Large Furniture grade miters?

[Hank Knight]

When they absolutely gotta be perfect I use a hand plane and shooting board. 
...
John

What John said.

There are two kinds of shooting boards for miters:

This one is for horizontal or flat miter joints:

Untitled by Hank Knight, on Flickr

This one is for vertical miter joints:

IMG_0508 by Hank Knight, on Flickr
Disclaimer: This is not my shooting board. It is an image I found on the internet. I used it to model my own vertical miter board, of which I do not have a photo.

With a shooting board, you can sneak up on perfect at .001" per cut. Hard to do that with any kind of power saw, regardless how well tuned it is.

[Rob Young]

When they absolutely gotta be perfect I use a hand plane and shooting board.  I don't have a special shooting board miter plane.  I use a Bailey's #6 that I got ground flat on the bottom and with the sides ground to 90 deg from Tablesaw Tom.  I made a shooting board after something I saw in FWW or someplace.  All that's important is that the angle is perzactly 45 deg.  Besides that, the most important thing is that the plane iron be really, really sharp and set for whisker thick shavings.  

I cut the ends maybe 1/32" too long and then finish on the shooting board.  The guys in the hand tools forum can school you far better than I, but I can tell you that it works well and isn't hard.  

John

This ^^^^

Cut them an RCH proud of your layout lines with a chop saw or whatever; then shoot the miters to the lines.

Even more of this ^^^^^^^

Cut, fit, adjust, repeat.

to add, my shooting board has a removeable fence that I can pivot and then clamp the other end. I use a good 45* square and set it then do a few test cuts and check them against my Starret. Easier to cut two pieces and make a 90* then check that. It will magnify the error x2 (x4 if you go all out and make a 4-sided frame as a test). I either tap the fence open or closed as needed or shim the face with blue tape.

[jasfrank]

I wouldn’t say that its that hard to tune a power tool to be as accurate as a hand plane. For instance look at how formica countertops were made with 45 degree angles. No hand tools involved and they always seemed to be right on. Its true they didnt sneak up on a cut but rather got it right the first time. Theres advantages both ways and a lot depends on what tools th OP has to work with and how much time he wants to put into it.

[jgourlay]

If you are saying you do not have a mitre saw that is accurate enough, or you do not wish to invest your time into making it so, there's other options. Tablesaw as mentioned with a homemade miter jig, or cut your pieces slightly long and clamp a pre- checked 45 degree mitre square on and use your router to trim them to exact. You could possibly stack the boards for this, depending on what length bit you had. Whatever method you use, try it out with some more pine or plywood first. Also, you need to have gun barrel straight edges on your stock to get decent results.

After reading the bulk of the comments, I'm leaning toward the tablesaw.  I was trying to avoid this as I have an incredibly tight shop, and no room to store even one more jig that size.  But for this project, it's probably worth it.

Actually, I would like to make my miter saw more (sufficiently) accurate.  That lack of accuracy has been an annoyance to me for a decade now, but not critical until this project.  It's a top end saw, but never seems to 'return' accurately.  I can set it at zero, then set it to 45, then back to zero.  When I set it back to Zero, it's a few tenth's of a degree off.  Which means that a key functionality is missing and I've relegated it to only the 'coarse' cut.  Noted another posters pointing out of blade deflection--which is an issue for me.

[jgourlay]

Even more of this ^^^^^^^

Cut, fit, adjust, repeat.

to add, my shooting board has a removeable fence that I can pivot and then clamp the other end.  I use a good 45* square and set it then do a few test cuts and check them against my Starret.  Easier to cut two pieces and make a 90* then check that.  It will magnify the error x2 (x4 if you go all out and make a 4-sided frame as a test). I either tap the fence open or closed as needed or shim the face with blue tape.

Shooting board was my first choice here as well.  I've got a triangle I use for my current shooting board, and it's good enough for the framed lid of small boxes.

This piece is going to consist of ~7" wide, 5/4" thick curly paduak.  My experience with 4/4", 3" wide bubinga and other similar woods makes me chary of the shooting board as I'm not sure I have the personal ability to "dump the clutch" sufficiently to run the plane through the whole board smoothly.  With cocobolo I've run into a number of circumstances on 5/4" pieces where a blade setting light enough to let me run the plane through the width in a controlled manner only removes a 'powder' layer and not a true shaving.  I think in this case I need a power tool.

If this were, for example, a 4" wide piece of 4/4" walnut, I'd hit that on the shooting board without hesitation.

[jgourlay]

J,
How much room do you have on this one miraculous board?
Are you looking to create a continuous grain frame?
Are you crosscutting the parts, then cutting your miters?
Or, are you cutting the miter from the whole board?

I've got four boards, each 70" long.  So, one side per board and then boxes for all the nieces later.  I do plan on crosscutting 1" over, joint, plane, scrape/plane (#4 HAF with close set chip breaker first, then scrape if needed).  Then cut the miters, shellac washcoat (twice) to cut off bleeding, then glue with hot melt.

After hot melt gluing the frame, then I set the chess board on top and scribe with a very sharp knife the side of the chess board.  The chess board LOOKS square from a distance, but it is not.  Handplane down to the line for each side of the inside.  Glue with Titebond dark wood glue.  Then drop in the chess board.  Then agonize over whether the board should sit flush with the frame, or sit 1/4 - 1/2" below the frame.

I bought all four of these boards at the same time, supposedly from the same log, about 15 years ago from Gilmerwood.  Haven't seen anything like them since, so this has to be right first time!

[jgourlay]

What John said.

There are two kinds of shooting boards for miters:

This one is for horizontal or flat miter joints:

Untitled by Hank Knight, on Flickr

This one is for vertical miter joints:

IMG_0508 by Hank Knight, on Flickr
Disclaimer: This is not my shooting board. It is an image I found on the internet. I used it to model my own vertical miter board, of which I do not have a photo.

With a shooting board, you can sneak up on perfect at .001" per cut. Hard to do that with any kind of power saw, regardless how well tuned it is.

You are making me feel more keenly the need to make a new shooting board!  I have for a few years been thinking about one where the track the plane slides in is tilted. I would loose some with of cut, but I never use full width now, and it would give a slicing action that may improve performance with the cement-hard exotics I like to work.

Heck it would help with with Home Depot fodder as well.  Cherry/Walnut/Maple do beautifully on my current one.  Southern Yellow Pine tears the heck of my blades after just a few strokes.

[Rob Young]

Shooting board was my first choice here as well.  I've got a triangle I use for my current shooting board, and it's good enough for the framed lid of small boxes.

This piece is going to consist of ~7" wide, 5/4" thick curly paduak.  My experience with 4/4", 3" wide bubinga and other similar woods makes me chary of the shooting board as I'm not sure I have the personal ability to "dump the clutch" sufficiently to run the plane through the whole board smoothly.  With cocobolo I've run into a number of circumstances on 5/4" pieces where a blade setting light enough to let me run the plane through the width in a controlled manner only removes a 'powder' layer and not a true shaving.  I think in this case I need a power tool.

If this were, for example, a 4" wide piece of 4/4" walnut, I'd hit that on the shooting board without hesitation.

LeeValley used to sell (maybe still does) a "sanding shooter".  Essentially, substitutes a fancy sanding block for the shooting plane.  Easy enough to fabricate something and use self-stick paper.  Probably slower than a plane but might work as well.

In general the heavier the plane, the easier it is to run through a big miter. A #8 should to the trick if you don't have a #9 or #51 handy...

[hbmcc]

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.

[danceswithticks]

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.