Mitering corners

All sorts of fixes if the miters didn't have to fit around the plywood - but all of them shorten the edging so it doesn't fit any more.

If your jig is square and/or you carefully mark mating pieces for cutting on opposite sides you'll come out pretty well, but I see one possible problem. I can only see one clamp on the piece. If there isn't another one out of sight, it's possible for the piece to rotate a tiny amount. Just enough to give the kind of result in your example. Use two clamps and make sure they're tight.

I'd cut a slot in frame and plywood and use a spline.

One trick I've seen on the corners is to cut one or two shallow (1/16" or so) plugs and cut a recess in the corners to hold them. Put the grain of the plug(s) across the joint.

Did you make your jig so you can make micro-adjustments? It looks to be just a touch off.

To secure the table top, you might try some of these bad boys. There are variations.

I'm sorry. I just re-read your post and I thought you were talking about something else. Please disregard.

While you have added another factor with the center section, I use a slight different technique to get good corners.

I make a lot of picture frames and stretchers for my wife, There are two factors to get good joints, the angle and the length. I use a fixed 45 miter gauge on my table saw. I then cut the sides about a quarter inch proud of the length of the final frame size. The using staples I fasten the oposite sides together, and slowly trim the sides to the correct length with multiple cuts. Once cut, I remove the staples and mark the bottom to identify the paired sides. I have also marked them on the inside of the frame.

I use a router table and slot cutter to cut the biscuit slots and glue them together with wrap around clamps (bars screwed in to corner blocks) I continue to check every thing with a good triangle square.

With the top as you have designed the length of the sides must be perfect. If I were doing it I would cut two scrape pieces of the side material, clamp them to the table around the center piece, Then trim one set of pair sides until they were the perfect length. Once I had them, I would clamp them to the table around the center piece and cut the other pair side to perfectly fit between the other two sides.

I need not tell you that to get the corners you have, will only require a couple of thousands of an inch trim of the paired sides.

I have found that the marks made by the staples are so thin that by the time you have completed the finishing they nearly disappear. Of course that will depend on the staples you use. I use Arrow 1/4" T50 Type Staples Box of 1250 or the slightly larger 3/8"

One more thing occurs to me about your miters. If you jig is a true 90 degrees, you should be able to cut a miter *from the other side* and so that they match perfectly, even if the 45 isn't perfect. Just a thought.

This is what I use, there are other manufactures, and while it takes a little more time to set them up, I prefer the type that can not be slide.

With this type of clamp you can pull the side together a little better that you can with your jig.

With above type of clamps you do not need an elaborate jig. With your jig or with the I indicated above, the primary determinate in the squareness with be the precision the center piece as the frame can not be any more square the the center section.

This is my third post on your corner. I looked at your corner again as I was closing the browser, and had another thought.

As I said in my first post you are within a couple of thousandth of and inch on the dimension of getting the gap closed.

I have you thought about using placing a strip of sandpaper on a long board and removing a thousandth of an inch from the center piece?

I think this may have been a source of error. I may have confused "Up" with "Down", resulting in two "left" cuts being mated rather than a left and right.

you'll come out pretty well, but I see one possible

I don't think that was the problem. I also held the piece against the jig with my hand, further from the blade. I considered adding two more clamps, and I may still do it, but I doubt that the pieces moved.

I could cut the slot OK (router table), but I don't think my tool complement is well-suited to making a long spline.

Hmmm. I'm having trouble visualizing this. Do you have a photo?

Assuming you have a hand plane ... Make a shooting board for the miter. After cutting the coarse miters with your chop saw (leave the boards a bit long) final trim to size &fit on the shooting board. If you start with boards that are wider than needed you can trim the inside edge to make the board "longer" should you over trim a miter. Art

Step #1: determine the cause

1. joints not cut at 45 degrees

1. assembly not square

2. both

There aren't grooves in the frame for the panel? Apparently not, judging from what you say. If not, I would make them and a tongue on the panel; or splines. Or matching rabbets on both frame & panel. Once done, glue, no fasteners.

Rip them out of a piece of hardboard or 1/4" ply. Accuracy in width is not required as long as it is less than the combined depth of the slots. They don't have to be long, either...multiple short pieces work just fine.

There's an ugly example on this page: