Locking Miter Joints

[Pete Staehling]

I purchased a couple inexpensive (Meihejia) 1/2" Shank 45 Degree Lock Miter Router Bits (1/2" and 3/4") and have been tinkering with them. I have done a little tinkering and figured I'd post some impressions and maybe a question or two.

After initially failing to get any decent results I worked out the details a but. I found that as reported by others setup was a little fiddly, but not as bad as I expected. It is a little bit of a pain to need to set it up for the specific thickness of the stock and mill it all exactly the same.

Tear out is an issue, but can be managed. Using a backer piece and maybe using wider stock than needed and cutting it to width after routing helps. Also the router speed needs to be slowed down. Feed needs to be carefully managed with firm pressure to keep things in place. Feather boards help a lot as does a miter gauge/sled. For end grain, cutting with multiple passes or precutting 45 degrees partial depth on the table saw helps.

The joint isn't especially strong, but it looks nice, squares up pretty much automatically, and with not much clamping effort.

I have not actually built anything other than test joints so far, but it seems like a useful technique that I will use in the future.

How much better are the much more expensive bits and in what ways are they better? With just some light use in testing the Meihejia 1/2" shank ones seem okay.

Any other hints on how best to implement and utilize lock miter joints?

[Ole Anderson]

Lock miter joints can be frustrating until you get it figured out how they work. Mostly I have used them to build large square legs from 3/4" stock. My first try on a mission style bed with expensive QSWO was a bust, I ended up going with a regular miter. When I realized how important consistent stock thickness was. So on the next project I milled my test pieces and final stock in the same run and recorded the caliper thickness on a template for future use during router table setup. Don't recall the brand, and they did not come with a setup block like some do now. Essential is an accurate thickness measurement, either a caliper or using a fingernail test.

This pic shows a test fit on my pool table legs, one side is long to bolt to the rails under the slate.

"Isn't especially strong."? On that I will have to disagree.

[Steve Eure]

I have used the lock miter bit and I do have the set up block for one set that I have. The other set, I haven't tried yet. One word of advice that I can give you is to have set up blocks for each thickness of material that you will be using. This will get you in the ballpark much faster and will limit setup time and test cuts. You will still have to test cut but it will be limited. As far as not being a strong joint, I find that not to be true. Much more stronger than a straight miter.

[Carl Beckett]

I have a couple different sizes and have used them successfully. I did buy the alignment fixture from infinity, which sits on the bit to help with initial setup (instead of blocks).

On QSWO I found the small cheap one off ebay did not cut that great. All your tips are good ones. A question I have: Does anyone know of a lock miter blade set that can be run on a shaper for wood under 1/2" thickness? (insert set for the profile pro block?)

[Pete Staehling]

Don't recall the brand, and they did not come with a setup block like some do now.

That raises another question. I kind of don't get the setup block unless it assumes you mill your stock to exactly match the thickness that the block was made for. Is that the case?


"Isn't especially strong."? On that I will have to disagree.
I'd guess that depends on what the application is and what joints you compare to. My impression was that they broke much more easily than box joints would and using those or dovetails when building boxes or drawers would be a stronger option. At least they seemed way easier to break than tests I did in similar materials with my box joint jig. I didn't do a fair and objective test though, so take that for what it is worth. For the applications you mentioned, I'd think you wouldn't be using those joints any way, so yeah lock miters are probably a lot stronger than plain miters or butted corners. Again just my impression. Has anyone actually done an objective test or have a link to one handy?

[tom lucas]

I have the MLCS set with setup blocks. I used it to join the corners on a QSO hope chest. Once setup using waste test cuts, it worked very nicely. Long, strong joints with sharp corners, and it squared up pretty much automatically. I did mine on a sliding router table, which helped a lot. But, yes, as with most coping you need a waste backer board and slow, accurate feed.

I used the set again recently on a shop cabinet made from cheap 3/4" ply. It did not work so well, but mostly it was the plywood quality, a lot of little pieces of broken tongues. But, it's a shop cabinet. So doesn't matter so much. I won't use the lock miter again, on plywood unless it's high quality material.

Seems to work best on hardwoods.

