Secret for Air-Tight Miters?

[Pat Germain]

Is there a secret for getting nice, air-tight miters? I've been making some large shadow boxes over the past few months. My miters are looking OK. And I can make them look better with some sanding. But they're still not air-tight.

Everything looks perfect during dry-fit. I'm using biscuits which help things to stay lined up. But as soon as I apply the glue, it seems the end grain starts absorbing it and the edges start to curl and warp. Thus, as soon as I apply the clamps, it's not looking so good any more.

I've tried using parallel clamps, strap clamps and combinations of the two. I've used packing tape at the corners and that seems to help a little. But the results are mostly the same.

Any suggestions? Thanks!

[Joe Scharle]

Merle band clamps made my life with frames very simple. They use a steel band. I bought mine from MLCS.

[glenn bradley]

How thin and what is your material? I do mitered boxes with cherry, ash, lacewood, walnut, shedua, padouk and such in 3/8" material without the reaction to glue moisture you describe. Since you are using biscuits I would think it is not real thin material. If the material is very soft and/or thin a lower moisture adhesive may be the fix(?).

[Pat Germain]

Joe, thanks for the clamp recommendations. I will be sure to check those out.

Glenn, I've used alder and cherry. My next box will be African mahogany. After machining, the stock is 3/4" thick and 4" wide. I'm using TBII glue. What would be a lower moisture glue? Formaldehyde resin, perhaps?

[David DeCristoforo]

The 4" dimension is the depth of the shadow box? Assuming that it is, you need to make some clamping cauls that are at least 4" tall so that the clamping pressure is distributed over the entire 4 inches and not "concentrated" in the middle. Simple wood blocks (attached to the clamp jaws with hot melt glue or double sided tape or whatever other method strikes your fancy) will work fine. Also, you may want to consider using a spline instead of a "biscuit". A biscuit will swell when moisture from the glue hits it and you need a lot more clamping pressure to close the joint tight than you need when you "dry fit". A spline cut from solid wood will not cause as much difficulty. In addition, you can make the splines the "full length" of the joint which will help hold the alignment along the whole joint line rather than only part of it as is the case with a biscuit that is only a couple of inches in length.

[Pat Germain]

Yes, the shadow boxes are 4" deep.

My boxes have a plywood back which fits into slots at 1/4" depth from the back. This allows me to use a French cleat to hang the box. The glass/lucite in the front slides in from the bottom. This allows it to be replaced if (when) it becomes scratched or broken.

The plywood back helps with keeping things square during glue up. But all it takes is 1/16" of movement to ruin a miter. . I thought about using cauls, but I wasn't sure if they would help. I'll give that a try. Thanks for the tips, David. Are you suggesting something like a dovetail spline for the corners?

[David DeCristoforo]

I was thinking more of a "simple" spline in a slot. A dovetail spline would be an "upgrade" but the point is more to have the corners held in alignment along the whole length of the miter rather than just part of it.

[glenn bradley]

After machining, the stock is 3/4" thick and 4" wide. I'm using TBII glue. What would be a lower moisture glue? Formaldehyde resin, perhaps?

I'm sorry, I am a vacuum of knowledge on resin glues. I was just hoping to shake out some ideas. With a 3/4" material and a 4" run I don't know that moisture is your culprit. I think the caul method suggested may turn out to be the fix you're after.

I use small parallel clamps (Bessey Uniklamps) but still sometimes get the gaps you speak of as there is not even pressure along the whole seam line. I'm glad for this thread as it has nudged me into trying the caul route I keep thinking about but, not acting on.

[Pat Germain]

I'm glad for this thread as it has nudged me into trying the caul route I keep thinking about but, not acting on.

I'm in the same camp, Glenn. I had hoped to try the cauls yesterday. But this has been the worse year from my hayfeaver allergies in my life. I've been pretty much incapacitated.

