The Devil's miter (Lock miter)

[Michael Schuch]

I just got in from the shop after 7 hours of tweaking the shaper for lock miters. It has been 15 years since I have done lock miters and those were in solid maple. Back then I made 3.5" x 3.5" square corner columns for our staircase railing that fit over the 1.5" x 1.5" square steel tubing I used as the skeleton for the main stair case railing in our house. I guess 15 years is long enough to forget how perfect the setup has to be and what a pain it is.

This time I was using the lock miters to make boxes out of plywood. Plywood is even tougher than hardwood because it doesn't machine as easily and there is no rounding over the edges, the face ply's have to meet perfectly.

A good couple of those hours were making a new fence face piece for the shaper and tweak it in to be a perfect 90. The rest was dialing in the lock miter setup and practicing how amazingly slow I can push a piece of stock through the shaper so the plywood ply's do tear out.

Anyone else use lock miters? Anyone else use them in plywood? 30 years ago I took a class in cabinet making and they presented the lock miter as a way to join the cabinet exposed side to the face frame with a nice clean look. (I didn't actually see anyone attempt this)

[Warren Lake]

patrick here past did that on his work. Looks nicer for sure than plunking a face frame on. At times it blends fairly well at times not then it can move. On some stuff past I used to rabbet then round, depended on the work but for that it worked well.

We did mitre and spline that is how the old guys did it. Its fast. The lock seems better maybe but as you said all has to be set up right. I have shapers, fences dont cut it. Very well made I think the way the wood fastens makes it go out of whack. I want to redesign how the wood is held in place.

[Mel Fulks]

I used to do a lot of lock miters , and I offered some help here that some found useful. But that was a long time ago before I retired.

One thing that stops the square corners from ripping off ,is to use a round over bit to climb -cut the corners. Then when you run the
lock miters they don’t break and fly off.

[Kevin Jenness]

No doubt lock miter setups are fussy. When dialed in they allow for a clean result with clamping in one direction. Most cutters require running one set of parts flat and one set on edge, which can be problematic when running long pieces where the parts want to lift off table due to leverage and the sharp corners can get crushed, plus a conventional powerfeed can be a pain to change from horizontal to vertical. I have a Freeborn lock miter set that allows for running everything flat on the shaper table and running a dado on the tablesaw on half the pieces, which I find easier. https://ballewsaw.com/freeborn-pc-28-020-carbide-lock-miter-set.html

I use miterfolds far more often. All that's required is accurately sawn edges held together with packing tape and folded up. Four-sided columns can be wrapped with stretch-film plastic twine to pull the last joint together. I nearly always do miterfolded finished cabinet return panels. For long grain solid wood or plywood they produce a very strong glue joint without much fuss. They do need flat material and straight edges.

Miters on plywood allow for no more than the slightest easing, that's the nature of the beast.

520372

[Stan Calow]

The thread title is apt. I have bought and thrown out (into the pond) three bits over the years. I kept thinking "now I'm more experienced and can do it". Nope. I could not maintain a consistent edge for long enough to get a usable workpiece. And plywood was the worst - the edges too fragile. Threw away a lot of wood. Yes, it's all my fault, but it should be easier. I settled for keyed or splined miters for small projects.

[Rich Engelhardt]

https://sawmillcreek.org/showthread.php?301623-Infinity-Lock-Miter-Master-Jig
Alan Schaffter a fellow SMC member invented that handy little jig.

I also picked up a Micro Jig Fit Finder that I haven't used yet to make it easier to find the center. Jury is still out on that.

[Jim Becker]

I bought lock miter tooling a number of years ago...for the router table, not a shaper since I don't have one. WAY too fidgety for me! And that's with a little setup thingie. There are way too many other alternatives for joinery so it sits in my tooling storage...

[stephen thomas]

Would have to see which version you are using, but i used to sometimes make sets of lock miters for some hardwood trim installations.
It made the installs go so much faster when you did not have to count on matching miters on the job, and accommodating the vagaries of many old plaster corners, in restoration work.

I would run the "difficult" ends on a pile of loose rectangular section trim (baseboards, e.g.) on the tenoner, and then leave the easy slot and miter for the field work, which can all be done on a saw.
Although there is still a miter router bit in my toolbox, ground & adapted to run the integral spline with miter end as well, which is one pass in a jig. You do have to then trim the integral spline back, but that is not really a precision/difficult issue.

I rather abhor miters for veneered and plywood work. Especially kitchen cabinets. One bump or ding and the whole thing looks a mess.
Then you live with it until you replace the unit or entire kitchen. Or you hire someone to let in and blend (as far as possible) a solid piece in the ding area(s)
I'd rather see (& I use) a solid mating piece with some overlap. Even if you get dings in a solid corner, they are easier to fix, and somewhat less unsightly to begin with.
Tactile makes a difference, too - a person should not constantly feel sharp edges. There is not enough veneer in modern ply to even really ease a mitered corner if one element is not a slightly proud (before easing) solid.

[Larry Edgerton]

When you get a setup that works make a couple of samples that you keep with the cutters.

On the vertical piece adjust the fence out to leave a slight square edge, about the depth of the plywood veneer. Even on solid wood. This stops the piece from sinking in the cut as it does when you try for a point. On plywood after I get the setup right I clamp on a 1//4" piece to the fence and make preliminary cuts, then remove the 1/4 and make the final cut. Hope this helps.

