How do I clamp box joints?



When making box joints, per a number of articles I've read, I've left the fingers a touch longer than the thickness of the board. So if I'm making joints on 1/4" plywood, my fingers might be 5/16" long, and then I just sand them down at the end. Some authors (Matthias Wandel as an example) even suggest cutting the fingers with a table saw.

An issue I'm encountering with these is that it becomes difficult to clamp the joint tight, since a clamp ends up pressing on the extension of the fingers sticking out, not the second board you are trying to clamp. I tried clamping a square to inside of the joint, which works well for keeping the joint square, but doesn't seem to apply enough pressure to pull the fingers tightly together.

Is making the fingers a tad longer poor advice, or is there another way to clamp this type of joint? I had considered making a jig with fingers and just a small amount of wood, but with 1/4" plywood, and only 1/16" excess material, it seems unlikely this would stay in place with clamping pressure.


Posted 2015-07-29T17:47:34.390

Reputation: 3 663



For clamping box joints, I've found that offsetting the clamps just to the inside of the fingers ensures you will be able to pull the corners tight. With the clamps there, you will still be pulling the joint closed while introducing relatively no bending to the boards. Picture below (source:

clamping #1

You could, as you suggested, make some jig with matching fingers that interlace with the box joint, giving you bearing on the portion of the joint you're trying to press. This can be rather tedious though, since you need to make a minimum of eight (two per corner) to properly clamp a box, possible more depending how deep the box is. As suggested by @keshlam, you can buy box joint cauls that serve this purpose.

Another way I've seen is to use offset blocks (or cauls) and then clamp around the entire assembly using ratchet straps. This is illustrated in the picture below (source: LumberJocks).

clamping #2

Per the suggestion of @keshlam, it's prudent to coat wooden cauls with packaging tape (or another surface that glue will not stick to) so that they don't become part of your box inadvertently. Also, you might try lining the very edge of your joint with blue tape to catch any squeezeout, similar to the image below (source:

enter image description here


Posted 2015-07-29T17:47:34.390

Reputation: 11 035

1"Matching fingers" cauls are available commercially for the most common small-box box-joint sizes. Haven't tried them, don't know whether they're any better than these solutions. Quick reminder: when using wood cauls for glue-up, coat their inside surfaces with tape or wax so they don't soak up spilled glue and get glued to the box. Also, some folks swear by putting blue tape next to the joint so squeeze-out winds up on that rather than having to be removed from the wood. – keshlam – 2015-07-29T18:32:07.497

I actually did try offsetting, and this would definitely work with thicker wood, but in my case, the 1/4" plywood flexes too much and ends up pulling the fingers out of the joint with even a bit of pressure – Steven – 2015-07-29T19:02:57.357

1Maybe you can put a piece of wood slightly shorter than your inside dimensions inside the box while clamping? This would keep your box from bowing in. – grfrazee – 2015-07-29T19:16:40.967

1I use plastic wrap for keeping my jigs and clamps from getting glued to the piece. A big roll from Sam's Club. I put a sheet under the project too so i don't get glue on my workbench. – Daniel B. – 2015-07-29T19:40:39.947


I tried clamping a square to inside of the joint, which works well for keeping the joint square, but doesn't seem to apply enough pressure to pull the fingers tightly together.

I think that's basically the right idea but could do with being implemented differently:

Right-angle clamping jig

[Source: Wood Magazine]

Another option that you might like to consider is something similar to this:

Corner clamping setup

As you can see the clamping blocks themselves support the thin stock and work simultaneously to keep it straight while providing the necessary clamping action to squeeze the joint firmly.

Is making the fingers a tad longer poor advice

I wasn't familiar with this prior to your question but I think I can see why this might be advisable in some situations.

I would however recommend that you not do this if your cuts don't result in unacceptable amounts of chipout, as it saves on later work while simultaneously makes clamping more straightforward during the assembly phase.


Posted 2015-07-29T17:47:34.390

Reputation: 38 608

I think I am going to make some of those ply brackets. Any hint on what thickness the ply is? – tl8 – 2015-07-30T01:24:22.237

Really like those jigs! @tl8 it looks like 1/2" or 3/4" – Steven – 2015-07-30T03:16:32.897

1@tl8, I don't think it matters once it's thick enough — a sufficient thickness to provide a good surface for the clamp head. So anything over 12mm / 1/2" should be fine; lots thicker than this would work great too (very stable) but might not really be needed. – Graphus – 2015-07-30T13:22:32.323


I wanted to elaborate a little bit on what grfrazee touched on with his answer. Gluing cauls or "Matching Finger" cauls could be a great idea. Indeed, as you have read, some instructions/teachers will advise to let the pins protrude the surface. This can make it easier to create a flush surface after but it can also be used as an aesthetic! If they were rounded off for instance.

Making these cauls could be considered a waste of time and wood on just one project. However, if you tend to make the same dimensions of finger joints, or even dovetails, you can still get good use out of cauls. They are simple and if created properly almost guarantee that pressure is being applied in the correct position. Risk is usually minimal just outside the joint but if being square is essential it would not hurt to consider.

There is another great video from The Woodright's Shop called "Pinch Rods and Squeezy Blocks". The latter being the name of how Roy Underhill's refers to his gluing cauls. A picture below show his dovetail "squeezy block" which took less than a minute to make using a chisel and one of the cut boards as a rough guide.

Squeezy Block

There are other examples as well like the ones from a blog post

Like grfrazee said

This can be rather tedious though, since you need to make a minimum of eight (two per corner) to properly clamp a box, possible more depending how deep the box is

but they are not really hard to make and if you can you them again then why not. From the post

It’s worth the 20 minutes it takes to make these.

As a bonus that same video show how to use (and make) pinch rods to use to make sure your box is square!


Posted 2015-07-29T17:47:34.390

Reputation: 13 424


Try this setup.Green is jig. Red is where clamps go.ClampJig


Posted 2015-07-29T17:47:34.390

Reputation: 21

1I would suggest clipping the inside corner of your jig to keep glue squeezeout from sticking to the jig (see @Graphus's post above for an example). – grfrazee – 2015-07-30T20:29:00.697

1I did try a square as I mentioned in my question but it didn't pull the joint together tight enough – Steven – 2015-07-30T22:42:59.153