Click on the photos for an enlarged view.
Compound miter sled for 12-sided 70-degree side angle.
Another view of the compound miter sled at left. Note that the saw blade is square to the saw table top.
This is a simple compound miter cutting fixture attached to a miter gauge.
Another view of the compound miter cutting fixture at left. Note that this fixture requires the blade to be tilted.
My sleds are made from 1/2" Baltic birch plywood. I like Baltic birch plywood because it seems to be dimensionally stable and stays flat. I make my fence and runners out of 1x4 oak or maple. You might note that my runners don't fit tight in the miter gauge slots. When I glue the runners to the sled, I hold the runners tight against the outside of the miter gauge slots so the sled assembly fits tightly in the miter gauge slots, but if the sled is too tight in the slots I only have to sand the outside of the right-hand runner to make the fit looser. Conversely, if the fit is too loose, I only have to modify the outside of the right-hand runner, such as by gluing on a veneer strip.
Instructions for making a sled for 12-sided 50-degree compound mitered rings:
1. First we'll make the sled platform. Cut the sled from Baltic birch plywood 15" by 20". The 20" dimension should be cut very accurately.
2. Cut two 15" lengths of 1x4 oak (or maple) runner. Set your table saw blade angle for 9-1/2 degrees and rip one long edge of each of the 15" lengths of oak runner. Readjust your blade angle to 0 degrees.
3. Measure the depth of your miter gauge groove. On my table saw, the miter gauge groove depth is 3/8". Rip one of the 15" lengths of oak runner so that 1/4" of its right-hand side of its mitered top extends out of the miter gauge groove. Refer to the above sketch. The 1/4" dimension should be cut very accurately.
4. Test fit the two runners into their respective miter gauge grooves. They should both fit loose. If they don't fit loose, then you should make them thinner by whatever handy means you have, such as by sanding.
5. Attach the right-hand runner to the bottom of the sled. I usually glue it on, then drive in a couple of brads for good measure.
6. Set the sled on the saw table with the sled's right-hand runner in it's miter gauge groove, tight against the right-hand side of the groove. Block up and shim the left-hand side of the sled so that the so that the left-hand lower corner of the sled is exactly 3-37/64" from the table top. Don't block up too close to the left-hand miter gauge groove because we need access there. Refer to the above sketch. The sled is now at the correct angle (9.577 degrees).
7. Measure for the height of the left-hand runner by measuring from the bottom of the left-hand miter gauge groove, on the left-hand side of the slot, to the underside of the sled. Rip the remaining 15" oak runner so it will fit exactly in this space. If you cut the runner a hair too wide, with the intention of fine-tuning its width later, this wouldn't be a bad idea.
8. Put the newly-cut runner in place under the sled and take out the blocks and shims. Make sure the left-hand runner is tight against the left-hand side of its miter gauge groove and the right-hand runner is tight against the right-hand side of its miter gauge groove. Re-measure from the table top to the left-hand lower corner of the sled. Again, this distance should be exactly 3-37/64". Cut or shim the left-hand runner to obtain this dimension. Finally, glue and brad the runner into place. The sled's fit in the miter gauge grooves should have no slop. If the fit is too tight, you can sand later. If the fit is too loose, now's the time to remove the left-hand runner, before the glue dries, and readjust its location.
9. Now we'll make and install the sled's fence. Cut a 24" length (not a critical length) of flat 1x4 oak. Make sure one one of the 3/4" sides is very straight. If it isn't straight, rip it, joint it or whatever to get it straight.
10. Make two marks on the sled, exactly 11" up from the near right-hand corner and exactly 6-57/64" up from the near left-hand corner. Refer to the above sketch.
11. Lay the 24" long 1x4 oak fence on the sled, with the straight edge oriented away from you. Line up and clamp the right-hand end of the fence at the 11" mark. Line up and clamp the left-hand end of the fence at the 6-57/64" mark. Attach the the right-hand end of the fence to the sled using a wood screw. Leave the left-hand end of the fence clamped so it can be adjusted. The fence can be permanently attached to the sled later. The fence is now at the correct angle (78.401 degrees). Glue a strip of fine sandpaper the opposite edge of the fence to prevent slippage of the boards you'll be cutting. You can add hold-down clamps and other accessories to the sled at this point.
12. Put the sled on the table saw top and check its fit. If it fits OK then you're ready to make a test piece to check the sled's angle settings.
