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.