This is one cutter I absolutely wish I had a repeatable computer spindle/fence setup for.

[Joe Calhoon]

Depending on the lock miter head and if it comes with good drawings it’s pretty easy to set on a manual machine using a height gauge and doing some math. Lock miters with flats on the fingers (first picture) are easier to set than ones with all rounded fingers. Like my old Freeborn cutter, second picture. Shaper fence with micro adjust is also very helpful setting these up.
Also a feeder that tilts easily to face the fence helps. Way back we used to keep 2 feeders on the Martin shaper just for lock mitering. One on the left side set for the vertical run.
520402520403
For most cabinet size work I agree with Kevin and think the miterfold- packing tape is quick, easy and plenty strong. I don’t miter much plywood but would use this method for that. High end veneer work has solid edge banding applied before the veneer is laid. The miter is cut through this to give solid backing at the corner where the miter meets allowing easing of the corner.
pictures are some recent miter folds in quartered oak.
520404520405

[Ralph Okonieski]

I bought the jig that Rich mentioned and also keep some samples. I also made a few “hold down” jigs (for lack of better terminology) to assist in the cutting. With the tool and samples, setup is fairly easy, but always cut a test piece just to confirm. The “hold down” jigs give more confidence in keeping fingers away from spinning blades and keep components at correct orientation.

[Michael Schuch]

Even though I supposedly only need to clamp in one direction I always end up clamping it in 2 directions which seems to work for me.

My plywood box actually turned out a lot better than I expected! It was more of a test but I am impressed with the results. I am not sure if it is the route I want to go to complete my project.


No doubt lock miter setups are fussy. When dialed in they allow for a clean result with clamping in one direction. Most cutters require running one set of parts flat and one set on edge, which can be problematic when running long pieces where the parts want to lift off table due to leverage and the sharp corners can get crushed, plus a conventional powerfeed can be a pain to change from horizontal to vertical. I have a Freeborn lock miter set that allows for running everything flat on the shaper table and running a dado on the tablesaw on half the pieces, which I find easier. https://ballewsaw.com/freeborn-pc-28-020-carbide-lock-miter-set.html

I use miterfolds far more often. All that's required is accurately sawn edges held together with packing tape and folded up. Four-sided columns can be wrapped with stretch-film plastic twine to pull the last joint together. I nearly always do miterfolded finished cabinet return panels. For long grain solid wood or plywood they produce a very strong glue joint without much fuss. They do need flat material and straight edges.

Miters on plywood allow for no more than the slightest easing, that's the nature of the beast.

520372

That is a cool shaper cutter... it is out of my price range for hobby work though. I like the idea that all the pieces can be run flat on the table.

I have seen the tape and folding technique on youtube but have never tried it myself. I might give it a try.


When you get a setup that works make a couple of samples that you keep with the cutters.

On the vertical piece adjust the fence out to leave a slight square edge, about the depth of the plywood veneer. Even on solid wood. This stops the piece from sinking in the cut as it does when you try for a point. On plywood after I get the setup right I clamp on a 1//4" piece to the fence and make preliminary cuts, then remove the 1/4 and make the final cut. Hope this helps.

I did make samples last time I used the lock miter bit... that was 15 years ago and I have no idea what happened to them. I will make more and hopefully store them better.

The 1/4" fence spacer for a first cut makes a lot of sense. I might give this a try.

Getting just enough flat on the vertical piece so it doesn't dive into the table after it passes over the cutter is the trick. I spent a long time and quite a few pieces of wood trying to get that just right. With hardwood I can just round over the corners after everything is together. I can't really do that with plywood so it took a while to get just enough flat but not too much.

I am using an old Walker Turner shaper and I am thinking of leaving it dedicated to lock miters. I have a lot of 3/4" bore cutters and I don't have a 3/4" spindle for my other shaper. I will have to see what happens.

[Edward Weber]

I bought a lock miter bit about almost 20 years ago at a wood show. It came with 2 UHMW setup blocks.
These little blocks definitely reduce the setup time about 90% for me. Ever since then, I now make setup blocks for all my mating profiles. Something I hadn't done before

[andrew whicker]

There's a good video on YouTube of a guy setting up his shaper w lock mitre. I used that for reference and thought setting them up was pretty easy.

I do have a manual up and down in addition to the electric feed. I think with just electric feed it would be more painful.

https://youtu.be/CEgv-AYj0oY?si=qN-IxuMxQdiDrrFY

Also,I think sacrificial fences are key to using my shaper properly. I don't have an aigner fence. I use a sacrificial fence to have as much support as possible infeed and outfeed.

[Steve Demuth]

I use them, but only for long grain cuts in applications like boxing a column or similar. Wouldn't even consider them for plywood unless the face ply on my lumber was fairly thick and long grain in direction.

I don't find dialing them in particularly hard to set up (assuming you have a square to the table fence, and both the fence position and bit height are truly fine-adjustable). Set the bit so that its close by eyeball to cutting the same height as width with the lock structure roughly centered on the stock. Then dial in the height When the height is right for your stock thickness, test pieces that are cut face-to-table, with one face flipped, will align perfectly. After that, moving the fence to get a perfect joint is just typical final adjustment. And of course, once you've done it. you can keep an offcut for future setting.