretical center distance at which optimal meshing occurs. By pushing the pinion into the rack, one can reduce the backlash level — but this also degrades tooth meshing, which can cause excessive wear and binding.

    The best way to reduce the backlash of a rack and pinion drive is to increase the quality of the teeth, as well as their mounting accuracy.

    To completely remove the backlash of a rack and pinion drive, a split-pinion or dual-pinion drive can be used, where one pinion drives while the other removes the backlash.

  17. #17
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    Re: how to adjust for bumpy rough parts

    ok so this has been quite helpful, I'll be in touch with legacy, hopefully with what has come out here I will better suited to explain the problem. Unfortunately we are too far away for the company to come fix it, we've had a lot of problems with it since the beginning and I'm not looking forward to have to get in touch with them again but what are you gonna do.

  18. #18
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    Re: how to adjust for bumpy rough parts

    With the machine switched off does the axis feel "lumpy" when you push it along ? If so, try adjusting the rack/pinion until it feels smooth.

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