13. For the mathematically-inclined woodturners that want to build a custom sled for a different compound miter, the sled equations are as follows:
D = E - C / Tan( MiterGaugeAngle ) and B = A + C * Sin( BladeAngle )
To download an Excel spreadsheet that calculates frame miter sled dimensions as well as compound miter sled dimensions: click here.
14. First we'll test squareness of the table saw blade. Unplug the table saw and raise the saw blade to its full height. Square the blade to the saw table as accurately as you can with a good square. Be sure that the table saw's blade insert isn't throwing off your readings. Plug in the table saw. As a recheck for blade squareness, using your miter gauge, cut the thickest board that your blade will cut (say, a 1x3 on edge, with the 3" side vertical). Check the cut squareness with your square. Turn the board over and recheck the cut squareness again from the other edge. Both square checks should agree. If they don't agree, your square is not square. Readjust saw blade squareness as necessary. This step is the reason that I make compound miter sleds using this design...because I hate to re-square my table saw blade.
15. Now we'll set up for the test piece. Use a length of the 1x4 oak (or maple) to make the test piece. The test piece consists of a half-ring of segments glued together. We could make a full-ring as a test piece, but a half-ring only takes half as long to make. Rip the test piece so its long side edges are parallel. This step is necessary for accurate repetitive segment cutting (the thickness should be reasonably the same, too). Place your new compound miter sled in the table saw miter gauge groves. Make sure that there's no sawdust in the grooves. Make sure the runners bottom in the grooves. Raise the saw blade to its full height. Push the sled into the blade, cutting just into the fence. Don't push downward on the sled platform to the left of the left runner because the sled will tip, binding the saw blade. Place the 1x4 board against the sled fence so that about 4" is to the right of the blade's sawcut. Cut off this piece, which will be used as the board stop. Flip the 1x4 board over edge-to-edge (do NOT flip end-to-end) and align against the sled fence so that about 1-1/2" is to the right of the blade's sawcut. Butt the board stop against the end of the 1x4 and clamp the board stop into place. It's very important to use a board stop so the segments will be cut exactly the same width.
16. Next we'll cut the test piece. Butt the 1x4 board against the board stop. Make sure that there's no sawdust between the end of the 1x4 board and the board stop. Hold the 1x4 board tight against the fence so it doesn't slip either direction (the saw blade will tend to push the 1x4 board to the left as you cut). Cut off the first segment. Don't push fast. You'll get a better cut if you go a little slow. If you put hold-downs on your sled, install hold-downs for both the board and the cutoff segment. Cut six test segments. Remember, we're cutting six segments because we're cutting a half-ring of a 12-sided ring. If for some reason, any of the six segments are miscut, then cut another one to replace it.
17. Lightly sand the six test segments, then glue them together. If you're interested in what kind of glue I use...it's Titebond II. Since it's very difficult to clamp these odd-shaped segments together for gluing, I use rubber-bands. I rubber-band pairs together, then I rubber-band pairs of pairs together, etc.
Click on the photos for an enlarged view.
Fig.1 Compound miter sled half-ring test piece.
Fig.2 Compound miter sled half-ring test piece.
18. Now, we'll inspect the test piece for sled fence adjustment. Place the test piece on your saw table saw top with the cut edges vertical, like in Figure 1. Using your square, check squareness of the test piece. It should be square. (Before you make any fence adjustments, mark the starting fence position by making a light pencil mark on both sides of the fence.) If the test piece touches the square at bottom but not the top, then move the left side of the sled fence away from you about 1/16" and make a small light pencil mark on the sled for new fence position. Conversely, if the test piece touches the square at top but not the bottom, then move the left side of the sled fence toward you.
19. Now, we'll inspect the test piece for sled platform angle. Place the test piece on your saw table top with the cut edges down, like in Figure 2. (If the test piece rocks a lot, then there is something wrong with the way one or more of the test segments was cut, such as different segment widths or angles or maybe the edges aren't lined up good enough.) The cut edges should completely contact the table top surface. In Figure 2, look at the 3/4" wide edge of the test piece in the foreground. (Click on the photo for an enlarged view.) Note that the right-hand corner of the edge contacts the table top but the left-hand corner of the edge is slightly off the table top. This test piece is actually pretty close and I would just sand the half-ring flat for use. But, if the left-hand corner had more gap with the table top, then you need to glue a piece of veneer to the bottom of the left-hand sled runner. Conversely, if the right-hand corner had a gap with the table top, then you need to glue a piece of veneer to the bottom of the right-hand sled runner.
20. Make your adjustments to the sled as necessary and make another test piece. Readjust as necessary. Good luck.
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