2019-08-10

New milling machine

I had a CNC 3018 PRO machine from Amazon.
  • It needed assembling, but I've done that now. It is something to bear in mind if buying one though.
  • It is a bit of a "home made" style, with the PCB bolted to the frame, etc.
  • The one I got had a bent screw for X axis, which limited the usable space a bit.
  • It has a working area 30x18cm.
  • I was able connect a Z-axis probe (well, contact to PCB copper clad board).
  • It works well, and I used bCNC on my Mac to work it.
  • It comes with a controller box, which is not needed really!
  • It is surprisingly cheap.
So, especially given the bent screw, I decided to get a replacement, and went for a CNC 3020 from Amazon.
  • This is a much more "industrial" unit - proper drag chains and cables. Looks good but costs a bit more.
  • Work area (you guessed it) 30x20cm
  • Comes pre-built
  • Has a boxed controller and power supply, with nice connectors to the machine.
However, it has a challenge or two - the main one is that it works with some very specific software (a pirate copy of which is apparently supplied with it). It has parallel (!) and USB, but the USB only works with that software and only on a windows machine - it does not even appear on a Mac even in a debug log! The software did not look too good anyway! This is a bit of a bugger.

The upgrade

My solution was to upgrade the controller to a TingG controller. This adds a bit to the cost (especially with the customs/VAT and admin charge from US). But was not actually at all hard to do.

The old controller is bolted to a heat sink, and can simply be unscrewed.

The new controller is slightly smaller. I needed to drill/tap an M3 hole in the heat sink for one of the screws so that I could screw it to the heat sink. I also cut an extra hole in the case for the USB connector.

Also, I decided to use the heat sink, which meant something between the new controller board and the heatsink. I got some small copper block / heat sinks and thermal tape - and I used them between the back of the new board and the existing heatsink.

The TinyG is designed to work without the need for a heatsink, but this can not really do any harm.


The connectors on the old board were not the same, though the new board has pads which means I could have changed them. I decided instead to use the screw terminals on the new board, fitting bootlace crimps on the wires. I had to swap the cables to make them fit, and use the motors 2, 3 and 4 as closest to edge of the case. But they all fit!


I also connected the emergency stop to the reset pins. I have not worked out how to connect the spindle drive to the new board yet - that is a challenge for another time. The case has a button and a dial for that which works for now.

I also got a 6 way (plus ground) chassis DIN connector and wired up all 6 end stops (X/Y/Z Min/Max).


This meant various micro switches, and super glue on the actual machine.


And also, the Z-Min I wired to croc clips to allow me to use contact to PCB for homing Z axis.

The TinyG has good documentation, covering setting the current, etc. I was able to set the axis to the right motors, and set the polarity and travel per rotation, etc.

However, the bCNC code I was using did not like the TinyG, and I ended up installing CNC.js which works nicely on my Mac. It does lack the multiple point Z axis levelling of bCNC sadly, but works.

I did have to configure the homing for Z axis a tad as it is designed for a switch from which you back off. For PCB contact you get the exact Z home, and do not need offsets. But this is all well documented.

Slack (backlash)

However, even though it is a really good solid construction and seems to have no play, I found there was an issue with the X axis. It seems to have around 0.1mm slack on it. This means if you move right to a point it is actually around 0.05mm short of it, same if moving left (i.e. is then 0.05mm right of it). Make a row of left and right moves and lines and you see the problem clearly. This test shows it well - centre is left and right moves before each line, but left is all moves from the right (apart from first, bottom left) and right is all moves from the left. Repeatable slack!

Oddly the Y axis is absolutely spot on!

This is not really enough of a problem to cause issues making PCBs, but is annoying so I wanted to fix it.

I checked for anything loose, and also checked the TinyG for any options, but to no avail.

My solution (now updated to the eps2gcode tools) is software to compensate for slack. This seems to work.

Overall I am quite happy with the result.


P.S. Tightening this nut a bit fixed it.



P.P.S. After reading the TinyG docs I am actually running at 4 micro steps not 8, so 1/200th mm spacing, which I think will be more than adequate for anything I am milling. I suspect 1/100th mm would be fine but actually that is a lot more noisy. Now that the backlash is sorted maybe I'll go back to 8.

3 comments:

  1. The 'slack' of which you speak is more commonly referred to as 'backlash' :-)

    Any mechanical drive has some backlash (has to otherwise it couldn't move) but having such a difference between the X & Y axis suggests something's not right.

    I believe this is a screw driven machine so it could be the whole screw jumping back and forth as it's not retained to the frame properly or a loose fit between the screw and driving nut.

    I'd check out the X axis hardware.

    ReplyDelete
    Replies
    1. I had looked for any problems mechanically, but with it being around 0.1mm it was not obvious. There is a nut at the end of the screw - I have tightened that, and magically it is fine now. Thanks for the tip.

      Delete
  2. Is this more solidly built than the 3018? I'm thinking I could literally use the 3018 controller with it to be honest.

    ReplyDelete

Comments are moderated purely to filter out obvious spam, but it means they may not show immediately.