Can you burnish plywood miters?

[FS7]

I thought of another potential solution.  You could tip the blade on your TS to 45 and cut a dado parallel with the joint about 1/2 way in, then glue in a piece of solid wood.  Then you have two choices.  Either bring it flush and square with a flush trim router bit, or cut it at a 45 until it's just proud of the plywood, or leave it completely untrimmed proud on purpose as a design feature.  Any of those options would eliminate the gaps and make the corner a lot more durable.  If you used a dado blade you could make the "spline" more substantial, like you always intended the corner to look that way.  

John

This is an interesting idea. I don't really see how it could be trimmed (bandsaw would have enough clearance) as trying on the router table or the table saw would always have something in the way. If one side were flush, it would be easy to trim the other, but getting that first side doesn't seem very doable. Leaving it entirely untrimmed is an interesting idea though. I don't think that fits with the style of the rest of the piece though. I could see it being slightly recessed (essentially chamfering the corners, though still with a solid wood spline) but then it's starting to get way more complicated than it needs to be.

Even if I end up going the simple route this is a way to learn. Covering the edges with larger trim means that there was really no reason to do a miter joint. I could use the small beech trim (or whatever it is) as it would hide the gap and provide just a bit of contrast. The spline idea is pretty interesting. That's probably the sort of thing I would want to test first though. 

On second thought, a flush trim bit in the router might work. The pillar is about 36" long and I think I need about 31.5" or so. This does not count the base part that will be covered with some sort of baseboard trim, so I could probably get away with 29.5" or so. With my luck though the flush trim bit would dig into the veneer or leave tracks or something.

[jteneyck]

Once you add the new "splines" just rip a piece of 1/2" plywood to fit between them.  Put that down on your tablesaw or router table with the column on top.  Now clamp a right angle fence to the fence on your TS or router table so there's an inch of clearance under it.  Adjust the fence until your cut rips/routs off the spline on that side, repeat 7 more times.  

John

[Cooler]

I would make some filler out of sawdust from the scrap and fill the voids.   The clear finish will almost certainly render the miters as "tight". 

I was surprised at the strength of the glued joints when plywood was mitered.  But then remembered that alternate layers of the plywood would be face grain to face grain, so 3/8" of the thickness is face grain to face grain.  Clearly not as strong as a solid wood miter, but not as weak as I might have thought.

To tighten the miters on my saw, I ran a couple of strips of masking tape on the surface of the saw to "adjust" the angle to make a tight miter.  It was fast and easy.

[FS7]

I would make some filler out of sawdust from the scrap and fill the voids.   The clear finish will almost certainly render the miters as "tight". 

I was surprised at the strength of the glued joints when plywood was mitered.  But then remembered that alternate layers of the plywood would be face grain to face grain, so 3/8" of the thickness is face grain to face grain.  Clearly not as strong as a solid wood miter, but not as weak as I might have thought.

To tighten the miters on my saw, I ran a couple of strips of masking tape on the surface of the saw to "adjust" the angle to make a tight miter.  It was fast and easy.

It's been mentioned in previous posts, but this "plywood" was made from 1/2" MDF (so a full 1/2") and 1/4" nominal plywood (so about 5mm). That means the wood contact area is far less, with most of it being MDF to MDF. It is holding well and won't be stressed in that direction (out), but my previous experience has told me that MDF just isn't very good at being glued. For that reason alone I am considering trim. With the larger trim, I could put screws in to strengthen the joint. With the smaller trim, I could use the brad nailer, but given that they only cover a very small amount that's probably not the best idea. I'd be putting the brads in about 1/8" from the edge, which means that any misalignment and that nail's sticking out right through the face veneer.

Still haven't decided. I can work on the cabinet sides until I do decide. Since it's large enough to require being done in sections that can be done last to give me time. Still, I might decide to do the corner tenon idea. I can probably start with a test cut on the edge (that would be cut off or hidden if I didn't like it) and even so if the cut isn't even, I can still use the trim.

[Cooler]

It's been mentioned in previous posts, but this "plywood" was made from 1/2" MDF (so a full 1/2") and 1/4" nominal plywood (so about 5mm). That means the wood contact area is far less, with most of it being MDF to MDF. It is holding well and won't be stressed in that direction (out), but my previous experience has told me that MDF just isn't very good at being glued. For that reason alone I am considering trim. With the larger trim, I could put screws in to strengthen the joint. With the smaller trim, I could use the brad nailer, but given that they only cover a very small amount that's probably not the best idea. I'd be putting the brads in about 1/8" from the edge, which means that any misalignment and that nail's sticking out right through the face veneer.

Still haven't decided. I can work on the cabinet sides until I do decide. Since it's large enough to require being done in sections that can be done last to give me time. Still, I might decide to do the corner tenon idea. I can probably start with a test cut on the edge (that would be cut off or hidden if I didn't like it) and even so if the cut isn't even, I can still use the trim.

It is not that MDF is bad at being glued, it is that it absorbs a huge amount of glue.   I apply a thin layer to both surfaces and then a second layer of glue (not letting any of it dry) and then assemble. 

I assemble by taping the miters together prior to gluing, so assembly is simply folding the pieces together into a rectangle and then taping the final corner. 

I find if I don't pre-coat with glue that so much glue gets absorbed into the MDF that it yields a joint that is glue-starved.  Too much glue will absorb into the MDF and could cause swelling.  So I go with a light coat which barely skins over and then another coat and then assembly.