Large Furniture grade miters?

[jgourlay]

For your purposes, there are two key concerns: Seasonal movement, and the finished miter angles. First, miters.

Sometimes the jig?/Fixture? gets more elaborate than the cut it is designed to assist in producing. A fancy hand gauge is not necessary. You need two of those aids: the electric saw you depend upon now, and the finish miter gauge for hand shearing. I assume you set the miter with the 4 or so cuts to check final square. (Three corners butted will reveal the truth of the angle on the fourth corner.) No. 4 corner fits flush with no gap, or it's gappy and the basic 45 angle is off. The 'trued' machine mitered scraps can then be used to true the hand gauge. Because, somehow the saw will miss the angle. Maybe, a chip or minute piece of sawdust got in the way. Probably, the saw or blade flexed. Hand fitting is more accurate and fixes the minute errors.

You true the hand gauge in the same manner, by registering the 2 gauge blocks to a machinist's square. Lock those gauge blocks to a level surface that will hold Precious Piece comfortably. USE SUPER GLUE to attach the gauge to that surface. (A Seattle specialty trim contractor does this for his super rich clients.) This procedure needs to be done with a fancy gauge too. You have a bunch of those 45-angle scraps so register shaving down the mitering blocks using a hand plane of #4 minimum mass with the sharpest squarest blade alignment you can make. You are truing the gauge blocks that might have slipped after gluing and expose light between the wood and metal square.  The hand gauge, which has two blocks of wood at right angles, is trued.

Take your scraps from the machine saw and shave gossamer pieces, or dust, from the miters on the miter gauge. Check the angle fit. Is the fourth corner tight or gappy? When tight, you are ready to cut Precious Pieces. 

When everything is done, pop the gauge wood off the table. It has served its purpose. Next time it may be a nonogon table you build.

Personally, I avoid a perfect corner termination. A flush miter fit is stressful enough. And, you still need to fit the interior field inside Precious Pieces.....

Seasonal movement of the wood in the table top will blow out those miters, unless you build to accommodate wood movement. That means, technically, every edge needs to slip and slide with a gap between the field wood and the mitered frame. Tongue and groove time. Give yourself working margins. Give the T&G joints a 1/16th, or so, reveal. If the field is plywood still do the safety joint. Wood is wood no matter what form it is in. 

Gads!! This went too detailed, too far; and, everything else. Really, you could avoid all this fuss with wood putty.

That's a great response, thanks!  The board (field), is already a stand-alone piece - made from 2 1" thick 24x24" pieces glued together and then veneered on 5 of the 6 sides.  What you're telling me is I need some form of gap to allow the frame to shrink without pinching on the board. Hmmm....

My 'lazy' idea was to sink the board 1/2" below the frame and use a moulding.  Then put a glass top on it!  Maybe that is less lazy than smart.

The frame:  thanks for the tutorial!  I was also planning to route "mortises" into the thickness and use a same species floating tenon to tie those corners together.  Which was gonna be a different thread.

[jgourlay]

If you're making miters with  7" wide boards cut at 45 degrees each to make a 90 degree joint, I would expect over time the miter to open up on the shortest [inside] corner no matter how carefully the miter is cut. That is a pretty wide miter joint to expect no seasonal movement imo.

Would mitered half-laps be better all around?  Simpler too?

[srv52761]

That's a great response, thanks!  The board (field), is already a stand-alone piece - made from 2 1" thick 24x24" pieces glued together and then veneered on 5 of the 6 sides.  What you're telling me is I need some form of gap to allow the frame to shrink without pinching on the board. Hmmm....

No, I think what he is saying is that the wood of the  frame will move and destroy the joint.  I don't know how much a 7" board will move,  the widest stock I have used for a frame is about 3".
But if it wants to move,  there is nothing you can do to stop it,  you need to accommodate for it in the design phase.  A miter may not be your best joint.  Or, maybe veneer over a stable substrate.

[hbmcc]

Sorry, I was not clear. Too many things going on in my response.

You have the mitered surround, and the interior body of the top. Both are the top but distinct parts that move differently from each other. The mitered surround, by itself, will shrink and swell without cracking the joints, like a bellows. everything in that part moves uniformly. The interior body piece moves differently so must be separated from the surround by sliding/slip connections to allow each independent movement. If you want to prevent the mitered edge surround (Precious wood) from separating at the joints when seasonal movement shuffles the dimensions, there must be a sliding connection and a gap to accommodate the differing expansions and contractions of the two parts. 

