Problems milling a pocket in plywood


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Thread: Problems milling a pocket in plywood

  1. #1
    *Registered User* CatalinM's Avatar
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    Default Problems milling a pocket in plywood

    I'm trying to mill a jig to hold down my back PCBs for milling and I've encountered some issues with my machine - a cheap Chinese CNC3018, with an Arduino based controller. I know this is a very low end machine, but my hopes are that with enough care and attention I can get something usable out of it.

    The jig is a actually a simple piece of plywood with 4 holes for fixing it to the machine table and a pocket of 170x120x1.5mm in which I'll place the PCB.

    Anyway, on to the problem details. First, the 1/8" end mill I'm using looks something like this:
    - maybe not the best choice, but I was trying to save the other end mill I have for some aluminium work.

    I did the CAD/CAM part in Fusion360 and had multiple strategies ready. First I tried a 2D Adaptive Clearing a in a single pass of Z -1.2mm and 300mm/min feed (spindle should be 9000rpm). Entry had a ramp at 150mm/min. It all seemed fine, but after entering the stock, it quickly started milling higher by about .5mm (eyeballing it). I think it lost some steps after that as well as by the end of the program it milled almost nothing. It seemed very strange to me that it looked like it was losing steps on pure horizontal milling...

    I then reset my Z position and gave it another shot with a simple 2D Pocket strategy (this running a bit faster, at 400mm/min). Same thing happened. After the initial ramp, it started milling horizontally and rather quickly went up almost 1mm, but it did cut more than the first run, and faster.

    Here's a picture of the end result, after these two tries:


    The second time around, I remembered GRBL had a setting to keep steppers locked at all times, so I enabled that. Seemed to have an effect as cutting was better, but my impression was that it lost steps when horizontal engagement was high.

    I looked on the Internet for answers but found nothing related, so I took another shot at it. Now, I thought that maybe I need to tighten the collet nut more, using a wrench - on previous tries I hand tightened it. Started the 2D Pocket program again and looked with amazement that it didn't seem to lose any steps now. I guess that was part of the problem... but not all of it. I noticed two things now: first of all, it now dropped a little with doing a 90deg turn so it actually started to go lower. Secondly, I felt the machine was kind of struggling having a more wood to mill out than before at the same speeds and feeds, so I manually set the feed some 30% lower. I still felt it was struggling, so I lowered it to 50% and now everything went south... literally. The bit started going lower and eventually went out the other side. The plywood is about 4.5mm and the cut was supposed to be only 1.5mm.

    I wasn't using a spoil board (as I didn't imagine this would happen and the end result was supposed to be a spoil. board of sorts itself). Luckily, it went through right down the middle of a T slot and didn't ruin anything. Here's a picture of the end result.


    My conclusions so far are these:
    - I do have to tighten the nut using a wrench (even if I didn't measure things before, I'm pretty certain the drill bit slipped up)
    - I have to get the feeds and speeds right... it seems that at lower feeds the bit I'm using is biting into the stock and driving down
    - Maybe I should use a 2 flute mill for this

    Thanks for the patience. I appreciate any and all advice on this issue.



  2. #2
    Member awerby's Avatar
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    Default Re: Problems milling a pocket in plywood

    It sounds like you're losing steps on the retract stroke. The machine thinks it's gone farther up than it really has, so it sends the bit deeper from its false starting point. That would account for the "struggling" you noted, since it's trying to cut more wood than was called for.

    Yes, you do need to tighten the collet with wrenches - hand-tight won't work. Yes, a two-flute bit is much better for cutting wood and other soft materials than a 4-flute, which is generally only used for steel.

    It doesn't sound like feeds and speeds are really the problem, since slowing down didn't help. Try reducing the acceleration on the Z axis by half, if that's the one that's losing steps. If other axes are also faulting, reduce the accelleration on them as well. See if that helps (using a spoil board this time), The other way to avoid losing steps on the upstroke is to fit the machine with a counterweight or gas strut to help offset the weight of the spindle (or whatever you're using instead of one). .

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  3. #3
    Member KH0UJ's Avatar
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    Default Re: Problems milling a pocket in plywood

    Is it a ball screw? or lead screw? if it`s a lead screw then yes, I faced that problems way back until I converted it to a ball screw, tiny backlashes will offset your machines coordinates specially in a looping program.



  4. #4
    *Registered User* CatalinM's Avatar
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    Default

    @awerby: For the most part of these programs (where it appeared to be losing steps), there wasn't any retract movement involved, just horizontal a circle (for the adaptive) or a straight line. I kept thinking about it last night and realised that it was going higher when I raised the feed and going lower when I lowered the feed... like the end mill either rode the stock (when going faster) or took a bite out of it (when going slower). Also, I noticed that depth changes occurred visibly when the machine made sharp 90deg turns (if you look at the last picture, at the corners).

    @KH0UJ I think it's a lead screw, having anti backlash nuts.

    My idea now is to switch end mills (I'm going to use the carbide 1/8" 2 flute) and use the adaptive strategy again, this time with lower depth as it seems that my machine can't handle 1.2mm.



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    Member peteeng's Avatar
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    Default Re: Problems milling a pocket in plywood

    Hi Catalin - The tool needs to be held in the collet much better. So spanners are mandatory as you have found. The first tool you show is a burr and by the looks of it, its not one intended for plunging. This would explain why it didn't like going down and slid into the collet. 2 flute router bits are much better for this work. The burr also produced poor edges. A router bit will give you clean edge and it will plunge nicely. Peter



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Problems milling a pocket in plywood

Problems milling a pocket in plywood