The Corners Don't Fit

[edflorence]

I should add that the Miter Pro will work for Dusty's example, but not for Bear's. For boxes or frames deeper than the capacity of the blade, we are back to cross-cut bevels.

[algale]

I have a problem building boxes]24135[/ATTACH]

My tiny .2 degree error accumulated to 1.2 degrees at the final joint. Mathematically, it is obvious but the question remains, "What can be done about it"?

Equally obvious - cut 45 degree miters. Treat that tiny .2 degree error as a big thing.

Get an Incra V120 Miter Gauge or learn to micro-adjust the Table Tilt.

Wonder how many thousandths of an inch that 0.2 degree error amounts to per cut.

[BuckeyeDennis]

Wonder how many thousandths of an inch that 0.2 degree error amounts to per cut.

Assuming 3/4" thick wood, and that the error is such that the corner would be acute if glued flush, it works out to a 0.0074" gap at the inner corners, if distributed evenly across all corners (i.e. a true 90 degree glue-up).

If glued one corner (flush) at a time in series, Dusty's example would generate an interference of 0.118" at the final corner.

Or if glued up into two "halves" first, a gap of 0.015" at the inner corners, when the two halves are joined.

And if my trig ain't correct, I plead guilty of mental fatigue after a long day, and of using an iPad to attempt engineering work with my behind planted comfortably on a sofa.

What does Sketchup say?

[shipwright]

A couple of solutions:
I use hot hide glue and assemble all four corners at once with masking tape, no clamps necessary. A fraction of a degree here or there is lost and they look perfect and are extremely strong.

If you really want perfection on the 45 degrees, use a vee grooving router bit.
It's easiest if you rough cut the angle first but you don't have to.
http://www.woodcraft.com/Product/208155 ... egree.aspx

[Mike907]

How about one of these:http://www.highlandwoodworking.com/mitertrimmer.aspx

[algale]

Also an old craftsman taught me a secret of the miter cut. Glue them together if there is a gap on the outside corner run the shaft of a screw driver over the corner. The wood fibers will bend and close the gap. No filler no noticeable difference when staining or finishing.

Ed, I learned this same technique thirty years ago when I was interning in an art museum in college and it really does work. We built a lot of frames every day. The miters didn't always come out perfectly. The frames were a lot of gold leaf and silver leaf that would have been impossible to touch up afterward with filler, etc., and this solved that problem. I'll add that we didn't just use this technique when the miters were slightly off; we used it even when the miters were perfect because it made the frame so much more pleasant to handle. Use a long, stout screwdriver.

We also built a lot of those metal frames that were popular in the 80s. On those, we'd just file over the corner.

[algale]

How about one of these:http://www.highlandwoodworking.com/mitertrimmer.aspx

We used this in the art museum I described working at in my post, above. Once it is set up and dialed in, it will make perfect 45 degree cuts and you can cut rice paper thin slices off the hardest stock if the blades are sharp. But it still is hard to get the pieces the exact same length, which I think is a bigger problem than getting a perfect 45 degree cut.

On the Shopsmith, I've used the trick of setting up the rip fence with a short block clamped to the front edge, to control the length and ensure duplicate length cuts are made. If that doesn't work, I've used the rip fence, sanding disc and quill depth stop trick shown in PTWFE to touch up the pieces. I think it is called sanding to exact length.

[bffulgham]

This works great for me on picture frames.

I set up a stop on the right side to insure I get the same length for opposing sides.

Granted, this will only work to create a shallow box.....

To do a beveled cross-cut to make a box with taller sides, I move to the bandsaw and tilt the table.
and spend a LOT of time fussing with getting 45-degree tilt exact.

Or forget mitered corners totally and use my router and dovetail jig to do half-blind or full dovetails

[Ed in Tampa]

We used this in the art museum I described working at in my post, above. Once it is set up and dialed in, it will make perfect 45 degree cuts and you can cut rice paper thin slices off the hardest stock if the blades are sharp. But it still is hard to get the pieces the exact same length, which I think is a bigger problem than getting a perfect 45 degree cut.

On the Shopsmith, I've used the trick of setting up the rip fence with a short block clamped to the front edge, to control the length and ensure duplicate length cuts are made. If that doesn't work, I've used the rip fence, sanding disc and quill depth stop trick shown in PTWFE to touch up the pieces. I think it is called sanding to exact length.

The chopper is great and produces a mirror smooth joint.

And as I said above you are right getting the exact same length is the hardest part. A small difference of even 1/32 will open the joint a noticeable amount.

[JPG]

Cut the pieces to almost correct length at a 45.

Sand one end to provide smooth surface using the disk sander.

Sand the other end to get both smooth surface and exact length.

Repeat for other three pieces.

Use whatever method you prefer to get them assembled.


This sounds good but I do not recommend it!!!

The smooth surfaces are not ideal for this type joint.

The teeny tiniest of angle deviations will be glaringly obvious.



By leaving a rougher surface on the gluing surfaces, the ends tend to interlock a small bit and a stronger joint will result. This also allows a small range of movement to position the pieces slightly.


Truth be exposed, a miter joint for a box corner is quite difficult to get 'perfect', and is not as strong a joint as others. I would be inclined to back it up with an interior cleat which adds to the degree of difficulty getting them perfect, but is much stronger.