Making dogbones on CNC

 Here's hoping everyone has a healthy and successful 2022!!! 

Obviously this blog is about my teardrop trailer build, so I won't delve into opinions on beer, people, politics, life, etc - you can catch that over in ¨Life in the swamp¨ if you are interested.  Progress has been kinda slow for the past week.  I have actually had to pull some extra hours at my day job, and that's been enough to occupy most of my time.  In my free time, I've been clearing out the build space, and working on learning how to use the Path workbench in FreeCAD. 

Let's talk a little bit about FreeCAD.  There are other packages that are probably easier to use for generating gcode, like Fusion360 and V-Carve in particular.  Our makerspace/shop has a license for both, and I was starting to learn F-360 when the pandemic hit. At that point, I lost direct access to the machines (makerspace was closed down tight), and the F-360 personal use license was becoming 'difficult'.  Oh, and I built a CAD workstation using Linux, and F-360 only runs under Windows, so - not gonna happen.  Looking at the options, I started investigating FreeCAD, and found it was a)very similar to F-360 in capabilities (and difficulties), b)runs great on Ubuntu Linux, and c)it's really and truly free!  Not a demo license or one that allows you to create but not save, etc.  This is really free, is moderately well documented (good, not great) and has a very active user community, and numerous Youtube communities.

FreeCAD is a 3 dimensional parametric modeling application, with multiple workbenches supporting things like architecture, sheet metal, metal tubing, and 3D printing/CAM/CNC.  

OK, now the bad news.  It's a pain in the ass to learn, crashes occasionally, and many of the videos make assumptions about your existing skills that may or may not be appropriate.  And, while the documentation is often marginal.  If your last drafting experience was paper, ruler, t-square and a compass, it's a complete and total mind warp.  And if you are an 'experiential learner', your brain _will_ hurt - often.  If I paid myself $10/hour for watching FreeCAD youtube videos, I could probably afford a nice windows machine and full pro license for FreeCAD or V Carve!!!

But, I've gotten pretty proficient, made enough 3D widgets and sketches that learning one more bench wasn't that tough.  Even if that one drives a 10K spindle and can destroy 160 bucks worth of plywood in the blink of an eye.  

After a couple of false starts, today was the first day I've cut anything meaningful on the CNC.  It doesn't look like much, but these are the major components of a CNC job - perimeter profiles, grooves and pockets.  The 'dogbone' cutout was the most interesting challenge.  If you work in wood, you've doubtless learned that you can't put a 1/2¨ board in a 1/2¨ rabbet.  It won't fit (without some sanding/chiseling anyway).  Getting a shape like the dogbone to fit is even more of a challenge.  

To make it work, I used a .250 (1/4¨) end mill on the slot, and used the same mill on the tab - but told the program that it was a .240.  This positioned it just a little closer to the edge line, and allowed it to fit very snugly.  I am not sure I could get 10 of these in a row to fit like this though! Nor would the joint contain much glue. For the wall and ceiling seams, I will (after more experimentation) use the same technique, but make it .220 or .230.

And why am I so wrapped up on this particular feature?  Aside from the mechanics of making it work, I think it will be a nice feature on the inner sidewalls of the trailer, and the ceiling.  Having two pieces of plywood simply butt against each other is not the most attractive, and they tend to 'work' against each other over time. You can cover it with trim, or use a scarf joint to make it blend better.  Using puzzle joint means that the two pieces can't move freely, there's more glue seam, and glass tape with epoxy on the back will lock it in for good.  Note: in thicker wood, biscuits, shims or dominos will make a strong mechanical joint.

At the end of the day, I'm a definite fan of FreeCAD, and strongly support the direction that it seems to be going.  It is probably a lot more difficult to work with than a program like SketchUp, but the price is right, and the end result is spot on.