Finger/box joint sheet of plywood.

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Forum topic by Josh posted 06-15-2015 01:05 AM 1204 views 0 times favorited 6 replies Add to Favorites Watch
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5 posts in 667 days

06-15-2015 01:05 AM

Topic tags/keywords: router tablesaw

I’m looking for a method for adding finger/box joints to a very large panel (think sheet of plywood size). Pretty certain it will have to be a router jig of some type. I’m open to any ideas or links to sites with templates or jigs.

6 replies so far

View BurlyBob's profile


3485 posts in 1688 days

#1 posted 06-15-2015 02:39 PM

Check out the Shopnotes box joint jig. It’s a fairly easy jig to build. It would give you the ability sandwich the plywood on the front side to avoid chipping which I expect would be a problem with plywood. Also using an upcut spiral bit might help reducing tear out. The Shop notes jig can be used on a router table or a table saw. I’ve done both with mine, but prefer the table saw and dado blades. There’s a You tube video where a guy demonstrates the one he built.

View splintergroup's profile


734 posts in 645 days

#2 posted 06-15-2015 04:22 PM

Sheet-of-plywood sized will limit you to a jigsaw or hand held router (or hand saw).

The jigsaw would be quick ‘n’ dirty, good ‘nuff if you aren’t after appearance. Basically lay out the tabs on one side, cut out with the jigsaw, clean up with a chisel, then transfer the mark to the adjoining side and repeat.

For router work, as BurlyBob suggests, select a bit to reduce tearout. Spiral bits are expensive, but will give a better cut than the cheaper straight cut bits. The cross grain plys will cause some grief with chipout, it all really depends on the quality and number of plys in the sheet (Baltic Birch takes box joints quite well).

You have two choices for cuts. The router can cut across the edge of the plywood which will leave a finished cut ready for joining if you use a square tipped bit. Chip out will be severe unless you can clamp sacrificial boards top/bottom.

Setting up a method to guide the router and index it precisely will be a whole different can of worms.

Alternatively you can use a template and a bearing tipped pilot bit. This will give you the needed precision, but your cuts will have a radius in their lower corners which will then need to be squared up with a chisel or a matching radius cut into the mating part.
The template for the router can be made from a short (maybe 16”) piece of 1/4” hardboard using the same techniques used for doing box joints on the table saw. You would clamp/double sided tape the template to the sheet and route away, then reposition it for the next set of cuts. Better accuracy (which is absolutely critical for wide panels) can be obtained if your template is the full width of the joint you are cutting.
Cut one edge of one sheet, then slide the template over the appropriate amount for the adjoining sheet.

Any way you do this is going to be slow and complex.

Things get much easier with the fewer (and wider) fingers. Finger widths in the 2” or greater range favor the jig saw and template methods. A bonus is fewer corners to square off.

View DKV's profile


3940 posts in 1927 days

#3 posted 06-15-2015 04:50 PM

Burley, are you sure an up cut reduces tearout? I would think a down cut spiral would be better. At least that’s my experience…I may be wrong :)

-- This is a Troll Free zone.

View Kazooman's profile


616 posts in 1375 days

#4 posted 06-15-2015 05:51 PM

DKV is correct. If you have the router on the finish side of the piece then a spiral down bit will reduce tear out on that face. A compression bit would be optimal for reducing (not eliminating) tear out on both faces.

View rwe2156's profile


2122 posts in 903 days

#5 posted 06-16-2015 02:42 PM

Plywood is a bear to try to finger joint or DT without tear out.

If you try to make one cut one side or the other will tear out. I think they only way to avoid it is to make the joint wider than your router bit so you can enter one way and exit the other (of course the other side will have tear out).

Make a jig and see what happens.

-- Everything is a prototype thats why its one of a kind!!

View splintergroup's profile


734 posts in 645 days

#6 posted 06-16-2015 03:18 PM

I was thinking more about this last night.

The main problem that disallows use of a traditional table saw jig is the size of the piece (sheet of plywood).

A table saw jig consists of a fence with a ‘key’ that is the with of the box joint ‘tab’ and is exactly one ‘tab’ width away from the blade. The (dado) blade is set to the desired with of the ‘tab’.

For the edge of a sheet of plywood, best efficiency is to use a router. Lets assume you are joining 3/4” plywood, although the thickness really doesn’t matter.
The ‘jig’ would consists of a 1/2” piece of plywood, maybe 12” square.
For the router, use a collar and a straight cut carbide bit. Let us assume you want the box joint tabs to be 3/4” wide. Use a 3/4” bit and for the sake of discussion, lets also assume the outer diameter of the collar is 1”.

You will need to dado a shallow slot across the underside of the jig plywood, exactly 3/4” wide (maybe half the thickness of the plywood deep). This slot will hold the ‘key’ for indexing the jig.

Perpendicular to this slot, you would glue/screw on two rails. These rails would probably be some straight hardwood, 12” long. maybe 3/4” thick by 1.5” wide. These rails are used to sandwich your plywood sheet and hold the jigs plywood square to the sheets edge. You want a snug fit to the sheet since these rails also eliminate any chip out from the router.

Now on the top side of the jig (opposite the rails), you need to dado/cut a 1” wide slot 5/8” away and parallel to the slot you previously cut. This is 3/4” away from the key with 1/8” allowance for the router collar. The dado will completely sever the jig plywood piece, but the rails will still hold everything together. Glue a 3/4” block of something into the first dado, between the rails. This is now the key for indexing. Place the jig onto the edge of your large plywood sheet and slide it over until the key rests against the corner. Clamp the rails securely to the sheet, away from the slot used by the router. Now you just run the router through the wide slot, with a bit depth set to the jigs plywood thickness plus the depth of the box joint tab (3/4”). Unclamp the jig, place the key into the just-cut slot, reclamp and cut again.
The process is the same as with using a table saw jig. I wish I could explain this with pictures, but hopefully I have provided a general description.

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