I need some advice on how to rout a locking miter joint on the 24” side of an 83” long 3/4” plywood panel for a built in oven cabinet. I have a router table, but also have a Freud plunge router and could make a jig for off table use. Any wisdom appreciated.
Marc the setup on those bits is pretty tricky. If you download some bit catalogs some of them will have the setup instructions in them. I can't remember if Amana or CMT or both had them in theirs. I remember one of them had a section with instructions for about a half dozen of their trickiest bits to use. One side of the joint gets run past the bit on the flat. The other side gets run through vertically. Even with small pieces that can be tricky but with panels that long it will be much trickier. The flat one you should be okay with but if you wobble up and down even a tiny bit of the vertical one you will probably ruin the mating surface. I've never heard of doing it free hand but that doesn't mean it isn't possible. Once again, the one that gets routed vertically would be the most problematic side.
You don't say what tools you have in your bio but with 50 plus years experience I'm assuming you have a table saw so I would be more inclined to go with grooves and a spline that I would be to try the lock miter on something that big.
I agree with Charles. That's a pretty big chunk of ply for that type of joint. Keeping it vertical will be particularly tricky. And, I hope you're using BB ply, the cheap stuff is unlikely to make a usable, strong lock miter joint. I think a simple rabbit is a better choice, with a face frame to hide the evidence. Also consider pocket screws with glue. Panels that size kind of limit options.
What was said above. I tried a lock miter on a 36" solid wood for a pedestal once and gave up and went to a rebated edge instead. Lock miters work good for drawer fronts and boxes. I never could get the set up quite right until I bought the setting gauge from Infinity Tool Co.
If you insist on using it, you will have to make a high fence for your table and make sure you don't tilt your board as it passes the cutter. Also be sure to use a perfectly flat straight plywood,otherwise you will be tearing out your hair.
The rear panel (you are incorporating a rear panel aren't you?) gives all the strength you need when used in conjunction with rabbeted top and bottom panel joints. The major concern with a tall, heavily loaded cabinet, is wracking; the back panel makes that virtually impossible, even if it's only 1/8" or 1/4" thick. Glue and nail or staple the back.
The fancy joint is difficult to make and impossible to see/appreciate after assembly.
The mitered/splined joint Stick posted by stick is pretty easy to make using a table saw. It is probably a little stronger than a locking miter joint too. I can, unhappily, imagine trying to keep a 24 x 83 vertical or properly aligned on a router table. I can see a simple sled on a table saw producing a strong, attractive splined miter joint.
This string makes the point there are always several ways to accomplish anything.
The INfinity jig is really great, but tiny and has a flair for disappearing in you shop. Been trying to find my set (for different sized bits) for a couple of months now.
I've not had good luck using lock miter bits on any kind of plywood. I've just had too much tear-out of the center wings of the joint, and yes, I have the Lock Miter Master Set up guides. I would use one of the other locking miter joints that have been suggested using the table saw to create them.
For cross grained splines, I use a table saw tenoning jig, and set it so that the spline is the board face and between the jig face and the blade. Doing it this way allows me to make two splines by just flipping the board over to cut the second, and then I can repeat the process on the opposite end of the board for two more splines. I then use my miter saw set to 90 deg with a stop to cut the splines to length (length being in the direction of the grain). If I need more than 4 splines, I just repeat the process to get 4 more splines, etc. For a 24" miter, multiple splines can be used end to end to achieve the 24" I've made many splined miter joints this way, never had a failure, and it's entirely done on the table saw.
Biscuits are another quick way to spline a miter joint easily, but build yourself a jig for the biscuit joiner to get accurate cuts for the biscuit slots.
Here's a video of how to cut the long spline joint on a table saw.
This is a very simple joint to make. Set the blade to 45 degrees, cut the piece, then reset blade height to cut the groove for the spline, 90 degrees to the mitered cut, so no need to reset the angle. I think the video maker overdid the thickness of his spline. He also demonstrates the strength of the joint by standing on it.
End grain joints should be sized first. Note that both splined and, not splined, broke at the joint faces.
Not at all surprised that the MDF did not break at the joint face. Gluing MDF is almost like welding it. The two pieces, essentially become one. It is just sawdust and glue, to begin with.
The other comments are certainly correct saying that such a long joint can be difficult. There was a description of adjusting the router bit in a router table in the March/April 2011 "Fine Woodworking" it uses a height gauge and works perfectly with no test cuts. I have used this technique in 5" high drawer sides and it really works. If you can't find the article I will send you the pertinent info.
When doing cross grained spline joints or biscuit spline joints, I like to locate them a bit more toward the inside of the L joint like a 1/3 - 2/3 split between the faces. Doing this reduces the tendency of the outer surface of the joint becoming weak because of the cuts. I once made a biscuit spline joint using the larger 20 size biscuits in 3/4" thick material, positioning the cuts at about the middle of the material, and when I finished the cuts I discovered that I could see the biscuit slots through the outer surface of the stock. They had almost broke through. Locating the spline or biscuit slots more toward the inside of the joint eliminates this potential problem and makes the joint stronger.
I'm like Charley, I like to locate them as close to the heel of the joint as practical. I also like to size my joints, sometimes with more than 2 applications of glue depending on how porous the end grain is. With only one application the glue gets sucked into the grain and starves the joint for glue. By the way, the absolute best glue I have found for gluing either particle board or mdf is melamine glue. It doesn't get sucked in like the regular glues do.
Thanks to all for the sage advice.
I have come to the conclusion that trying to move an 83” panel horizontally on my router table, even with an extension table, could be a very difficult, if not dangerous proposition. Instead, I will use a rabeted joint reinforced with glued blocks for added strength and stability. And yes I am going to place a back panel into a rabet, glued and stapled.
I can tell you that The March/April 2011 "Fine Woodworking" magazine described using one to position a lock-miter bit in a router table. This is normally a trial and error procedure. Using a height gauge as this article suggests really works. No practice cuts are needed.
However, I have only used it to attach drawer sides to draw fronts and backs about 6” tall. You do need a dial indicator height gage to do this set up. If you can not find the article I can write you the steps. [email protected]
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