How Do I Do This Glue Up?

[Derek Arita]

First, let me say that the Leigh Jig does great Box Joints. Ok...now, how would you clamp this up? The box is 15" tall and all the fingers protrude by about 1/32". I'm using MDF and it's not dead flat, so in order to get a consistency tight joint, it will take some convincing in some areas.
Aside from slightly bowed clamping cauls, are there any other suggestions on what I could use?
In fact, if you have any suggestions on glue application, that would be great too. Thanks for the help!

[Bert McMahan]

You could make some cauls on the box joint jig as well to press down between the fingers, just make the fingers on the caul thinner than the fingers on the actual setup.

[Mike Hollingsworth]

I sure wouldn't try gluing all 4 corners at the same time.

[Derek Arita]

I sure wouldn't try gluing all 4 corners at the same time.

Yeah...too much to deal with. I actually just glued up 2 opposite corners. I used 4 pinch clamps per corner. Works ok, but they're not real strong, so they don't pull the joints together by themselves. I had to really work the joint together along the length, which for some reason, was no easy task...no where to grab in the middle parts. Anyways, once together, the clamps held long enough to cure. Sure wish the pinch clamps were stronger.
For glueing, I laid the joint flat, slightly engaged, then just slathered glue all along the joint, trying to work it into the surfaces with a brush. What a mess, but with so many fingers, it would have taken too long to do each joint individually.

[Gary Ragatz]

Maybe line your cauls with some styrofoam? It will conform to the protruding fingers and create some pressure between fingers. The styrofoam will stick to any glue squeeze-out, but will come off easily if you're going to plane or sand those fingers.

[Mike Henderson]

I made some cauls that have fairly thick cork on their faces. Then I cover the cork with packing tape to keep the glue from getting to the cork. The cork "gives" so it presses the joint fully. I use it on dovetails but it will work fine on finger joints. If you can't get cork, other materials such as fairly thick rubber will probably work fine.

I'd use a very slow glue, such as West Systems with the slow hardener, and do all four sides at one time.

Mike

[Derek Arita]

I made some cauls that have fairly thick cork on their faces. Then I cover the cork with packing tape to keep the glue from getting to the cork. The cork "gives" so it presses the joint fully. I use it on dovetails but it will work fine on finger joints. If you can't get cork, other materials such as fairly thick rubber will probably work fine.

I'd use a very slow glue, such as West Systems with the slow hardener, and do all four sides at one time.

Mike

Mike, good idea on the cork. I really do need to make up some cauls for this very situation, as I've faced it before.. That said, I've never done such long joints with either dovetails or finger joints, so I was always able to handle them.
So for cauls, do you make them flat or curved?

[Mike Henderson]

Mike, good idea on the cork. I really do need to make up some cauls for this very situation, as I've faced it before.. That said, I've never done such long joints with either dovetails or finger joints, so I was always able to handle them.
So for cauls, do you make them flat or curved?

For your situation, where you can put clamps anywhere on the cauls, I'd make them flat. Curved cauls are indicated when you can only clamp the ends of the cauls.

Mike

[Simon MacGowen]

The box is 15" tall and all the fingers protrude by about 1/32". I'm using MDF and it's not dead flat, so in order to get a consistency tight joint, and ]

I may have missed other hidden complexities of this glue-up which looks pretty standard to me as a carcase with finger joints.

Assuming protrusion is part of the design, not due to jig setting errors, and the fit is decent (not overthight), you can use clamping cauls close by, not over, the fingers to close each joint.

I would not glue up two halves, wait for their curing and then glue up the two halves to form the final carcase, because if one or both halves are not dead square, you would be in trouble.

Instead, I would apply glue to all the fingers within the open time of 20 mins?, keeping the joints apart. Once glue is in all fingers, proceed with the assembly. While glue is wet and joints are not set, I can check the diagonals and move the clamps as necessary to restore squareness.

If you have not done anything similar in complexity, get a helper to assist in applying glue and assembling (holding the other end of a clamp, etc.).

If the protrusion is a mistake, sand or saw it flush before gluing up.

Simon

[Prashun Patel]

I would do two joints at a time with the other two just dry fit.

Are you concerned with the joints swelling during glue application?

There may be some flex if you don’t clamp directly in line with the side, so I vote for cauls.

[Derek Arita]

I would do two joints at a time with the other two just dry fit.

Are you concerned with the joints swelling during glue application?

There may be some flex if you don’t clamp directly in line with the side, so I vote for cauls.

Yes, I am concerned about the joint swelling once glue is applied. It's a nice fit with room for glue, however I'm not sure if MDF swells or not. If it does, then it will be a tight fit. In addition, I probably should be using a slower set glue, however it's one of those things that I never got around to buying. I just would use it so infrequently that it might just go bad between uses. I should probably have a small bottle around just incase.
I'm a little less concerned about squareness, as I did a dry assembly and it seems like all corners are pretty square as it is. Also, the top and bottom will be inset and it'll be a tight fit there, so that should pull everything into square.

[Edwin Santos]

One more opinion here - I learned from Frank Klausz to use white glue (Elmer's Glue-All) for glue ups like this because it has a longer open time. Titebond yellow glue is technically stronger, but it sets up very quickly and here where you have a lot of glue surface area and tight joints, white glue is more than adequate and the box will not come apart.

I think it is too much brain damage to glue two lone corners and then combine them into a box in a second glue up.
Here's what I would do - Treat two opposing sides as your glue targets. Moving briskly brush glue on to the sockets of both of these opposing sides, lay one side down, insert the left and right, then insert and tap on the remaining side on top. Turn the assembly up, square it, clamp it and you are done. If you're going to make these types of things again, I like the suggested idea of having the packing tape covered cork faced cauls. A FWW author demonstrated a similar concept using soft pine as a caul for dovetails. Brush the glue liberally because this is MDF which will want to drink it up.

As a total aside, years ago Lee Valley Tools advertised a tip using their low viscosity Chair Doctor glue where they assembled a box joint dry and brushed the Chair Doctor Glue on the fingers from the outside. The glue wicked into the ends of the fingers causing them to swell ever so slightly and lock the joint. Of course this was a box that was intended to be sanded afterwards so yours may not be a good case for the idea but on the other hand, MDF will swell more than hardwood so it would be interesting. I've always been meaning to try the idea with thinned glue and now I'm reminded to do so. Good luck.

[Al Launier]

Have you considered the use of corner assembly squares (https://www.rockler.com/clamp-it-assembly-square) to clamp the entire assembly together when gluing, as a a supplement to cauls? You can make up a bunch of these sized to suit from 3/4" plywood with steps to position clamps as needed. Handy to have around in different sizes.

[Simon MacGowen]

One more opinion here - I learned from Frank Klausz to use white glue (Elmer's Glue-All) for glue ups like this because it has a longer open time. .

Edwin,

Yes, white glue is better regular yellow glue. But working time/assembly time, which is more important, is not the same as open time. This has been covered before: https://www.wwgoa.com/article/measur...assembly-time/

With the protruded edges, it is easier to glue up the "box" as one than handling a "L" shaped glue-up -- twice. For more complex glue-ups, it is advisable to break down the assembly job, but this isn't one of them. I have done many finger joint drawers (plywood) for the shop, and as long as the fit was not over-tight, swelling had not been an issue.

If the OP has some scrap finger parts around, glue up a sample to see how bad the swelling is.

Simon

[Jared Sankovich]

Glue with a long open time (liquid hide in this case) and a lot of clamps with cauls. Additionally I would glue it all up at once.
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