This is not a new concept nor is it my idea but I thought is was worthwhile sharing. My son recently wanted to make some frames for his university degree & other certificates. He had a little trouble getting the mitres accurate with the basic mitre gauge for the table saw even though I had made an extension piece for it.
One of Murphy’s laws says that after you do a job you think of a better more accurate & easier way. Murpy is also that pain in the neck that makes you get it wrong if you have only a 50/50 choice, or if there is anything that will go wrong it will. If I catch up with Murphy one day I will give him a bit of lip. Anyway we are right off the track now.
As I said this mitre jig is not my invention. It is fairly simple but consistently accurate if you set it up correctly. It works on the same basis as a T/S cut off sled. You need a flat board with a couple of runners underneath to guide the jig over the T/S surface. I used softwood runners cut to fit fairly snugly in the T/S grooves.
You will need to make sure the front edge of the sled is dead square to the saw blade and remains true when sliding back & forth on the runners. This should not be difficult if you install the runners at 90 degrees to the front of the sled.
Attach the runners then run the front of the sled through the T/S blade only as far as is needed to cut mitres, about 6” is ok for what I need it for.
Next you need to set two mitre guide supports at right angles to each other on the sled. The right angle is critical and the alignment to the T/S blade is also important. I found that a large builders square was handy to establish the right angle between the mitre guide supports. Using the front edge of the sled as a reference you then need to draw a line on the sled at 45 degrees to the front edge at a distance back from the front edge that will accommodate any stock you wish to mitre. (A 3,4,5 triangle can be used here to get accuracy over the longest possible distance to create the 45 degree mitre).
Then screw the mitre guide supports firmly to the sled at right angles to each other and one of them along the line drawn at 45 degrees to the front edge and adjacent to the slot cut by the T/S (boy that was long winded but it is really a simple concept).
Before you set the guides at 90 degrees cut a 45 degree mitre on each one and butt them together.
Lastly, run the front of the sled through the T/S and through the mitre guides. You should now have a perfectly lined up system that will cut accurate mitres every time. Tearout will be minimal because all of the workpiece is supported.
I have added toggles for added holding ability & also quick release.
The advantage of this jig is that provided you have the mitre supports at a perfect right angle all the mitres will be accurate because you use both sides of the jig.
If anyone has any questions I will happy to help out.
-- Grumpy - "Always look on the bright side of life"- Monty Python