Lock-mitre Joint On 3/4" Plywood?

[Howard Rosenberg]

Hi all - The garage is finally starting to warm up a little here in Toronto.
I'll be building an entry hall bench in the style of a Tansu chest out of furniture-grade 3/4" plywood for the carcase.
Don't want the plys showing - as in butt joints - would a locking-mitre joint work in plywood?
On something that's going to get a fair bit of use? With my two boys, Genghis and Atilla?
Can one set up the bit so precisely there's NO gap between mating sides?
Thank you. Howard

[George Summers]

Saw a tip somewhere recently about this situation. Suggestion was to use standard mitered joint and then after the joint was together to cut a 3/8"-½" rabbit down the corner and inlay a strip of matching hardwood. Trim the inlay just like you would edge banding. It will take more abuse than a plywood corner no matter how it is constructed. Plywood would just feather too much and the first bump would show the interior plys. You can even give the corner a slight roundover or some other treatment if you wnat.

George

[Justin Florentine]

Anyone else try this in the last 10 years? I'm planning a foosball table and am thinking of doing the carcass with mitre-lock joints in 3/4" oak plywood. I wanna just try it but not if it costs me $40 for a bit.

[Bradley Gray]

+1 on the standard mitre/solid wood in rabbet/roundover.

[Nelson Howe]

I made drawers in half in ply using the lock miter and the infinity setup jig. It worked and it would work, I'd say. But I agree it might not hold up to Attila.

Nelson

[Lee Schierer]

The problem with doing lock miters is that one corner is cut with the work piece horizontal and the other piece is cut in the vertical. Pieces made from plywood are generally large pieces so holding one in the vertical position may be problematic from a material handling point of view. Also, depending upon the quality of the plywood any bowing or warping will tend to misalign the cuts. I'm not sure how the 1/32" thick face veneer is going to perform when making the cuts.

[Phil Thien]

If you're going to use a solid wood corner in plywood, then just rebate one piece of the plywood so when the corner is joined there is a 3/8" square recess into which you can install your solid piece. This is how speaker enclosures that had solid wood corners have been produced.

[Justin Florentine]

Sounds like not much has changed. I'll look into the speaker joining technique and try to scale it up. Thanks for the input all.

[Chris Padilla]

Your plywood will blow out when cutting it in the vertical position. Trust me, I know this. :)

[Alan Schaffter]

In addition to solid wood-solid wood joints, lock miters work quite well in ply-ply and ply-solid wood case work construction. You need to feed the ply more slowly. The advantages of the lock miter joint over a standard miter is that there is more glue surface and the joint stays aligned during clamping. But, as you know, lock miter bits should only be used with a router table and there are two main issues using a lock miter bit. The first that has confounded users for years is the set-up which must be done precisely and correctly, which can be a pain. The second, as someone mentioned above, is how big of pieces of ply you will be using and can you adequately support them in the vertical position on your router table. Also, there are considerations if using a solid wood-to-ply joint- you will want to make sure the solid wood is milled to the same thickness as the (typically non-standard) ply unless you are able to adjust your setup to allow for the mis-match.

That being said, many of you know I designed a patent pending, little setup jig, the Lock Miter Master, that is being sold by Infinity Cutting Tools. It makes setup (of any brand of lock miter bit) a breeze, whether you are using a standard lock miter joint, a lock miter joint in stock of un-equal thickness, or an off-set lock miter joint. The Lock Miter Master online manual (http://www.infinitytools.com/PDF/Lock%20Miter%20Master%20Manual.pdf) shows these variations.

http://www.infinitytools.com/images/00-LMM_Add_1.jpg

I built a 4' X 4' X 14" shelf/cubby system for my grandson (little Hulk :D ) using a ply box with a hardwood faceframe all assembled and attached with lock miters. Unless you look really closely at the grain of the (big box store) birch ply and soft maple faceframe, you can't tell they are separate pieces. Sorry about the dust in the photos. FYI, for years, I was afraid of this bit. This was my first attempt using it. I actually developed the Lock Miter Master specifically for this project and used an early prototype to set bit height and fence position. I was so pleased with the results, I decided to see if I could license it- the rest is history.

Front corner where ply top and sides meet maple fraceframe (all lock miter joints!):

http://www.ncwoodworker.net/pp/data/2580/medium/IMG_4439.JPG

Ply top and maple faceframe lock miter joint:

http://www.ncwoodworker.net/pp/data/2580/medium/IMG_4435.JPG

Ply top and ply side lock miter joint:

http://www.ncwoodworker.net/pp/data/2580/medium/IMG_4434.JPG

[Mel Fulks]

Like Chris I've had big pieces blow out. Was making MDO exterior box columns. Didnt really hurt anything as I added glue
blocks to inside of of the horizontal run pieces ,holding back almost 1/8 from corner and using yellow glue and staples.
When assembling used yellow glue on the shaper cuts and a bead of construction adhesive on the glue block. Messy.

[Mel Fulks]

For those who can afford them ,the larger diameter cutters work much better, especially the ones with insert tooling.

