Gluing Tight Miter Joints

Gluing and clamping techniques for an excellent joint. July 6, 2005

I was wondering if anyone had suggestions on how to glue up mitre joints with a nice tight joint? I am using 5/4 stock. The mitre cuts are a true 45 degree and I have tried biscuits with pin tack and clamping with bessy frame straps. Any help would be appreciated.

Forum Responses
(Cabinetmaking Forum)
From contributor T:
Not sure what the bessey is, but I've glued up some pretty good mitres with a cheap Pony strap clamp and the metal corners it comes with. The finish carpenters seem to favor those wire clips that pinch thins together - for casing and such.

From contributor P:
If you are pre-assembling mitered joints before installation, the Senclamp gun would be a good choice to draw the joint tightly together (same principle as a corrugated faster, but uses smaller wedge shaped fasteners called Senclamps). Use fasteners on the back side for a tight joint with no visible fasteners. However, with the Senclamp, you can also shoot a fastener into the edges of the outside corner and/or inside corner to draw the pieces together.

From contributor F:
Mike, as furniture maker type I shudder at all this steel these guys are pumping into their joints. I do it like this: I never assume that because a corner is supposed to be a certain angle that it is truly that angle, whether 90 degrees or 45 degrees. The trick is to test each corner with miterd pieces so that at least one of the two is long enough to re-cut. Then when you can hold both pieces together on the attachment surface and the miters mate perfectly, brush a super thin coat of glue on each joint and let it dry some. Now brush on a wet coat and rub the two joint faces together until you get a vacuum seal started.

Fine tune the alignment of the two, and grab a roll of regular masking tape. Apply 1.5" or so of tape to one side of the joint running parallel to its length. Rub it down good with a finger. Now reel out some more of that same strip of tape around the corner on tear it off. Stretch it (this is key, stretch it) and stick it down and rub it with your finger.

Do this across the width of the molding and be sure to reverse the starting point of the tape clamps every other strip. If it is a small to medium joint, the tape will be enough. On large joints, I use cauls and any type of clamp I have that will work. Even when I am using clamps, I rub the joints to get a vacuum seal.

You asked how to get a tight joint, and the key for me is to cut it accurately first, then hold the pieces together after applying glue and rub the joint back and forth until it starts to get hard to move (vacuum). I rarely use nails, usually just on in the field stuff like crown molding.

From contributor F:
After re-reading, I thought maybe you were making a four piece frame, like a picture frame. In that case, a biscuit works good for alignment along with two bar-clamps underneath, and two bar-clamps on top. Tighten the clamps slowly and adjust the pressure to keep the tips of the miters aligned.

From contributor D:
To add to the method of priming the joint with glue, I would suggest that you practice anytime you are gluing end-grain. The capillary action of the wood has the ability to suck the moisture and the effective gluing properties, or viscosity, out of any adhesive.

From contributor R:
I've been doing mitered cabinet doors and have found that erring towards leaving the joint ever so slightly open on the inside really helps. For glue, Titebond 3 seems to form a more plastic, glue-line, and it seems to hold miters very well. When I switched over to it, I made a 1" wide frame and dropped it repeatedly on the corner the next morning and the glue held. This was a production speed glue joint without a sizing coat wiped on.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
When gluing up any miter joint, I recommend first spreading a thin layer of glue on each miter and letting it dry. This is done because when you glue up a miter joint, the glue will go into the pores of the wood, making the bond weaker (wood is porous), as opposed to first putting the layer of glue down to make the glue and wood bond much better. Depending on the size and type of the project, you won't need to use nails.

Comment from contributor B:
An easy and effective way to make strong mitered joints in box-type corners is given in "How To Master The Radial Saw" by Wally Kunkel. This is a short version (without diagrams unfortunately).

1. Get a roll of filament reinforced packaging tape.
2. Miter the corners accurately, leaving about a 1/64" flat area at the tips.
3. Lay the pieces down on a flat table outside up with the flats abutting, and strap them in this position using pieces of tape about 10" long running perpendicular to the joint.
4. Turn the pieces over and put a fairly thin line of glue in the bottom of the v groove formed by the two pieces.
5. Fold the pieces to form the right angle between the pieces.
6. Strap in this position using more tape. If you had the right amount of glue in the groove, the folding operation will spread it evenly in the joint. To get a tight joint, accurate miters are essential. You may need to adjust your saw.