Laser cutting is a great technique for quick fabrication, in part because of the very small point size of the laser beam. The minuscule kerf of a laser cut allows for very tight 90° angles on interior (and exterior cuts).  This makes it easy to laser-cut thin panels of material for joining, as oomout demonstrates in this post on instructables.com (example pictured below). One of the drawbacks of laser cutting is the limited depth of materials that can be successfully through-cut; the laser cutter that I have access to can get through about 1/2″ of MDF or plywood, but the cuts are very charred and not perpendicular due to the hourglass shape of the laser’s focal point.

T-Bolt connection using laser-cut 1/4" panels

CNC routing is another great technique for quick fabrication. The range of material depth is much greater, there’s no burning, and through-cuts are perpendicular. One of the biggest drawbacks, however, is the inability to route interior corners due to the cylindrical shape of the cutting tool. This can require tedious hand-finishing to file or chisel out the interior corners to 90° angles, which is not worthwhile for projects that don’t have to look pretty, i.e. prototypes.

The technique pictured above is achieved by simply routing an extra bit of material out of interior corners to allow for 90° butt-joints. To set the paths for the corner knock-outs, a circle is created at each interior corner using a 3-point definition; the first point is set on the corner’s intersection, the second two are placed on each edge of the corner at a distance equal to the diameter of the router bit you plan on using. This will make sure enough material is taken away to allow for a 90° joint.

Vector path for 1/4″ laser-cut t-bolt joint (.dxf)

Vector path for 1/2″ CNC-routed t-bolt joint (.dxf)

Colors in the screenshots are only intended to distinguish between steps. Enjoy!

The final product should look something like this Webcam file with all steps (.3dm, 16mb)
Step 1.a Start a new file (small obj/inches)
Step 1.b In the Top viewport, draw 2 concentric circles, centered on 0, 1″ and 2″ diameters respectively.
Step 2 Copy and paste the smaller circle in place.
Step 3 Turn on Osnap and activate Cen only. Activate Move command, snap to Cen of copied small circle, then type <45 to add an angle constraint.
Step 4 Move small circle outward so that it intersects with the outer circle.
Step 5 Activate the ArrayPolar command. Select the small circle to array. Set 0 as the center of the array. Set the number of items to 4. Set the angle to 360. Hit Enter.
Step 6.a Select the four small circles. Activate the Split command, and split the large circle.
Step 6.b Select the large circle, then activate the Split command and split the four smaller circles.
Step 7 Select the four larger segments of the outer circle, and the four smaller segments of the outer circles–they’ll form a rounded cross-shape. Join these curves with the Join command.
Step 8 Turn on grid snapping. Select the innermost circle, and move it up one inch in the RIght or Front viewport by clicking and dragging.
Step 9 In the Front viewport, turn on grid Snap. Activate the Interpolate Points curve tool. To draw the first point, snap to the grid intersection on the x axis that is two inches to the right of zero. Then turn Snap off. Draw a line that is the approximate shape shown – it does not have to match perfectly. To draw the last point, turn Snap back on, and snap to the grid intersection that is 1″ above the x-axis and 1″ to the right of the z-axis.
Step 10 Select the curve you just drew, then activate the Offset command. Set the Distance to 0.04″, and offset below the original curve.

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.