Flattening a large board

router flattening jigHaving my large boards now jointed and glued, I now need to flatten them. Mulling over the problem in the small hours, I remembered an idea I’d seen in a book on homemade jigs. It’s a very simple idea; just make a gantry across the board that lays flat on the bench and has a slot just wide enough for the widest guide bush I have. The gantry was made from 25mm ply – the 18mm wasn’t quite rigid enough not to dip a tiny bit in the middle under the weight of the router. The jig sits about 5mm above the board and holds the router at the same height relative to the bench, so all I have to do is insert the dish cutter and set it to the right depth to skim the bumps off the top.

router flatteningAt first it didn’t seem to work very well; I tried going along and across and generally scribbling around with the router, but the result was a very uneven surface to which the belt sander was hardly equal. Since this is the ‘wrong’ side of the board it doesn’t matter – it won’t show – so I was undeterred.

I decided to try going across the board, moving the jig a little and coming back across. I got the hang of it quickly enough and did a couple of feet at a time. I let the router cool down while going up the grits with the random orbital sander to smooth the bumps, which were now much more predictable and recognisable. It turns out that I didn’t need the belt sander after all. smooth with ROSNo bad thing; the ROS is much gentler, even on a coarse grit.

I started with P80, then P120, P180 and finally P240. It took me just over an hour to flatten the whole 2.4m board. A sigh of relief was heaved. I know how to proceed, and I’m confident that it’ll be a good result. The only thing to watch for is the depth stop on the router, which does occasionally slip when I knock it accidentally.

It’s also essential to clamp the corners of the board down on the bench for this job. The flattening is relative to the bench, not the board, so the board has to be flat on the bench. For some boards it will also be necessary to clamp along the sides too; that just means I move the clamps as the jig is brought across. Similarly the corner clamps have to be moved when the jig is brought to each end of the board.

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