How you connect the two parts needs to be included in the design. It sounds like the field piece is a fixed dimension. So: Groove all four of its edges, and all four of the surround's interior edges, and use slip strips (tenons) in the full length mortises, because you don't want blue sky showing through the expansion gaps if you try stopped mortises. Be sure to make the strips a tight fit and allow for the gaps when you cut the surround pieces. 

The straight miter is a fairly weak joint. If you have sufficient wood length, consider a lapped miter; it's sort of a hybrid half full lap on the bottom and mitered lap on top.

Edit, addition: Two laminations plus one veneer. .... This might do a lot of warping. Normal is a minimum of 3 laminations with the two faces aligned. A pretty face veneer will have a corresponding back veneer of cheap wood.

[srv52761]

Sorry, I was not clear. Too many things going on in my response.

You have the mitered surround, and the interior body of the top. Both are the top but distinct parts that move differently from each other. The mitered surround, by itself, will shrink and swell without cracking the joints, like a bellows. everything in that part moves uniformly. ... 

Now I am the one confused.   
 The mitered frame is made of solid wood,  and will expand and contract with changes in humidity.  But the bulk of the change will be across the grain.  The frame's grain will be going in perpendicular directions  at the corners.  I don't know  how much 7" width of that species will move (14" if we count  both sides) so I don't know if it will be an issue.  But if it does move  it seems as the sides swell and make the frame wider, the top and bottom pieces will stay the same width and try to make the frame taller.

Is this not right?

[Cooler]

I would cut the 45 degree cuts on my miter saw.  I would then glue the joints together using hot glue and an acccurate square to assure that the finished product is 90 degrees.  Then I would put the mitered joint back in the miter saw and cut through the joined line.  

The joint will then be a perfect match and will be 90 degrees. 

Repeat for all four joints, making sure to mark the pieces so you know how to put it back together.  

Then glue up as usual.

[danceswithticks]

Would mitered half-laps be better all around?  Simpler too?

Mitered half laps may help to some degree, but I think it still will open up because of its width. I base this on some of the trim work in  our house. Previous owner had used wide trim pieces around  a newer sliding door he had installed,
 mitered at the corners. When we moved in, the miters were tight. After just a couple of years, the inside corners started opening up and have gotten wider and wider over time. I think that's why wide doorway trim pieces are typically butted up with the horizontal piece "resting" on top of the two verticals. Same applies to crown molding in corners. Both pieces are not cut at a compound 45 degrees but one piece overlaps the other and is coped to fit. The inevitable wood movement in wide miters is hard to account for. Miters in narrow trim is not so bad.  Perhaps veneer would be a solution instead of solid wood?  Just my 2 cents.

[hbmcc]

Now I am the one confused.   
SNIP

Is this not right?

You and me both...... I forgot your OP 7 inches dimension. Yes, the longest dimension (outside) will stretch significantly more than the short side at whatever the longitudinal expansion property is for your wood. It's not much, but the inner corners of the miter joint will separate. 

My parting thought is to do bridal joints. Expansion will be more consistent in that joint. But, then you may not have a means to cut out a 7-inch mortise. You only need to contend with inside perpendicular cuts--no wonky corner points. Third option is breadboard ends, with shorter tenons, and mortises; but breadboard connections aren't very compatible with most home shop machines.

[srv52761]

Ultimately , I think with 7 inch widths,  mitered joints may not be the best solution.  

A  bench I made had 6" aprons and used mortises and tenons. The plan called for double tenons at each joint to mitigate the effects of expansion.

[Alan S]

The length of the solid wood will not change enough to cause any problems, but the width will.  Since the length of the mitered end stays constant and its width varies, that causes an angle change as it dries out (angle decreases and tends to open inside corners) or takes on moisture (angle increases and tends to open outside corners).  The mitered corners will stay closed only if they are strong enough to bend the wood to the angle change.  The wider the mitered pieces, the larger the angle change on expansion and contraction, and the stiffer the wood, making it hard to bend.  At 7" wide, you are getting to difficult territory.  That's why people are suggesting very strong joints that might be enough.

The separate problem of dealing with a panel that expands and contracts across its width (but not length) within a frame that does not significantly vary, is a very traditional one that usually involves a floating panel.  Or it can be sidestepped with a dimensionally stable plywood panel.