Recently, I’ve been working on a new set of boxes for the Minnesota Woodworkers Guild’s project supplying burial ceremony boxes for families experiencing neonatal loss. Last year, I went with a blind finger joint design, which you can check out here, and this year I wanted to try something different. That is, after all, the whole point of the project through the CNC group. I was aiming for a clean and classic mitered look that still utilizes the CNC for some pretty much all the heavy lifting.

The Approach

I’ve always liked the look of a nice clean miter, but sometimes they can be a bit finicky to get aligned and kept strong. The intent of this design is to solve both of those problems by using a completely hidden square spline that provides additional glue surface and mechanical alignment without showing the joinery on the outside.

It’s pretty easy to make boxes flat on the CNC with exposed joinery, but I’m not as much of a fan, and the challenge of working with the CNC limitations to come up with box corners that don’t look like CNC made joinery is what I’m after.

Right, on to the design. Everything is handled in a parametric model in Fusion 360. This is super helpful because I can quickly adjust things like the stock thickness, the overall box dimensions, the spline size or offset depending on what material I have on hand and the size of box I want. For these, I’m using 3/8″ poplar for the sides and 1/4″ material for the top and bottom panels. My splines are 3/16″ square dowel. In the above image, we can see the section of how the box is made, with the top and bottom panels in groove, and a larger groove that will later have a piece glued in to hold the lid in place.

The CNC Strategy: Order of Operations

When it comes to the toolpaths, I found that the order of operations is the key to a clean finish. I start out with a 90° V-bit to cut all the mitered ends first. By doing the miters before any of the other grooves or pockets are cut, you avoid a lot of the tear out that can happen when the bit meets an existing gap in the wood.

After the V-bit does its thing, I swap over to an 1/8″ roughing bit for the rest of the work. This bit handles the grooves for the panels, the rabbit for the lid liner, and the pockets for those hidden splines. If I was more concerned about visible areas, I would use a standard, probably down cut, 1/8″ bit, but I prefer the speed of this rougher of a finer cut, since nearly nothing is visible once done, apart from two edges that I’ll sand later. If you’re using thicker material or wider grooves for the top and bottom panels, you could use a larger bit. All depends on your box size parameters.

The “Hidden” Spline

The fun part of this box is definitely in the corner joinery. Instead of a traditional spline that you cut from the outside after glue-up, these use a 3/16″ square dowel that sits in a pocket cut directly into the mitered faces.

I just used some scrap material sanded down to exactly 3/16″ x 3/16″ for the splines. This makes the mitered corners stronger with more non endgrain glue surface and essentially self-aligning during glue-up.

I also included a rabbit on the long sides for a wood liner that allows the lid to seat perfectly once it’s cut off later, as well as grooves for the top and bottom panels.

Speaking of top and bottom panels, they really aren’t anything too special. In the box I’m building today I skip the chamfer around the top panel, and am using 1/4″ stock instead of 3/8″ stock as modeled. This results in a top that sits flush rather than proud (where I want the mitered top panel is if it’s sticking up above the sides. I like the look). Essentially, it’s just a rectangle with a rabbet all the way around it. I also clip the corners so I don’t have to worry about any imperfect spots when assembling the box from jamming things up.

Assembly and the Welder’s Miter Trick

For the glue-up, I used some 3D-printed 90-degree brackets and a handful of heavy-duty rubber bands. If you’ve never used rubber bands for box glue-ups, I highly recommend it, they provide great even pressure all the way around without the bulk of traditional clamps or strap style clamps. You can get the file for the ones I printed free from here.

I’ve mentioned before that I’m not always perfect at every step, but I try to be good at cleaning up in the next one. If you end up with tiny gaps in your miters, you can use the old burnishing trick; take something smooth and hard like a screwdriver and run it along the corner to gently roll the wood fibers in to each other at the corner. Once you sand it back and get finish on it, those gaps virtually disappear.

Final Touches

Once the glue was dry, I took it over to the bandsaw to cut the top off. I marked the lid line at about 1″ down to make sure I cleared the internal grooves. In my box, I had that wide groove start 3/4″ down, so I left around 1/4″ for the liner to hold the lid in place. After a quick session with the hand plane to flush up the liners, I finished these with Real Milk Paint Co. wood wax (a walnut oil and carnauba wax mix).

I ended up with four of these ready to go, and I might try a larger version next, depending on time.

Thanks for checking it out, I will catch you in the next one!