Lock Miter bit size choices

[Alan Schaffter]

I never tried lock miters in standard ply- too many voids and inconsistent thickness, but Baltic Birch or other better hardwood ply works just fine. I built a book case from Birch veneer ply with poplar (faceframe) from the borg- it was assembled with lock miters- the sides to top/bottom and even the faceframe was joined to the sides/top/bottom with lock miters- that took some figuring out, because if you don't do it right you can't attach the faceframe (hint: the faceframes should be cut in the vertical position)- see photos below.

The Infinity Lock Miter Master is fantastic setup jig for lock miter bits (all brands). I know the guy who designed it and he's a great guy- he also invented the INCRA I-BOX box joint jig which is another option for making boxes. The Infinity website has a very detailed set of instructions for using the jig to make standard and specialty lock miter joints.

All pieces in this photo were joined with lock miters:

[Mike Cutler]

Charles, there is a way to stop the tear out. Use a 3/8 round over on the edge that will be mitered.

I don't use this bit alot Mel, but thank you for that tip. I'll think I'll try it out today for practice.

[Alan Schaffter]

The only problem rounding over a mitered corner in ply is that you reveal a few of the inner plies.

[Mel Fulks]

The only problem rounding over a mitered corner in ply is that you reveal a few of the inner plies.

The round over I referred to is before the lock mitre cut. I've used it mainly on solid woods like maple and birch which can
often have long pieces as much as 3/4 inch wide torn off by the mitre cutter. I think of it as removing a loose thread before it is pulled. It doesn't change the look of the finished work.

[Rick Potter]

Alan,

Who was it that told you the inventor was a really great guy? His wife, maybe?

[Alan Schaffter]

Alan,

Who was it that told you the inventor was a really great guy? His wife, maybe?

Yup, the person who spends his royalties!

[Alan Schaffter]

The round over I referred to is before the lock mitre cut. I've used it mainly on solid woods like maple and birch which can
often have long pieces as much as 3/4 inch wide torn off by the mitre cutter. I think of it as removing a loose thread before it is pulled. It doesn't change the look of the finished work.

There is another technique described in the Lock Miter Master manual, called the off-set lock miter, that allows you to round over the corner when you have a solid wood panel, like a drawer front or faceframe meeting a piece of ply. It drives the miter "around the corner" so no plies are exposed. There are two sets of special set-up marks spaced in 1/8" increments on the LMM that allow you set up the bit and fence to do this. You can also do a double offset- so that you can use corner, square or rounded inserts of the same or contrasting wood. Of course you can cut the corner rabbet in a finished joint as well. You'll also notice, that contrary the claims of most lock miter bit manufacturers, the LMM allows you to use a standard lock miter bit to make a lock miter with stock of different thicknesses. There are some side effects to doing this, but they are not serious.

[Warren Lake]

I dont see the second one a rabbet will do the same and done it many times then rounded it as it disguises where the line is from the face frame that has been rabetted back there is nothing gained form doing the lock mitre that way that comes to mind first time seeing that. Do you have photos of the first one before you sanded and put the finish on it it looks very good what you have shown in terms of being tight but know that once sanding and finishing can take things up a level or two as well so curious what is off the cutter.

[Alan Schaffter]

I'm not following what you are saying, but with the dual offset setup, you only have one operation, not two. One definite advantage of the offset lock miter illustrated in the first drawing- using a 1/4" offset, you can match up 3/4" solid wood face frames (or drawer fronts) with 1/2" ply sides. I don't have any before shots of the cabinet, but the joints were darn tight- the end result is a function of good milling (bit/fence setup, sharp lock miter bit and steady feed rate) and clamping. With borg ply which has very thin face veneer you don't want to do much sanding to achieve a good corner- you'll expose the next ply.

Though nobody mentioned them, some of the benefits of a lock miter over a plain miter or even a plain miter with spline, is that it requires a single setup/operation, yields a perfect 45°, stays aligned during glue-up and clamping, and has more glue surface for a stronger joint. It is not as strong as a box joint or even possibly a dovetail joint, but it has its place and is just another weapon in your arsenal.