Blowout on box joint jig

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20 Dec 2019
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I'm having my first go and box joints on a jig I made based on William Ng's video.

After a fair amount of tweaking, I'm fairly happy with the resulting fit. The problem I'm having is blowout on the rear of the fingers when using birch plywood. As you can see here:

This didn't seem to happen when doing test cuts on cheap plywood, but I think because the birch has more, and therefore thinner layers, the final layer is not as strong.

I've improved on the results in the above photo, by adding a zero clearance backer board, using tape on the rear of the pieces and cutting much slower, but I'm still getting a bit of blowout.

Does anyone have any other ideas?


Pete, I would have said that fundamentally your joints are on the wrong edge. The grain direction (of the outer layers) should be 90 degrees from where you have it. Doing that and having a sacrificial backing board should solve your issues.

When I used to do this with a router I would always make 4 plunge cuts before pushing the router in each corner in and out. These were with half the cutter diameter (ie looking down it took out a semi-circle from the front and back edges, left and right corners).
I agree with Mike, but, having played with BJJs a bit, I came to the conclusion they are largely a router tutor's party trick, and almost impossible to get to work nicely.

It's very hard to get good quality furniture-grade ply here now (in contrast to the USA, it seems). Usually the veneer is really thin, and easily sanded through or poorly stuck down, which makes matters worse in this context.

Consider what your cutter is doing when it exits the stock: Initially it's pushing hard to one side, then outwards - tearout is almost inevitable. You _might_ improve it with a backer board of hard material (I mean the fence with the locating peg), or working out some way to score the exit side (it will do it at the left of the outer side, and the right against the fence/backer), or simply hiding the spelched sides inside the joint (probably the most practical). The cutter needs to be really sharp too.

A while back I tried doing octagonal boxes with box joints. After a lot of messing around I did get it to work, but never very nicely. Tearout was one of many persistent problems, and the whole exercise really put me off. If I was doing octagonal boxes again, I'd use mitres and let in splines instead (and it probably looks nicer, as the sides and corners are symmetrical).

If you do want to machine-cut them, consider the tablesaw jig equivalents. you should get a much better finish.
Thanks both. I had another think and replaced the backing board with something thicker, and also started clamping my workpeice to the board rather than just holding it, which I think has made a big difference. I've just done the first few pieces and no tearout yet…!
Ah, systematic experimentation - way to go, that man!

It's interesting that my old router plate was an Axminster one, with holes tapped to take their box joint system. I never bought it, but IIRC, it had a hard plastic backer board/fence, something like Tufnol, or the Bakelite that many router plates are made of. That would trim to a fairly sharp line, so in turn would give great support for the edge as the cutter exited the back of the wood.

You might try something of fine grain, or even possibly Perspex or similar (beware the propensity to melt & go gooey if it heats up though), and you might also try a front "backer board" too (index against the near edge of the peg, so the notch stays crisp). Clamp up as a sandwich...

... who once seriously embarrassed himself, trimming down a motorbike windshield with a jigsaw - I followed the line perfectly, but the cut closed up again, well and truly behind the saw blade (blunt blade and feed rate too slow = too much heat). The result was OK in the end, but took a lot of sanding along the edge to make it look neat.