Cutting Mitres on the table saw is not trick ... getting them accurate is a bit more challenging, especially if you are doing things like picture frames. I seem to make most of my angle cuts on the table saw at 45 degrees, which by chance is the same angle as picture frame sides, so this jig will do double duty for me.
The one thing I should add here in terms of making picture frames, getting the 45-degree angle is only half the battle, the other thing that is crucial in making picture frames is that each opposing side of the fame needs to be EXACTLY the same length or the corners will never line up.
Watch it on Youtube: https://youtu.be/3czW9oJS1Kc
This jig will make accurate 45-degree angles, and if you want to make picture frames you can use the stoppers to make sure the horizontal pieces and the vertical pieces, however long they are, can be cut at exactly the same length.
To make this jig I started off with some re-cycled 3/4 inch plywood, good quality, flat and still with plenty of life for these smaller jobs.
The base of the sled would be 24 inches wide and 16 inches side, but of course, you can make it fit your own needs.
I am also using smaller pieces of 3/4 plywood for my 45 degrees angle measuring board, plus I will need a couple of solid wood arms for the sides, these are 18 inches long and 1-1/2 inches high, and last but not least the plastic mitre slot blanks that I cut myself from white UHWM (Ultra High Molecular Weight) plastic ... the same stuff they make plastic cutting boards from. You can purchase this from your local plastic supplier and cut your own miter slot blanks and save some $$$s or you can order it pre-cut and delivered ... UHWM (Ultra High Molecular Weight) plastic Mire Slot Blanks.
The first place to start is to measure the distance from the table saw blade to the center of the right mitre slot and be sure to leave some extra room because once you attach you mitre slot blank you are going to want to cut off the edge of that plywood blank so you will have a place to measure from when attaching your 45 degrees measuring board.
Attaching the plastic blank material can be a challenge. If you have an air nailer, that will make it easy, otherwise you will need to use double-sided tape to align the plastic with the plywood base, MAKE SURE you have pre-drill and countersunk holes in the plastic blank material because driving screws into without pre-drill holes will make the plastic extrude it'self and create a hand plastic lump on the surface or worse, between the plywood base and the mitre blank.
When the miter blank is secured, flip the base over, check to see how smoothly it rides in the miter slot, then when satisfied, cut the right side edge of the base off and now you have a perfect side edge that is aligned with your miter slot.
Next, position your base on the table saw in its normal usage position, and with the table saw blade all the way up, make a cut into the top of the base about 2 inches long. this will be used as a reference mark to help align the 45-degree plywood marking board you will attach next. Align the 45-degree marker board as best you can and attach a screw near the top where the point touches the cut you just made. Now use whatever method you like to align that 45-degree board, but make sure that it is absolutely aligned ... you could even double check it with a different marking device to make sure it is aligned and when it is fastened the back of the board with another screw so it is firmly attached.
Next cut the ends of positioning arms to their 45-degree angle and now with a bit of glue attach them with screws to the base and use the 45 degrees measuring board as your guide. Once they are attached the center measuring board can be removed. The only thing left to make this jig as a picture frame makes is to cut a couple of stopper blocks for the side arms. One side should be at 45 degrees the other cut off square and now you can use whichever side works best for any application you are working on.
If you decide to add the plastic safety shield, just make sure to give it some extra height above the highest point of your table saw blade. Attaching the safety shield will help shield you from chips and sawdust being thrown up from the table saw blade cuts.
This is a quick, easy and very accurate jig and should give many years of work and is much quicker than setting up a mitre gauge every time you want to make a 45 degree angle cut ...
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Copyright Colin Knecht