Chatter cutting 4mm plywood

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This topic contains 11 replies, has 5 voices, and was last updated by  Dui, ni shuo de dui 1 year, 2 months ago.

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  • #39164

    Azza
    Participant

    Hi,

    I’ve built an MPCNC – got it up and running and built some toys for my kids out of 6mm marine plywood.
    I’m using a 3mm ballnose endmill, I was having a few minor issues with chatter on the 6mm plywood, then I bought some 4mm plywood and its much worse.
    Everything appear to be fine until the final pass(es) where the endmill breaks through the plywood on the side and then there is a lot of resonance.
    Then things tend to go wrong with either the X or Y Axis missing some steps and the mill cruises off in the wrong direction.
    I’m using a 500W Spindle Motor ER-11.

    Going to try a different bit to see if it makes any difference. I’ve tried clamping the plywood done more firmly and placed a MDF board underneath.
    I did upgrade my X / Y drivers to DRV8825 / 32 microsteps and properly adjusted the ref voltage. Maybe I should have reduced the steps?
    I’m after some suggestions on what to do next?

    #39167

    Ryan
    Keymaster

    What hardware are you using? You say you upgraded to 8825’s so I am assuming you didn’t get it from this site. Please be as detailed as possible.

    A picture of the machine and how you secured the work would help.

    Some more info on your bit, it’s size, the speeds you ran it at, how you made your Gcode, if the dimension were correct.

    #39171

    Azza
    Participant

    Hi,

    I’ve build my own 3d printers. I’ve basically got 3 machines, a Prusa i3 which built the Kossel Delta and that printed me a MPCNC.
    They’re all RAMPS 1.4 hardware. I’ve got plenty of parts / spares etc. for when the magic smoke escapes.
    I pretty much know my way around 3d printing, calibrated etc. the hardware, firmware etc, but CNC is new to me.
    Steps are set correctly on XYZ axis, although I’ve not done fine tuning like I would on a 3D printer to get the dimensions super accurate.
    From memory the machine is approximate 800mm x 800mm with ~150mm Z.

    The bit is a 3.125mm ballnose, running 12000RPM (max speed of the spindle). GCode came from ESTL CAM.
    I started with 1mm depth per pass, dropped it to 0.5mm to see if it helped. It did a little bit.
    The is a picture of the machine before it went into the Shed.

    This is what I’ve made using 6mm plywood. Dimensions are correct otherwise it wouldn’t have slotted together.

    I’ve been clamping the plywood to a 3/4 inch sheet of MDF to see if that helps.
    Seems the plywood I’ve been using lately is harder that the first sheet… or at least the vibration is only bad when it breaks through.

    Sorry will have to get a picture later. I’m a bit stuck for time to investigate as I’ve got young kids and its a struggle to do anything.
    This is a photo of carving the toy parts – I was pretty slack with the clamps but it was happy with that material.
    The second sheet of 6mm ply I bought was much harder and I started having issues with that.

    Thanks for your help. The machine is a great design.

    #39176

    I have a similar issue, but mainly with aluminum. Most of the chattering occurs on the very last path, when it goes through the material.
    I think this is a clamping issue: when the bit goes through the material, it tends to pull it up from the table, and I guess this creates the vibrations.

    Maybe you could try to secure your plywood in the middle, by directly screwing it onto your waste board at several places. Should reduce your shattering.

    Your model looks great, where did you find it?
    One remark though:: it seems like you are wasting quite a lot of material while milling, try to better organize your parts before making a cut.

    1 user thanked author for this post.
    #39177

    Kevin Lopez
    Participant

    The chatter you are hearing is likely from either A, a lack of rigidity or B, your workpiece is resonating. If going to a thinner stock made it worse, then yeah it is probably that it is moving too much during cutting. I see you clamped it quite well on the sides, but try using some wood screws in the center to hold the center down more. Do you find that the chattering goes down a little when you cut closer to a location of a clamp? Also, your z axis is rather large so that doesn’t help much.

    Is your bit perpendicular with your workpiece?
    Have you faced/leveled your base mdf?

    #39179

    Azza
    Participant

    I did think about screwing it down. I’ll give it a try – thanks.
    I’m definitely seeing some movement around the middle and the thinner ply would not be helping there.

    Yes, wasting too much material – just got the thing working, got a bit excited rushed off and starting making the Front End Loader.
    I was finding that trying to carve too much in a single go I was having issues.
    So I ended up doing one or two pieces at a time, reset and go again. Hence the poor arrangement.

    Initially I though the issue was steppers over heading / skipping / poor adjustment etc.
    I found the model at:
    http://www.wood-jigsaw-puzzles.com/category/free-wood-jigsaw-puzzle-plans/

    1 user thanked author for this post.
    #39190

    Jeffeb3
    Participant

    They aren’t skipping steps. That would end up with funny shapes.

    What travel speed are you cutting at (mm/s)?

    #39191

    Azza
    Participant

    I’ve made some progress tonight.
    I used some thicker 12mm plywood, clamped it down tight and put a few screws in strategic spots.
    Probably should have used a solid board underneath?

    I worked out while it was milling the holes that it was resonating in the steel tubing.
    Stuffed a piece of foam in there dampened it.

    I also noticed some slop in one of the top bearings on the Z axis, I need to reprint one of the brackets as its got a small crack forming.
    The other thing was that when it broke through, it really plunged through… like dropped a couple of mm and I really had to slow things down.
    Suggests to me that the Z axis isn’t making its depth on each pass.
    So I’ll look at adjusting the Z axis driver – properly.

    Feedrate is: 20mm/s XY and 10mm/s Z
    I’ve been slowing things done to 50% or lower when the wheels start to fall off.

    So I’ve got plenty to look at now and making progress.
    Thanks for the help everyone.

    #39199

    Ryan
    Keymaster

    The z axis has a firmware limitation of 8.4mm/s, and plunges are best done at 2-4mm/s.

    20 is also fast I suggest going for deeper cuts nut faster, try slowing down to 10-15mm/s and doing a deeper cut per pass. The idea is to use more of the cutter as to not wear put the tip of all of them and to get better accuracy buy dragging the cutter through the material so fast and putting incredible amounts of force on the machine.

    It is a fine balance but when you get it right there are very minimal forces on the machine.

    1 user thanked author for this post.
    #39255

    Azza
    Participant

    I’ll see what I’ve configured in marlin and adjust the setting in ESTLcam.
    Do you think that the slower plunge rate will fix the Z-axis issue I observed?
    I have a geared stepper one of NEMA 17 – 5.x ratio ones – that would provide more torque but possibly break the cutter?
    Kind of goes against what your saying.

    #39256

    Ryan
    Keymaster

    You would have to go 5x’s slower.

    #39265

    The geared motors usually have a lot of backlash. Do not use them, unless you have a good method to compensate from that.
    If you lack torque, you could go for 1/8 step instead of 1/32, won’t make any difference in terms of quality, but will improve torque.
    But as Ryan stated, if everything is set up correctly, you should have very little force exerced on the machine, it should cut through wood like butter.
    Try different feed speeds, different cutters.

    It looks like you’re using the same spindle motor as mine (brushless 500W), so I guess it is running at 12 000 rpm. Try different numbers of flutes, to lower the feed speed, the spindle speed and once you get the less audible noise, increase progressively the path deepness. Each machine have different sweet spots, these spots depends of feed rate, spindle speed and many other things, take a look at this great article which explains in detail how chatter works:
    http://www.cnccookbook.com/CCCNCMillFeedsSpeedsChatter.htm

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