An overview of free & open-source toolpathing for the DIYLILCNC. Turn a raster image into a G-Code engraving pattern with GIMP, Inkscape, Pycam and EMC2 in Ubuntu.

Taylor and I have been working on a DIY 3-axis CNC mill over the last few months. I’m posting this mostly to brag :), but also as an excuse to do some documenting on the process that might be useful to others. We’ll keep posting as things progress.

Some video of testing

The original plans use 1/2″ MDF for all the panels. We decided to go with with clear acrylic for the visual effect. Also, we were able to use a laser cutter for most of the cutting to the edges are nice and clean.  The downside is the brittle nature of acrylic, which likes to crack when drilled from the edge (and this design requires a lot of that).

CONTROLLER & CONFIGURATION

We’re using linux-based Enhanced Machine Controller to run the mill, with a modified version of the standard stepper configuration. Here’s our EMC configuration files. So far, I’ve been really pleased with the performance. It was trial-and-error figuring out the scaling– I ended up attaching a pencil to a dremel collet, then manually jogging each axis until I could draw a one-inch line in each direction. The scaling factors I came up with are: 520 for X and Y, and 16000 for Z, and that’s with quarter stepping set on the hardware controller board.

We kept getting a “Joint 2 Following error,” indicating that the z-axis was losing its position. I found that this was happening only when using  G00 (rapid-positioning) codes. I’m still a little unclear as to whether we need to even worry about positioning in this case, but for now I’m just replacing G00 codes in my toolpaths with G01 (linear motion). It’s a little slower, but for now the error is not happening anymore. See Feed Speeds for the permanent fix.

TOOLPATHS

We use Rhino3D and RhinoCam to generate our toolpaths. I’ve found that a number of different post-processors to work just fine, but mostly we’re using .NCD’s out of habit.

FEED SPEEDS

There doesn’t seem to be a way to directly set the feed and plunge speeds in EMC. The NCD post-processor defaults the feed speed to 3.7 and the plunge speed to 7.3 and the units are inches per minute. So we’ve been doing a search and replace in the post-processed files setting plunge to 14 and feed to 10 (this is easier than going back to RhinoCam to re-generate the toolpath with new feed speeds). We also then modified the MAX_FEED_OVERRIDE setting in the stepper_inch.ini file to 3, meaning we can increase the feed override up to 300%. We now limit the max override to 150%, and more deliberately set feed and plunge speeds in RhinoCam, under the Feeds & Speeds tab — these vary depending on material.

ERRORS

We kept getting a “Joint 2 following error” while testing, and found–after consulting the EMC documentation wiki–that changing the FERROR setting to 5.0 and the MIN_FERROR to 1.0 (that’s percent) for all axes in stepper_inch.ini alleviated this error. There’s the potential with increasing the FERROR settings of losing some accuracy, but that’s not our main concern at this point.

The only downside to this setup is having to reboot every time I switch OS’s. Boot Camp only allows one partition to be the default startup disk. I keep my default set to OSX, since that’s what I use most. But if I want to reboot in Windows or Linux, I have to sit and wait while I hold down ‘alt’ and the Boot Camp bootloader comes up. I would rather spend this time staring out the window, or getting up for coffee.

That’s why WinRestart is awesome.

WinRestart is a [painfully] simple two lines of Applescript (thanks to Taylor for the code) that passes a shell script to the Terminal, using the bless command. This sets your Windows/Linux partition to be the boot disk on the next restart only. WinRestart is saved as an application, which means you can keep it in your Dock. Just click to launch, and you’re on your way to automatic restart heaven!

After downloading and extracting WinRestart, open it up in the Script Editor (Applications -> AppleScript -> Script Editor.app) and change the name of the boot volume to your alternate partition. For example, if your Linux partition is named ‘linuxpartition,’ you would change the default Untitled:

do shell script “bless -mount /Volumes/Untitled/… privileges

to ‘linuxpartition’ (no quotes, though):

do shell script “bless -mount /Volumes/linuxpartition/… privileges

UPDATE: Andy writes with this tip about using WinRestart with an NTFS partition:

I just wanted to thank you for your WinRestart post on rainbowlazer. It was pretty much exactly the information I wanted — however, using a NTFS partition for boot camp did cause some problems with using bless.

Bless seems to require statfs, when you use it with the –mount option, which apparently isn’t supported with MacFuse/NTFS-3G. Instead [of bless -mount /Volumes/Untitled/ -legacy -setBoot -nextonly], the following bless command worked great for me:

bless –device /dev/disk0s3 –legacy –setBoot –nextonly

Thanks for the input, Andy!

UPDATE: Tim writes with this tip about adapting WinRestart to work with System Events to prompt saving of open documents before restart:

Thanks for developing the WinRestart application. I wanted to comment that I have used it a lot and have found it really helpful. I made a version of the script that might suit most users a little better though, maybe you would like to post it as an alternative.

do shell script “bless -mount /Volumes/Your_Volume_Here/ -legacy -setBoot -nextonly” password “Your_User_Password_Here” with administrator privileges
tell application “System Events”
restart
end tell

It doesn’t ask for a password and lets you save unsaved documents/close programs.

For users of this, I also recommend turning off the startup screen in the WinStart File menu.

Many Thanks Again,

Tim

Thanks for the tip, Tim!
The addition of your password in plaintext is a little iffy in my opinion, but if you’re not worried about it then it’s a good time-saver.