Our day began at Rockler woodworking in Somerville where we went to pick up some epoxy. We were suffering from sticker shock on even small quantities of the epoxy when Rockler employee, Woody (yes, we’re serious), came around the corner to offer some advice. After some discussion about different methods of filling our kerfed joint, we decided to test out his method of mixing sawdust and wax-free shellac. The advantages of using this method (aside from it being a much cheaper option) is that it should be easy to sand and it will bind to itself in case we need to apply it more than once. We then went back to the shop to test this method and finish our pieces.
The shellac method seemed to work and we left it overnight to dry. The color of the joint was much darker than our nearly white birch ply but it should be okay for our purposes. Unfortunately, the pieces which we had cut with the kerfs a bit shallow were completely unusable, even after we sawed out the excess wood with handsaws. The kerfs would not bend into themselves and so the corner cracked when we tried to fold it together. Also, we realized that although only six kerfs are required geometrically to bend the piece, it may be a good idea to add more to accommodate for cnc error or material variability. We decided to take the opportunity to cut another piece on the CNC and troubleshoot our workflow a bit.
Here were the problems with the last round of cutting:
1. Kerfs were too shallow
2. Kerfs became damaged when the router did a profile cut.
In this test, we addressed this by doing the following:
1. Added an initial step of cutting halfway into the profile cut (.25” depth) in order to “break the edge” before cutting the kerfs.
2. Taped the kerfs down after cutting.
3. Added two kerfs for a total of eight.
4. Because we suspected that an inconsistent maximum z-dimension of the plywood on the CNC bed (due to warping) may have contributed to the shallow kerfing, we ripped the plywood down to a 14” width before CNCing this piece. This enabled the wood to lie flatter (with points screwed down at a lesser distance apart).
The results were much, much better and we were able to fold the piece up immediately. We also had almost no broken kerfs or outside kerf edges. We will need to route the back of this one for the lap joint again before we can fold and glue it into a loop, so that will be our next step.
A closer look at the CNC as it mills out the regions into which the steel plate for the legs will be set.
Here’s the CNC mill in progress, using a 1/8” bit to cut the kerfs.
Some more photos of the CNC process today.
We finally had success cutting out first three pieces of plywood for the final pieces. We had minor problems when some of the plywood strips between the kerfs became detached during cutting, but we’re planning to repair those few areas before bending tests. Next we need to route matching depressions for the lap joints on the opposite side of the plywood as the CNC-ed grooves, so that we can bend each piece into a flat, closed loop.
Today was a bit disappointing. We spent a long time wrestling with the CNC setup, which turned out to have a controller problem unrelated to our files. When we finally tried to start cutting, we broke a bit. In the end, no pieces were produced today, but we certainly learned a lot about how the CNC does (and does not) work.
We used the CNC to cut the first prototype of the plywood loop. A central slot and strategically placed kerfs allow the entire piece to be folded from a rectangular flat sheet, with the kerfs facing the interior of the finished loop.
Here’s a video showing the CNC in action, milling the kerfs on our plywood sheet.