[Phil Thien]

Alan brings up a good point: While the size of the panels and thickness (or lack thereof) of the veneers complicates matters, it is the case that lock miter joints have been used in industry, for large products, for decades. JBL and Altec and others used them fairly extensively on "large" panels for speaker enclosures.

Alan's jig seems like it would be handy, what they did at the enclosure facilities for JBL/Altec/EV/etc., was to draw a fine line on the bits using a small straight edge, and this was used to setup the bits. Nowhere nearly as handy as Alan's method, but still pretty decent.

Also these outfits left those large shaper bits in the shaper for a long time, they were only removed for sharpening. If they were changing them quite often (as we do router bits) they would have appreciated one of Alan's jigs, that is for sure.

BUT even in the case of JBL and Altec, they still had assembly people that were very good at burnishing the corners. Two guys would clamp-up an enclosure, then another guy would make sure all the joints were closed using tools that often looked like steel rolling pins. This step had to be done before the glue set, as once the glue was set you would not be able to close any gaps.

Some outfits are still using lock miters on some cabinets, I hear, but I imagine they're doing it with fancy CNC gear.

[Ralph Okonieski]

I've used the lock mitre with 1/2 inch baltic birch ply for drawers all the time. There is no gap once setup properly. I cut the ply a little wider to allow for the blow out and then trim to size.

[Vince Shriver]

Alan, Your miter do-dah is brilliant. The results shown on the Infinity website speak for themselves. Well done.

[Scott Stafford]

Good tip on cutting the ply to desired size after milling the joint!

Scott in Montana

[Alan Schaffter]

Alan, Your miter do-dah is brilliant. The results shown on the Infinity website speak for themselves. Well done.

Thanks!

What pleased me almost as much as when the initial idea worked, was that during testing I discovered it would easily allow you to do two variations on the standard lock miter joint- (a) lock miters between stock of different thicknesses, a situation that might occur if you have a case made with 3/4" faceframes and thinner ply sides (or visa versa), and (b) a case where you might intentionally want to shift the miter line "around the corner" leaving you with a "hard" corner or so you can round over corner without exposing inner laminations of the ply.

http://www.ncwoodworker.net/pp/data/2580/medium/Thicker_FF.jpg http://www.ncwoodworker.net/pp/data/2580/medium/IMG_4473.JPG


http://www.ncwoodworker.net/pp/data/2580/medium/Thinner_FF.jpg http://www.ncwoodworker.net/pp/data/2580/medium/IMG_4472.JPG

http://www.ncwoodworker.net/pp/data/2580/medium/Offset.jpg http://www.ncwoodworker.net/pp/data/2580/medium/IMG_4470.JPG

[Chris Parks]

I have a question for Alan. Does he know how the original developers of this type of router bit intended for them to be set up at the correct height. Surely the developer of the bit must have given some thought to how it would be used and how it would be best set up. To not do so seems totally stupid in the extreme. I have developed stuff in a different hobby and the first thing that was paramount in my mind was how user friendly and easy to use it was.

[Alan Schaffter]

I have a question for Alan. Does he know how the original developers of this type of router bit intended for them to be set up at the correct height. Surely the developer of the bit must have given some thought to how it would be used and how it would be best set up. To not do so seems totally stupid in the extreme. I have developed stuff in a different hobby and the first thing that was paramount in my mind was how user friendly and easy to use it was.

I am not clairvoyant, so can't say what the original developer(s) intended, but you might think, if he/they had given any decent amount of thought to how it would be used, and how best to set it, he/they would have developed an easy method to do so.

A review of the instructions supplied by various manufacturers and vendors (including Whiteside, Lee Valley, Freud, CMT, Grizzly, etc.) clearly illustrates this was not done. Instructions differ greatly- many devolve into nothing more than trial and error, while some are just flat out wrong! Case in point- I've seen a number of instructions (like these from Whiteside (http://www.whitesiderouterbits.com/LockMiterInstructions.pdf)) and WW magazine articles, that include instructions and drawings on how to correct a setup error- they recommend moving the fence in/out OR raising or lowering the bit only, not both. If you understand anything at all about the geometry of the lock miter bit and router table setup, you know that adjusting only one setting will result in an error to the other setting when you attempt to cut a joint in a fresh set of boards (A & B)- you end up chasing the setup! They are inter-dependent! Making an adjustment to one setting only works if you have already cut the profile in ALL (A) boards.

If you read WW forums, setup has been a big problem with this bit for years. The Lock Miter Master's main purpose is to give average woodworkers a near fool-proof method to set a lock miter bit correctly, the first time. The fact that it allows you to make other useful settings is just a bonus. To limit a tool to its primary intended purpose and only one setting, when others are safely within its capabilities seems "totally stupid in the extreme." It would be like saying a tablesaw, which was originally designed to rip, should not be used to make cross cuts, bevels, dadoes, coves, mouldings, rabbets, should not be used with miter a gauge, and never used with tenoning, box joint, or other jigs.

[Chris Parks]

Thanks Alan, you have just confirmed my suspicions.