Last weekend, I had left myself ready to do some construction on the base for the torsion box. The CNC-cut parts are really a very small portion of the total build, although it seemed like a lot of work at the time.
In case you havent been following along, the CNC gantry (all the parts that I cut out on the Book Machine — I really need to name my CNC machines, but thats another show) moves back and forth along the X axis table, which in the case of the new machine is a bit over 4 feet wide (the bit over making it a real pain in the ass to construct from standard sheet stock) and a bit over 10 feet long (ditto). Its constructed out of 3/4 MDF (remind me next time to do it out of ply). In order to keep the table flat with the 80# gantry running over it, I built a torsion box for it to sit on, essentially a 4 high grid of MDF that resists bending and twisting. The torsion box took the better part of 3 sheets of MDF by itself, and MDF weighs about 100# per sheet, so were looking at about 400# of wood (probably closer to 700# once you add in the spoil board and workpiece) that needs to be rigidly supported, and wouldnt it be nice if it wasnt sitting on the ground, too?
All this is to say that I needed a base for the torsion box to sit on, and it had to be tough. I already mentioned that I rummaged my scrap wood pile for parts, and the construction came together as framing made of 2×4 and legs made of a mixture of 2×6 and 4×4. Because of the way I built it, I decided that I was going to make the base have 8 legs, with the 4 corners done in 4×4 and the 4 middle legs made of 2×6.
I cut all the leg pieces as long as possible, given the wood that I had on hand, so that I left myself as flexible as possible for mounting options. Also, if I want to attach the torsion box to the base, I knew I would need some extra height on the legs to accommodate. Since Im doing design on the fly, things end up getting attached and then removed and then redesigned. Suffice it to say that this happened some during the day both yesterday and today.
By the end of the day Saturday, I had screwed and glued all four frames together, and had cut all the legs to length. My task Sunday, then, was clear; get the legs attached.
I decided that 2×6 would help to stabilize the middle joints, where I will have extra pieces to maintain the correct spacing between the frames. Attaching the 4x4s presented a challenge, but I decided to go with Simpson Strong-ties (A34 did the trick, two per leg) and that went pretty smoothly. I am on the fence about Simpson strong ties, because I am very much a screw person, and they are clearly designed for the nail type builder. Which reminds me, I need to find a real air nailer, because my brad nailer was not helping with this. Anyway, I suffered through, learning that 1-1/2 nails are a perfect size for attaching Simpson string ties to 2x4s. I am always a little unclear on the whole size of dimensional lumber and the size of nails and stuff, since they come in 1/4 increments at that size. Shrug. I have convinced myself that 1-1/4 are perfect for attaching 3/4 ply to things, and that 2-1/2 is perfect for attaching 2×4 in a butt joint.
By the end of the evening, I had two sturdy (as in, could easily support my weight on a single frame member) legged base stands.
I still need to install the lower frames (which will provide stability and also storage, once I line it with plywood – did I mention that the frame is perfectly 4 wide at the opening?), then I need to cut and install the spacers, then the bottom of the torsion box goes on ( I had an epiphany, I can maybe lay the bottom of the torsion box in the frame, put glue on it, then just drop the torsion box in place on top of it! Hmm… ). Once thats done, I move the frame to where it is going to live, and install the torsion box in it.
Once the torsion box is in situ, I can work on getting the rails installed onto it, and then I am into assembly of the new CNC machine!
There is still a bit of work to be done. Probably another 2 sessions to get the base finished, another one to get the bottom of the box installed, the frame moved into place and the box installed into the frame, then another one to do the rail installation. I would do it next weekend, but I think were out of town. Lets hope I can get this all done in February. It is going to be a photo finish.
Good work this weekend.