Miter jointing plywood? Can I do this?

[Chris Atzinger]

Hi Everyone,

A friend of mine asked if I could help building a TV console similar to the pic below. He wants to use walnut plywood, but miter the corners to get a more seamless look. I am hesitant about miters that long. I'd also have put biscuits in there, but there isn't a lot of material to work with that that point and alignment would be critical. I am also worried that the weight of a TV would want to push the joint apart.

My suggestion would be to cut a 5/8" deep rabbet in the sides and treat like a but joint. There would be a very minor seam in my opinion, but you'd get added strength.

Is this doable? Is there a better way?

Thanks in advance!
Chris


250723

[Rich Riddle]

You could use a lock miter bit such as the one below:

http://www.rockler.com/product.cfm?page=2142&utm_source=Pricegrabber&utm_medium=CSE&utm_campaign=V9129&utm_term=Rockler

Some members discussed it here:

http://www.sawmillcreek.org/showthread.php?12491-Lock-Miter-Bit-amp-Large-Cabinet

Difficult but possible. You would of course have to edge it or build a face frame.

[Jamie Buxton]

You're right, that long miter would be challenging to build in veneered plywood. If you're off only a hair anyplace, the light-colored substrate will show through. On top of that, the knife-edged corner will be difficult to live with. If you ding it just a bit, the substrate shows through.

Suggestions...

1) Build it with miters in solid lumber. That's more feasible construction, and more durable. Furthermore, walnut is one of those species that looks a lot better in lumber than in veneer.

2) Build it with plywood, but edgeband the top, and butt the sides to the underside of the top, kinda like your pic.

[Kelby Van Patten]

If you miter the plywood, you could easily use a spline to add strength.

[Alan Schaffter]

I recently completed a 36" X 36" X 15" deep bookcase using birch ply sides and top and hardwood faceframes- all joined with lock miters and glue. If you will make your piece from ply it will be much stronger with lock miters which have almost twice the gluing surface of a simple miter, are mechanically self-aligning, and won't slip when clamped- a perfect choice for your project.

Once you get the bit height and fence position set on your router table (which can be difficult), lock miters are easy to cut. They always come out 45° for a 90° corner which makes assembly a snap!

http://www.ncwoodworker.net/pp/data/2474/medium/IMG_43691.JPG

Lock miter profile- the one on the left is cut with the board held vertically against the fence, the profile in the board on the right is cut with the board flat on the top of the router table. For assembly, faceframes should receive the profile cut in the walnut piece on the left in this photo.

http://www.ncwoodworker.net/pp/data/2580/medium/IMG_4469.JPG

Close-up of lock miter joint between solid wood faceframe and ply top. Tight enough for you?

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

Close-up of lock miter joint between ply sides (sorry about the dust in the photo):

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

Close up of corner - three lock miter joints!:

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

As I said, lock miter bits can be difficult to set up properly. I know it won't help you now, but to get these good joints I designed and used a new little jig that makes it simple to set lock miters in any thickness stock, even if the boards in the same joint are different thickness. I licensed the design to a manufacturer and it should hit the market sometime this spring.

[Chris Atzinger]

Hi Alan,

Thanks for posting the pics with the lock miter. The finished product looks great. How easily can this be done on a standard router table? My understanding is that keeping the panels perfectly flat is paramount. From the pic provided by my friend, the top panel is maybe 30"-36". I am not sure how easy I could keep that in that flat/in the same plane. I also read in the link from Rich Riddle that you can't really use these bits by hand.

I am open to any suggestion to make this work.

Thanks!
Chris

[George Bokros]

You cannot use lock miter bits by hand, a router table is required. I would do them so the shortest piece is the one done vertically as the fence on the router table is not that high so the shorter the piece done vertically the better.

[Steve Juhasz]

If you just use one layer of 3/4" ply, the weight of the TV will over time sag the upper board and it will be garbage. No matter what joint you use it will fail. Try to laminate up 2 layers of 3/4" and make the whole box 1.5" thick all around, then you have nearly 2 inches of glue area per miter and you will have a sag-proof shelf, without building a torsion box.

[Prashun Patel]

Is there a reason he's married to walnut ply? For one or two sheets, I bet it'd be cheaper to purchase 4/4 walnut. Walnut ply will likely also have less variation than the picture. That piece seems to capitalize on sap/heart wood combination.

I would be concerned about nicks on the corners that would reveal the core.

If you wanted to hide the end grain (almost), you could do a pinned rabbet or pinned lock rabbet joint.

