As I mentioned elsewhere I have decided to make a 45 degree miter jig for my BT3 somewhat along the principles of this Rockler miter slot sled Jig.

The idea is that it is easier to make a perfect 90 degree corner than a 45. Errors in a 45 will accumulate 8 times around a picture frame so a 0.12° error in each cut on a miter saw will result in a 1° error on the finished frame. 0.12 is hard to see but 1 degree is pretty obvious in the finished work.

If you get a perfect 90 and cut the two joined pieces on the complementary sides then the errors in 45 will cancel out because the joint will always add up to 90 degrees, i.e. the errors cancel out so that 45.1 + 44.9 = 90 degrees. And all four corners will be square.

First thing I needed was a near perfect 90 degree square corner. I have built these right angle assembly squares, So I checked them for square (still right on to my protractor and all my Try squares) so I used my existing squares to square up and glue the new one (middle of the sandwich) and checked it, after the glue set I put some deep screws into it for long lasting strength:

Now you may ask why a butt joint (ugh, right) and why didn't I use mitered corners or other various jointer like half laps etc. I needed most of all perfectly square; strength is secondary, so simple leads to the fewest errors. This wood was planed and edge jointed and cut as square as I could. Glued between a pair of good assembly squares I then put 3" screws into the butt joint to hold it.

Hints as to the further plans:

The idea is that it is easier to make a perfect 90 degree corner than a 45. Errors in a 45 will accumulate 8 times around a picture frame so a 0.12° error in each cut on a miter saw will result in a 1° error on the finished frame. 0.12 is hard to see but 1 degree is pretty obvious in the finished work.

If you get a perfect 90 and cut the two joined pieces on the complementary sides then the errors in 45 will cancel out because the joint will always add up to 90 degrees, i.e. the errors cancel out so that 45.1 + 44.9 = 90 degrees. And all four corners will be square.

First thing I needed was a near perfect 90 degree square corner. I have built these right angle assembly squares, So I checked them for square (still right on to my protractor and all my Try squares) so I used my existing squares to square up and glue the new one (middle of the sandwich) and checked it, after the glue set I put some deep screws into it for long lasting strength:

Now you may ask why a butt joint (ugh, right) and why didn't I use mitered corners or other various jointer like half laps etc. I needed most of all perfectly square; strength is secondary, so simple leads to the fewest errors. This wood was planed and edge jointed and cut as square as I could. Glued between a pair of good assembly squares I then put 3" screws into the butt joint to hold it.

Hints as to the further plans:

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