Mitering Carcase Corners

[John Stevens]

I saw a pic of a really cool looking free-standing cabinet the other day. The carcase was formed from four slabs that were mitered together at the corners. I'd like to do that, but I have no confidence that I'd be able to cross-cut boards that are, say, five feet long and two feet wide, with the kind of precision needed to get gapless miter joints all the way around a piece of that size. How would you do it?

What tool(s) would you use to cut the mitered edges?

Even a tiny amount of warping or cupping will leave a badly visible gap in this type of joint. How would you get the carcase sides to remain flat from the time you finished milling them until they were mitered and ready to be glued together? Or if you're willing to concede a little warp and cup, how would you deal with it when cutting the miters?

Would you glue all four sides together at once? Would you glue two sides together, then glue the third to the first two, then glue the final side? Or would you glue two sides together, then the two other sides, and then glue the two "L" shaped halves together?

Would you just assume that you're gonna get gaps no matter what, and focus your effort on trying to fill or disguise them after glue-up?

So far, I've made two small test pieces, but I wasn't satisfied with either. As you can see, I've been trying to think of ways to get good, tight joints all around, but there are a lot of possibilities, so I'm hoping some of you can save me from testing each and every one. Thanks in advance.

[Brett Baldwin]

When you say slabs, how thick are you talking? I'm picturing 8/4 thick or more. In any case, I'd think the table saw would be the ticket. I'd go with an infeed and outfeed tables if you are going to try to handle a 5' slab of wood of any significant width. Also, featherboards (strong ones) to keep it all tight against the fence. Actually, while I was writing this it occured to me that a jointer could also give you a nice, straight miter if your fence has that capability. Of course I'd think a rough-cut on the tablesaw would be your first move.
As far as keeping it all tight, I'm not sure if anything will if you get a lot of movement happening but a splined miter may have a better chance than nothing. Biscuits at the very least I'd think.
This advice is worth at least as much as you paid for it, maybe more.

[Mark Rios]

What about a lock-miter bit? I hear that they are hard to use but it should give you a good even miter. I'm sure others here have more info on this type of bit than I do.

[Charlie Plesums]

If you want mitered corners on a larger carcase, you probably need biscuits or a lock miter or key to keep the sides aligned during glue-up.

The lock miter mentioned is a good choice, but be sure to have the padded jacked ready the first couple times you try to align it... They are black magic until you get the hang of it.

[tod evans]

lock miter and a straightedge....02 tod

[Dino Makropoulos]

Try with a 45-1/2" degree cut.
You can fix-fill the inside of the joint later.

[John Stevens]

Thanks for all of your suggestions. Just to be clear, when I was talking about "slabs" I only meant slab-side construction, as opposed to frame and panel. I should have realized that when woodworkers refer to "slabs," they're usually talking about very thick planks.

The lock miter bit is an interesting idea, but it would require some way of clamping the case sides with cauls to eliminate cupping while machining the joint. (A 21" wide, glued-up board is practically guaranteed to have some cupping--at least in my shop ) Not sure how that can be done.

I'm thinking of ways to use a guided circular saw to cut two adjacent sides simultaneously, using a jig similar to Dino's M&T fixture. (Dino, you probably know where I'm going with this.) It would have to be a multi-step process per side: cut each side to approx 45 degrees, then clamp one to the top of the fixture, one to the side of the fixture, and run the saw across the joint to shave about half a saw-kerf off each board. If everything worked just right, it would produce complementary angles that add up to 90 degrees. In order to get the opposing sides to come out to the same length, all four sides might have to be clamped simultaneously at some point in the process. Gotta think about this some more. If I pursue it and it succeeds, I'll report back.

Edit: Somehow the thought occurred to me that it might be faster, easier and just as accurate to make the final cuts with a hand saw instead of a circular saw. You can tell I don't use hand tools often enough.

[Charlie Plesums]

...The lock miter bit is an interesting idea, but it would require some way of clamping the case sides with cauls to eliminate cupping while machining the joint. (A 21" wide, glued-up board is practically guaranteed to have some cupping--at least in my shop ) Not sure how that can be done...

It seems that all of your options will require getting the joint straight while machining. The Lock Miter cuts one face flat on the table, and cust the other face vertically on the fence...the difficulty is that the setup must be correct for both parts at the same time, and that setup depends on the thickness of the wood. Can you eliminate your cupping problem enough to allow the cuts along the table and fence?

If you solve the cupping, once the lock miter joint is cut, it is trivial to keep aligned during the glue-up, and easy to clamp. Therefore, if you can solve your cupping problem, and have a plentiful source of either valium or liquor to support the setup, use of a lock miter may be your best solution.

[Alan Turner]

JOhn,
The secret mitered dovertail will glue up easily, and still present the look of a mitered corner. A bear to cut, but good neander work.

[Dan Oelke]

Oh Alan - that isn't even nice! He admitted not being that conversant with neader tools and you suggest the mitered dovetail.

To borrow a phrase from a old professor of mine - "implementation of this is non-trivial" which is an understatement!


There was an article in one of the magazines a year ago or so - but my memory is failing me as to which magazine and or even how long ago it appeared. If I remember correctly it kicked off a series of letters to the magazine about the appropriatness of publishing such an article. Some said that it wasn't appropriate because the technique was so advanced and there were always "better" ways of accomplishing the joint. Of course others thought that it was a wonderful article. I *think* it was authored by Ian Kirby - but I could be wrong.

[John Stevens]

The secret mitered dovertail will glue up easily, and still present the look of a mitered corner.

Thanks, Alan. It's becoming very clear that the winning strategy for me will be to find a job that pays more, and then hire somebody to design and build the furniture

[Steve Wargo]

No need to hire a designer when you get all of this good information. On a side note... Mitered Dovetails. Now you're just showing off.

[Alan Turner]

The closest I ever came to cutting a set was thinking about it. My comment was pretty tongue in cheek, but I know John, and I think he knew that. Or, at least I hope he did.

I thought about it for the feet for the Newport Kneehole Desk, but just splined them instead.

[Lynn Sonier]

I make small boxes (about a half inch thick) and miter the corners and use biscuits. Even though I dry fit, I always have trouble closing up the corners after I apply glue - don't ask me why. I use band clamps to pull the corners close and still have trouble. I would hesitate to use biscuits on an application this large. You'd probably need 4 to 6 band clamps. The spline suggestion would probably work a little easier. I think what I would do is use long setting epoxy, with a thickening agent the same color as your wood, to give you more time. I use sawdust to thicken. If its a light wood, I use baby powder and, with the color of the epoxy, it does a pretty good job.

[Curt Harms]

If I could find a 45 degree chamfer bit with adequate capacity, cut the miters close, clean them up with a router and chamfer bit, cut biscuit slots (sorry) use a slow set glue. I built a plywood microwave cabinet like that and it has held up well. I had some minor gaps in the outside of the miters; I just moistened the edges a little and took a screw driver and worked it sort of like forming a burr on a scraper. I think I used polyurethane glue. The gaps closed up and have stayed closed for about 8 years so far. This also resulted in edges that are rounded slightly so they're not sharp. Worked for me

Curt