Finger/box joint sheet of plywood.

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Forum topic by Josh posted 06-15-2015 01:05 AM 4073 views 0 times favorited 13 replies Add to Favorites Watch
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5 posts in 1442 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.

13 replies so far

View BurlyBob's profile


5973 posts in 2463 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 (online now)


2422 posts in 1420 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 2701 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


1236 posts in 2149 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


3163 posts in 1678 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 (online now)


2422 posts in 1420 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.

View groland's profile


211 posts in 3609 days

#7 posted 06-13-2017 10:28 PM

My box joint experience is limited to making three drawers. On each corner was about an 8” length of 1/2 inch box joints. I used 1/2 inch Baltic Birch plywood and the kind of table saw jig covered in William Ng’s excellent YouTube video on making box joints. I backed up the dado cuts with 1/8 inch masonite that did a good job of preventing tear out.

If you are making a large box, my biggest concern for you is gluing the thing together. I bought Tightbond III glue because of its supposedly greater open time, but still had difficulty getting the box sides together before the glue began to set up. And in one instance, I had to use a Bessy K-Body Revo clamp and a lot of force to get the joint to close up.

It’s tricky because you do not want the joint to be sloppy, but when every glued surface swells just a little, the joint can resist coming together.

I thought carefully about how to create a sequence for applying glue as speedily as possible and my joints were not super tight. I did clamp the joints together.

I applied glue to all touching surfaces. Perhaps someone with experience with methods of gluing large box joint structures could comment further on where to apply glue and what kind of glue is best for this application. (I was too lazy to wait on shipment of liquid hide glue which supposedly has better open time that TB III.)

I used blue masking tape on the insides of the joints to prevent messy spill-over of glue. This worked exceptionally well. Once the box was together, i just peeled the tape off and the drawer insides were perfect.

View Dark_Lightning's profile


3339 posts in 3306 days

#8 posted 06-14-2017 12:33 AM

Depending on what the box is expected to be loaded with or what other kind of abuse it will receive, stingy with glue is probably better, and then drill and pin through the fingers, as Todd Clippinger detailed in a post awhile back.

-- Random Orbital Nailer

View unbob's profile


810 posts in 2101 days

#9 posted 06-14-2017 12:47 AM

I have done very long box joints, I found slow dry epoxy works best for me, does not swell the wood near as much as water based glue, gives plenty of time to get the box together.

View Loren's profile


10477 posts in 3845 days

#10 posted 06-14-2017 03:27 AM

The joints can be cut from the face of the
board using a router, bearing-guided bit,
and templates. The corners will be rounded
and have to be squared up by hand but
the considerable blowout that can occur
when routing plywood from the edge
will be avoided.

Prior to routing with the template the waste
can be jig sawed out to save time with
the router and make less mess.

View Woodknack's profile


12430 posts in 2577 days

#11 posted 06-14-2017 03:42 AM

- groland

Why are you answering a 2 year old question?

-- Rick M,

View ArtMann's profile


1135 posts in 1013 days

#12 posted 06-14-2017 04:12 PM

Your question is a little ambiguous since a finger joint and a box joint are two very different things.

View AlaskaGuy's profile (online now)


4760 posts in 2506 days

#13 posted 06-14-2017 05:13 PM

Your question is a little ambiguous since a finger joint and a box joint are two very different things.

- ArtMann

Well it’s no wonder finger joints and box joints get misused.

Scroll down this page and take a look

-- Alaskan's for Global warming!

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