Tight Fitting Miters

foxtrapper · Post by **foxtrapper** » Thu Dec 08, 2011 7:12 am

Another trick is to simply not do miters.

I don't know of a joint more prone to showing errors on initial assembly, and to seperate in use from swelling and shrinking of the wood due to heat, humidity or other variables.

A lap joint, or a modified lap joint, does not show those problems. That's why they were used on windows and doorways in old houses. Leave a little overhang and the eye never notices the changes taking place as the wood swells and shrinks.

mbcabinetmaker · Post by **mbcabinetmaker** » Thu Dec 08, 2011 7:53 am

Hoffman key!

http://www.hoffmann-usa.com/index.php?c ... 9058594115

Not for your average hobbyist.

dusty · Post by **dusty** » Thu Dec 08, 2011 8:13 am

mbcabinetmaker wrote:Hoffman key!

http://www.hoffmann-usa.com/index.php?cPath=6&osCsid=f4f6b9fa89aa63c85d705f9058594115

Not for your average hobbyist.

It looks like it would make a strong joint but difficult to align if not cut using precision machinery.

Maybe you could do one of your tutorials some time.

foxtrapper · Post by **foxtrapper** » Thu Dec 08, 2011 8:32 am

I've never done one, though the house is going to force me to soon.

None the less, look at this thread about using a bowtie dutchman to repair a split.

http://archives.woodworkslive.com/index ... pic=2853.0

dusty · Post by **dusty** » Thu Dec 08, 2011 8:52 am

foxtrapper wrote:I've never done one, though the house is going to force me to soon.

None the less, look at this thread about using a bowtie dutchman to repair a split.

http://archives.woodworkslive.com/index.php?topic=2853.0

Back when I could see real well this would have worked. Now the light has to be just right and I have to be in exactly the right position. Nope, free handing this is no longer an option for this guy.

I must face the reality - there are two or three things that I just cannot do anymore. This is one of them.

bffulgham · Post by **bffulgham** » Thu Dec 08, 2011 9:04 am

dusty wrote:I was curious about what degree of precision is required to get really good fitting miter joints. Since I did not have enough cutoff in the shop, I decided to play with sketchup and to build a couple picture frames there.

It was interesting and a bit informative. I suggest that everyone should give it a try. Create four pieces with perfect (45°) miters and four pieces with 44.9° miters.

The conclusion is obvious. A perfect frame cannot be constructed with anything less than perfect miters (and perfectly dimensioned sides - meaning opposite sides must be of equal length).

Side note: In sketchup, make each frame piece a component. If you do not, once they are joined, the union is permanent. Just like real shop work. You can dry fit and still make corrections but once you glue the joint you have no further opportunity to make correction.

The angles on the mating pieces must add up to 90 to get a 'perfect' corner on a 4-sided frame. If you have one that measures 44.9 the other can measure 45.1 and still get a perfect 90. But, at some point the differences will be noticeable to the eye. At what point does it become noticeable? I dunno...... This is the concept that makes things like the Miter Pro or miter sled work so well. With these assumptions:[INDENT]The 2 fences on the Miter Pro are exactly 90° to each other
The alignment of the Miter Pro to the blade is off .5°
Fence 1 will cut at 44.5°
Fence 2 will cut at 45.5°

[/INDENT]So if you cut the pieces of the frame in sequence and alternate fences, you get a perfect 90° when you join the 2 pieces together. [INDENT]

[/INDENT]It's kinda like using the jointer to prep 2 boards to make a panel. You can get a closed joint and flat panel if you keep the face of the first piece flat to the fence and the back of the second piece to the fence. Even if the fence is not perfectly 90 degrees to the bed, you still get a flat panel because the angles are complementary to each other (I think "complementary" is the correct term....it's been too long since geometry class for me.)

mbcabinetmaker · Post by **mbcabinetmaker** » Thu Dec 08, 2011 9:49 am

dusty wrote:It looks like it would make a strong joint but difficult to align if not cut using precision machinery.

Maybe you could do one of your tutorials some time.

Yes dusty it requires their machine. I will add that to the tutorial list for later.

pennview · Post by **pennview** » Thu Dec 08, 2011 11:08 am

I'd just add to this discussion that adjusting the Shopsmith miter gauge to 45 degrees is not that difficult if you use the Stop Screw and if you always turn the screw in the same direction. This way you can sneak up on the 45 angle without much trouble. And, you really need to cut all four pieces of a scrap frame while adjust the miter gauge to ensure you've got the angle right because just cutting two and usings a square doesn't work very well, as most squares aren't exactly square.

If you don't have the necessary scrap to make a frame, you can always pick up a length of poplar or other clear 2x1 or 3x1 wood at the Home Depot, etc., for a couple of bucks and that will last quite a long time if used exclusively for setting up miters on the saw.

In addition to those corner reinforcements mentioned, you can use a biscuit (the Porter Cable FF size), splines or feathers installed in the outside edge of the frame after the corners have been glued, splines set into grooves cut into the adjacent edges of the miter joint, or brads/finishing nails.

BigSky · Post by **BigSky** » Thu Dec 08, 2011 11:52 am

This might come as a surprise to some but the miterpro is about as dead on accurate as you can get. Using a digital angle gauge, it measured 90 degrees 82 times out of a hundred measurements. The other 18 times it measured 89.9 degrees.

I zeroed the gauge while resting on one leg of the miterpro and then measured the other expecting to get 90 degrees. After doing fifty in that manner, I zeroed on the other leg and then measured the other for confirmation.

Also, I measured ten times by not zeroing at all and then checking that the sum of two readings was 90 degrees. It was, all but once when the two sides totaled way off at 89.8 degrees. I discarded that reading from the statistics table.

charlese · Post by **charlese** » Thu Dec 08, 2011 12:45 pm

It's been a great discussion! Thanks to Bud F. (post #26) for pointing out exact 45s are not needed for a perfect miter, so long as both angles add to 90 degrees. Also thanks to BigSky for his patience in checking the angle of the MiterPro.

Let me add another feature of the MiterPro that is not often mentioned. If you will disconnect the long leg, it can be attached to the miter gauge and will serve as a 24" extension. I used it in this duel configuration for a couple of years before buying a second leg.

Foxtrapper has a good point in exposing the wood movement difficulties of miter joints. I have a couple examples of this issue in wide mirror frames. The insides of the joints have separated, while the inserted splines hold the frames together. Actually I believe this issue mostly applies to miter joints of wide pieces, as wood movement of narrow pieces is almost negligible. Maybe one of these days, I will try a mitered half lap joint.