lock miter joint in plywood?

[bubba]

Hi all,

I am about to build an enclosure for a wall mounted bookshelf. What I would like to do is build the plywood carcass with mitered corners and a face frame.

The first thing that came to mind for making the miters is a lock miter joint. I have seen some comments to the effect that it won't work in plywood.

This is to be installed in the kitchen, so I want something that will give a good finished appearance.

Have any of you used a lock miter joint on 3/4" plywood?

Any other suggestions for making a strong, neat mitered joint in plywood?

Thanks for your help!

[handi]

The lock miter CAN work in plywood, but the quality of the ply is a big deal.

Cabinet grade ply will give decent joints, poorer quality plywood tends to splinter too much along the thin wedge end.

Pre cutting the waste off the edge before milling the lock miter is a big help in getting better quality joints.

Ralph

[Anthony W]

I would expect you will take a hit on the life expectancy of the bit from the glue in the plywood. But it can be done. I'm not sure if the setup jigs will work for plywood as plywood is not a typical "thickness" on setup gauges. Setup will likely be trial and error.

[Arlin Eastman]

Anthony W said:

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I would expect you will take a hit on the life expectancy of the bit from the glue in the plywood. But it can be done. I'm not sure if the setup jigs will work for plywood as plywood is not a typical "thickness" on setup gauges. Setup will likely be trial and error.

---

I never thought of that. So maybe another bit like a Chamfer Bit to help it?

Arlin

[Robert Adams]

Not sure how strong it would be in plywood but infinity has a lapped miter joint on the cover of their new catalog. Looks interesting and looks similar to a method I saw a guy doing on youtube.

www.infinitytools.com/1_2-Shank-Lapped-Miter-Router-Bit-Set-w_-3_4-Setup-Block/productinfo/55-505/

[Guest]

I've made a bunch of lock miter joints on plywood. It works fine and produces a very strong joint. As others have mentioned, the quality of the plywood has everything to do with the end result. I've used cabinet grade birch ply with good results. BB ply is the best, but it's expensive. One thing to watch for is splintering of the face ply, especially when the cut is perpendicular to the grain direction of the face ply. You can mitigate this problem by scoring the face ply deeply before running the panel through the router. Earlier comments about plywood being hard on the router bit are correct. I like the fact that you generally only need to clamp a lock miter joint in one direction.
My $.02.

Hank

[BillN]

I haven't used a lock miter bit on plywood but have used a drawer lock bit. The ply does tend to tear so I use a Japanese style cutting gauge to score prior to routing. I get a nice crisp cut.

[jteneyck]

I would not bother with a lock miter bit. I did 9 of these tapered porch columns in MDO.





As you can see, the joints are perfectly tight. I cut them using a template on my TS, and used biscuits to keep them aligned during glue up. There's no fooling around required to set up the lock miter bit and you can handle large parts w/o having to do a hair raising balancing act. Here's how my template worked. The part to be cut is held under the template with the screws you see in the end block.



After cutting:



It's a very simple process and the parts are guaranteed to be identical from the same template. If you want variable width parts, just re position
the part on the template.

John

[bubba]

Thanks to everyone for their replies.

I will see if I can find my lock miter bit, and try some test pieces. If I can't find it, I will look into the miter/biscuit method.

[Anji12305]

I do this by hand, with a fenced backsaw and router plane.

If you cut back the groove no more than 1/3 the thickness of the sheet, it's amazingly strong. More so if you can use a resorcinol glue as was used to make the sheets.

I recommend this in Baltic Birch or Apple Ply, as the veneers are uniform in thickness and make control of groove depth simple.

The mechanical aspect of the joint makes gluing up easier, but
it's still clumsy - I use slow setting Titebond glue, as resorcinol glues
require mixing just prior to use and I build alone mostly.