[Mel Fulks]

The lock mitre joints are used mainly to make newel posts and similar stuff
look like one piece of solid wood. Those who don't use the lock mitre ,usually use something equivalent or weaker.

[Rod Sheridan]

I have a couple different sizes and have used them successfully. I did buy the alignment fixture from infinity, which sits on the bit to help with initial setup (instead of blocks).

On QSWO I found the small cheap one off ebay did not cut that great. All your tips are good ones. A question I have: Does anyone know of a lock miter blade set that can be run on a shaper for wood under 1/2" thickness? (insert set for the profile pro block?)

Hi Carl, if you have a tilting spindle this one would do.

406793

[Mel Fulks]

Saw tear out mentioned. Running birch, maple,etc the cutter can tear of big strips and pieces that you can't even find.
To prevent that "pre condition" the stock by climb cutting a 3/8 round over the inside edge. You can use the fastest
shaper speed for that. When cutting the mitre don't make a sharp point, leave a small flat ,it will disappear in sanding and
slight corner rounding. Do not use climb cut for the mitre cut !

[Carl Beckett]

Hi Carl, if you have a tilting spindle this one would do.

406793

Thanks Rod, my spindle doesnt tilt unfortunately. But that is close... Maybe I will just have a set ground somewhere.

I guess I have had good enough success and use over these joints that I want to continue/improve the capability. Am thinking the shaper might give better results.

Attached is a leg of a QSWO cabinet I did not long ago, looking down from the top. After a little practice the fit was good - hard to see the glue lines in this pic.

[Charles Lent]

The Lock Miter Master setup gauges from Infinity Woodworking will make the setup of these bits easy. You will find that only one test cut needs to be made to assure that the setup is good before doing your work. The manual provided for them even shows how to join two different thicknesses of boards so that either the inner or outer surfaces are perfectly aligned. I have these gauges and they do a great job. But I've never been happy with the tear-out that frequently occurs when cutting these lock miter joints. I've even tried multi pass methods to reduce the tear-out, which helped, but didn't totally solve the problem.

So I've gone back to the old way of cutting the 45 miter joints on my table saw, and then cut a slot in the 45 at 90 to it on both mating board ends. Then I make cross grained splines from the ends of a donor board, using my tenon jig on my table saw. I set the jig so that the distance between the surface of the jig and the saw blade produces a thin spline that perfectly fits the spline cuts that I just made in my work pieces, using the tenon jig on my table saw. I can then flip the board over and make a second spline with the opposite surface of the board clamped against the tenon jig surface. Then I can flip the board end for end and repeat the cuts for two more thin splines. I then use my miter saw set at 90 with a stop block to cut the splines free from the ends of this board. This produces 4 thin splines with the grain running crosswise to their length. The tenon on the board resulting from these cuts becomes the waste and is discarded. If I need more splines, I can repeat the process and create 4 more cross grained splines. When used these splines can be shorter than the miter slot in your work and you can just stack them end to end during glue-up to fill the length of the joint. When your project is assembled and these splines are inserted in the spline cuts of your corner joint with glue, they serve the same purpose as the barb in the canter of the lock miter joint, but with no tear-out, and for me, they are much easier to make. The assembled and glued joint is also very strong and well aligned, with the spline completely invisible the finished joint, invisible except when looking at the top and bottom ends of the joint, but I don't mind the look of it there.

Making a strong miter joint that stays perfectly aligned during glue-up is my goal and I can repeatably get better results when making them this way using only my table saw and tenon jig. For the needed accuracy I use a Wixey angle gauge to set the saw for cutting the 45 degree joint, and a Wixey Digital Caliper to get the tenon jig adjusted to make the splines the thickness needed to fit the spline cuts in my work. A few thousandths under size leaves room for the glue and makes inserting them easy. It should fit much like a biscuit does when assembling using biscuits and glue.

In fact, you can biscuit join mitered corners for good results too. For both the splined miter joint or the biscuited miter joint, it's best to make the spline cuts closer to the inside of the joint, to leave a little extra space toward the outside of the joint. The joint will be a little stronger this way and there will be less chance for cutting the spline slot or biscuit slot too close to the outside surface of the joint.
Charley