Facejoint big slabs with CNC?

[Jamie Buxton]

I'm a furniture builder who's pretty ignorant about CNC, but I think it might help me on a job. I have two walnut planks which are nine feet long by 2.5" thick by 22" to 25" wide. I'm going to bookmatch them to make a big dining table. They're presently roughsawn, so the first task is to facejoint them. 25" jointers are rare these days, so I'm thinking that maybe a CNC machine would do the job. The CNCs I've read about are the kind which are used as computer-driven panel saws; they cut up full sheets of plywood. I believe they rely on vacuum to hold the sheet. It seems to me that if my planks could be held mechanically on the machine's bed, and if the ride height of the router tip can be set just below the face of the plank, one of these machines could make short work of face-jointing the plank. Before I get all involved in finding a shop which would tackle this, can anybody tell me if I'm thinking reasonably?

[Steve knight]

works fine I have done it several times for people. works great for cutting boards. just make sure they raster (cut back and forth in one axis) along the grain. so in your case lengthwise on the board. the tricky parts if the wood does not sit flat it needs propped up to get that first accurate face. the weight may be enough to keep it in place on it's own if not some cleats would hold it well enough.
the only hassle is finding the low spot so you can get it all flat.

[Matt Zettl]

I do this all the time. On a really large slab like you are talking about, you need to take some time setting the piece up. Like Steve said, it may be heavy enough that it won't move during cutting. For a one time operation, you can use a fixed fence along one edge, and wedges along the other side to hold it in place. If it's something that you're going to be doing with some regularity, then you can build a holding fixture that speeds up the process. Both Steve and I have examples of such a fixture on the ShopBot Forums.

Also, setting up the piece is important and well worth the time spent, as you're trying to remove as little material as possible. I also use wedges underneath to prevent any rocking. If you're not careful with the set up, the difference between the high and low points can be significant, requiring the removal of more material than necessary.

I use a 2.75" surfacing bit, and remove about .03" per pass on dense hardwoods like jatoba or bubinga. With softer woods, more material can be removed in a single pass. I surfaced a 48" x 84" bubinga slab on my 48 x 48 machine, and it turned out beautifully. If you know someone with a CNC machine that is willing to help you do it, and is not familiar with the technique, I would be glad to provide more details.

-Matt

[Jamie Buxton]

What's one of those panel-saw machines called in the trade? If I call up a shop and ask for one, what do I ask for? Is point-to-point CNC the correct term?

[Doug Jones from Oregon]

Jamie, if you go to the shopbot forum you will find a category that allows you to look for a owner in your area. I would be surprised if you did not find someone in your backyard.

But, if you want to look around locally first, cnc router is the term you are looking for.

doug

What's one of those panel-saw machines called in the trade? If I call up a shop and ask for one, what do I ask for? Is point-to-point CNC the correct term?