Prototype 22.5 deg mitre sled
I wanted to have a play with octagonal boxes, but can't saw accurately to save my life. I have a Triton mk3, but can't swing the blade over that far, and now that I've got it accurately set to 90 I didn't want to stuff around with it.
I decided to have a go at a sled to do the cutting. On the first attempt I made angled stops to set the angle, but they were slightly out. Over the 8 joints, the error cascaded and the end result was terrible.
So I had another go, but this time I fitted 5 hinges to the jig base, then used turnbuckles to set the angle. I used a Nobex Octo (marvelous bit of gear), adjusting each side with the turnbuckle until I thought that I had the angle pretty well. The intent was to make the first set of cuts, then fine tune the angle until I got a good fit.
I then made the sled that slides along the top of the base, with 2 cutting positions on it. he theory was to cut the box side blanks square, and slightly oversize. These go into the first cut position on the sled, and a fine first cut cuts the first end. This is then put into the second cut position, which has a stop block (with reverse angle edge) to make the second cut to size.
Pic 1: The jig in position, showing the tunbuckles. The base is set against the mk3 fence, and clamped into position. The sled has a piece of pine on the back, which runs along the top of the base. The sled was passed through the blade to get an accurate cut guide.
Pic 2: Shows the 2 cutting positions, and the stop block. I originally bolted a 4mm piece in a the blade edge of the sled. The concept was to have a sacrificial edge, that I could use to reset the jig after romoval/replacement. The problem was that it was hard to set the blank to the edge. So I moved the rails forward, a took a cut. This gives a good visual reference for the cutting position.
Pic3: Chucked some pine through, and cut 8 sides. These were to be my test cuts to refine the angle. ie check the fit, adjust the angle, run both ends of each piece through again, and recheck the fit. Do this as many times as neccesary to get it right. Bugga me!! The fit was perfect first time.:cool:
Pic4. Decided to make a box from the pine bits I had cut. Put the sides back through the sled, and also cut a top frame. I think it has potential.
Things to do on the final version.
1) Work out a guard.
2) Add a sliding block to the each cut position, that is 90 deg to the work to stop any tendency for the work to twist.
3) Add work clamping to each position, so that I can concentrate on keeping the sled down on the base, and to get my fingers as far from the blade as possible.
4) Route a channel for the work guides to slide in. This means I can hold the in position with a couple of screws from the back. When I need to reset the jig onto the mk3, all I have to do is move the guides forward by a couple of mm, and refix. Put the jig into position so that the edge of the sled is just touching the saw. Take the sled off, make sure the fence is parrallel, then move the fence toward the blade by a couple of mm. Lock the fence, clamp down the jig, put the sled back on and take a cut. This gives me an accurate cut line again. After several resets, I will lose enough cutting height that I can no longer get through the thickness of timber that I want. Not a great problem, just reset the top sled guide position, and reset the jig.
5) Make a second sled for 45 deg cuts. The angle is so easy to set with with the turnbuckles, I can use the setup for square/rectangular boxes, with the minimum of adjustment time changing. Would like to find some shorter turnbuckles, so that I can get them more perpendicular to the angled part of the base.