[Michael Schwartz]

My advice would be not to sand the joint as you just cant control what you take off. If you are using a properly setup table saw or miter saw with even a half decent blade your cuts should be perfect and ready for glue up. If your cutting the joint by hand use a sharp well tuned hand plane and shooting board to make it perfect.

Test cuts are your friend, Keep making them and dialing your saw in until the cut is perfect. It might take time and if your not familiar with setting up that machine and if it takes you an hour to get it right, stick with it.

Of course it helps too to make sure any edges on your stock your referencing off of when making any cuts are dead strait or flat.

Any good crosscut or even combination blade should work pretty well. I use more expensive industrial blades in my shop but I have had very good results with off the shelf freud diablo blades from home depot, and even used a ridged combination blade in my saw for a few months. (no affiliation)

Edit*

For glue up if your dealing with a mitered carcass, make panels out of thin plywood or mdf scrap just under the dimensions of the case sides, top and bottom. On the table saw or bandsaw rip 45 degree blocks and glue and screw them to the panels on the end so you have a parallel clamping surface.

For a mitered frame the above method could be adapted or you could buy a set of miter clamps. Another option is to just glue and braid nail the frame to a case if your not using it as a structural face frame and if your miter cuts are clean you should have no problem getting a tight fit without clamping pressure.

[Josiah Bartlett]

Another option would be to use a lock miter shaper head or router bit.

[Eiji Fuller]

I like to temporarily glue on some angle blocks to the outside of the miters so I can actually clamp across the joint with regular clamps. The miters come out super tight and I just knock off the blocks after Im done. Make sure and use a softer/weaker wood for the glue blocks otherwise they may take some of your project with them when you go to knock them off.

It takes seconds to make the blocks and it also makes glue ups of miters for different angles a breeze. I use 2p10 to glue on the blocks then sand off the residue.

Another huge benefit of this method is, is that there is no stress of trying to get all 4 joints aligned then clamping them all without any shifting while the glue is setting... You can work one joint at a time or do opposing corners then put the 2 halves of the box together.

[Ralph Barhorst]

I like to use a piece of rubber innertube to clamp my boxes. I had a couple of old truck inner-tubes and I cut out some 1" wide strips that are about 8' long. Stretch the rubber strip and wrap it around the box. It provides good even pressure and I have never had a problem with mitered joints.

[John Thompson]

Here's my suggestion... After you assure you have a butt kiss on the miters themselves... lay them edge to edge with the inside facing down. Apply a few strips of package tape across the width to hold it there. Then turn it so the packaging tape is down the inside of the project is facing up.

Apply the glue and then wrap up your package as the packaging tape with allow it to just roll up in a square or rectangle. Another piece of pack tape to keep it wrapped temporarily. Then I wrap my entire package in either 1/4" or 3/8" surgical tubing depending on the size. The strips of inner tube Ralph mentioned will work well also.

I use this on a small boxes and I build a lot of Quad-linear legs from QSWO for A & C pieces. The quad-linear is QSWO 1/4" thick glued to a cheaper core wood to give me fleck on all 4 sides and they are around 30" long on average. All eight legs on this computer desk-hutch are 45* degree mitered pieces as I spoke of.

See if you see a miter line in the pictures below on them. Sorry about the reflection on the top miter on the box but a quick camera shot to get the picture a few minutes ago.

BTW.. I mainly use hardwood but.. when I do use soft-wood there will be a faint line as softwood compresses easily. I take it out by burnishing the miter after glue up with a screw-driver shaft on any hard metal object.

Click on picture to enlarge...

Good luck...

[Stephen Musial]

How far are the biscuits from the face? Could the biscuits be swelling and causing the warping/twisting? (Have you tried it without a biscuit?)

For perfect air tight miters, I cut it rough (+1/32 or so) on the saw and then sweeten it on a Lion Miter Trimmer. You can't beat a tool that takes off shavings of wood that you can read through...

Got an original on Ebay for 75 bucks or so. You can buy Chiwanese knock offs at Rockler for 200.