[Ole Anderson]

As long as the feet are near the edge, sag shouldn't be a problem. Pics are what I did to adapt an existing store bought TV cabinet so I could put a recorder and the cable box under it. 10 years ago it got relegated to the basement, which explains the pressure washer behind it. If I remember correctly, I used biscuits for the joinery. I was pretty happy how the add-on was able to match the joinery and finish of the original cabinet. Should have dusted better before I took the pic...

If you go with a lock miter, it is CRITICAL that you practice the bit set up on wood that is the EXACT thickness of the stock that you will be using for the final cuts. I keep a piece of wood with a profile cut on each end to help with the setup the next time. And that piece is marked with the wood thickness, in my case 0.730". So next time I want a lock miter, I mill the wood to 0.730" thickness first. Setting up a lock miter bit is frustrating unless you have instructions, so once you get it set up, make a jig stick to make it easier to set it up next time. Google "lock miter bit setup" for instructions.

[Chris Tsutsui]

Another option is to use a rabbet edge so that the plywood layered edge reveal is small. Then you can use a router to cut away that edge and replace it with solid wood edging. The color might not match perfect but the edges will be more durable.

I've worked with a shaper to make lock miters in boxes using factory laminated plywood. Even if you make a lot of test pieces and everything seems perfect, well when you put all 6 panels together you might end up with a hairline crack on one more edges. TO fix this I would burnish the edge to roll the two veneer edges together.

250758

[Alan Schaffter]

Hi Alan,

Thanks for posting the pics with the lock miter. The finished product looks great. How easily can this be done on a standard router table? My understanding is that keeping the panels perfectly flat is paramount. From the pic provided by my friend, the top panel is maybe 30"-36". I am not sure how easy I could keep that in that flat/in the same plane. I also read in the link from Rich Riddle that you can't really use these bits by hand.

I am open to any suggestion to make this work.

Thanks!
Chris

I used my DIY NYW router table with 4" tall fence (see pic). To help support the boards, keep them tight to the table, fence, and bit, and limit bending, I clamped an accessory fence (a straight board) to the table then the fence to "trap" the boards I was cutting.

http://www.ncwoodworker.net/pp/data/2580/medium/IMG_43322.JPG

http://www.ncwoodworker.net/pp/data/2580/medium/IMG_43312.JPG

Lock miter bit- table use only!!!!

[Alan Schaffter]

If you go with a lock miter, it is CRITICAL that you practice the bit set up on wood that is the EXACT thickness of the stock that you will be using for the final cuts. I keep a piece of wood with a profile cut on each end to help with the setup the next time. And that piece is marked with the wood thickness, in my case 0.730". So next time I want a lock miter, I mill the wood to 0.730" thickness first. Setting up a lock miter bit is frustrating unless you have instructions, so once you get it set up, make a jig stick to make it easier to set it up next time. Google "lock miter bit setup" for instructions.

When I was researching my new jig I found the available instructions were confusing, incomplete, often boiled down to little more than the "trial and error" method, and many were just downright wrong!!! The best instructions I found were on the Lee Valley website.

As you say, most setups require BOTH boards be the exact same thickness as the setup block. The problem with retail setup blocks- they are usually made for 3/4" thick stock which is fine if you are milling your own solid faceframes and cabinet sides. But what if you are using ply sides? How often do you find ply that is 3/4" thick?!?!?! 11/16", 23/23, etc. maybe, but rarely 3/4"!!

My simple, new lock miter setup jig makes it easy to set up a lock miter joint with with stock of the same AND dissimilar thickness. It will also allow you set up a joint that offsets or "shifts" the miter line "around the corner" where it is not as obvious and where you can ease or round-over the corner without sanding through the surface veneer and exposing the plywood sides' interior laminations. To illustrate what I mean, here are drawings and photos of actual joints made with a lock miter bit set using my jig.

(Walnut) faceframe) is thicker than sides:

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

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

(Walnut) faceframe is thinner than sides:

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

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

Offset joint - ease or roundover the corner without exposing ply's interior laminations (1/8" roundover shown in drawing, not yet done in actual joint):

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

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

[John TenEyck]

Of course you can use beveled joints with biscuits. That's exactly what I did on this cabinet, where I had to match the style of an existing piece:

250769

The cabinet is 24" deep and cutting the bevels on the ends of the 7' long top was a bit of a challenge, even on my wide top TS, but the corners fit together beautifully. However, they are very fragile as others have mentioned and if you ding them before you get it glued up they will likely be ruined.

John