Saturday, October 16, 2010
The Project As Bench
I'm finishing up a dining table this week and it's one of the larger projects I've ever tackled. The top finished out at 120"x44"x1-5/16. I'm guessing it weighs north of 150 pounds. I don't usually write about furniture-making on this blog, but when it relates to benches and workholding, it's fair game.
This table presented some workholding issues that I'd like to address.
This top is made of five boards. The center board is about 15" wide, and the outer boards are about 9" and 7" (on each side of the center board). I glued up each outer pair, then joined each of those pairs to the center board.
Here I have one of the outer pairs (already glued up) held in an 8' Roubo bench. Notice that I'm using both the deadman and the right leg to hold a support batten with holdfasts. This board is heavy. I don't want it sliding down. The leg vise is not enough to hold this board, it's simply acting as a clamp in this case, the batten supporting most of the weight. It's important that the batten be made from softwood. This is an offcut from a pine 2x4. I don't want to damage a carefully jointed edge. Notice also that I didn't bother mounting up my sliding leg vise. I'll say it again, don't bother with a sliding leg vise. They are totally unnecessary. In fact, in this instance it would be very difficult to mount this board with two leg vises. You'd have to lift the entire board into the air to get it in the jaws. With the batten arrangement, you rest the board on it then slide it into the leg vise from the side. It's stress-free and virtually risk free. And you don't need to side-step around the sliding vise when jointing the edge. So if you're thinking of building a sliding leg vise I say don't waste your time and money. Build a massive deadman instead. Mine is just under 3" thick.
So now I have the center board in the vise and one of the outer assemblies on the bench top. I joint the edge of the latter as well as I can (I'm not fitting it to a mating edge), and stage it on the benchtop. The center board in the vise is then jointed until the edge is square and I get continuous shavings from one end to the other. Then the first test is done. I carefully lift the side assembly onto the center board and test the joint. I then adjust only the board in the vise if the joint needs work. I don't touch the board on the bench again. That way I'm fitting a fixed surface (the edge of the board on the bench) to a changeable surface (the edge of the board in the vise). This workholding arrangement works for me. The sizes of these boards is just about my physical limit though. Moving boards of this size without damage the joint's arris takes diligence.
At some point a workpiece becomes large enough that a bench becomes almost unnecessary. The board itself is massive enough to stay put on its own. It simply needs a basic stop to resist planing forces. To stabilize the board, I clamp a handscrew to the bottom edge, then clamp the handscrew to the leg of one of my low horses.
To resist planing forces I drive a nail right into my floor in front of the horse. If you have concrete floors in your woodshop, start saving for wood right now.
The low horses also allow wider assemblies to come together with greater ease. A low bench here is not as versatile. You want an interrupted surface here, for clamping access, and for hand access. I also like to fit these joints with the boards vertical. The weight of the boards allows you to detect minute twist in the edge and correct for it quite easily with a sharp jointer plane. Try tweaking the fit of edges this size on a power jointer. I can't do it. I like to be able to control the joint fit by a thousandth or so at a time. I think it's important on boards and joints of this size. You could draw a sprung joint together with clamping pressure, but why try to build in compensation for a less-than-ideal joint when you can dial it in with precision? I figure if the boards sits dead flat and light-tight (I sight along the joint and look for light from the window behind the joint) you can't get a better glue bond. I don't like to build stress into joints (unless it's specifically designed for it, like a drawbored tenon).
Bottom line. You don't need a bench to make a top this size.
For those still reading, I'll post some pics of the finished table soon.