So after some research I thought I'd share how we solved this problem.
First of all, one must understand the safety concerns. Removal of the guard is likely required (we didn't have our on) but its possible it could be left on. The largest concern is with kickback. We'll come back to this.
- Make a sled that can be used for cutting more than one radius, adjustable.
- Make a sled that can be used for other cross cuts
- Make it as safe and sturdy as possible
This is a one sided sled. That is, it is not extending to both sides of the blade at the same time. As a cross cut sled we can use it on the right side of the blade with the table to cut square ends on longer pieces where we don't care about the cutoff. This is not intended to be used to cut through the middle of a material, only the ends.
Material for the Sled
We used some nice White Oak we had to make the strip for the runner to insert into the table saw. A hardwood is definitely suggested. If you have something even harder (hickory, heartwood, etc) go for it.
We then cut a 3/4" Birch Plywood into a 26" x 36" rectangle (if you have a harder plywood available use that, this was the best available to us.) Being our prototype, we weren't concerned, but we may get something better for future sleds. We used 3/4" to give us extra beef for the next step.
Adjustable Radius Slot
I took an idea from some sliding adjustable router compass jigs. We cut a 15 degree dovetailed slot into the plywood, perpendicular with the blade.
We used a router table to do this, but to prevent tearing, we first dadoed a 3/4" wide slot 5/16" deep. Then we used the router to cut the dovetail precisely at the edges.
Then, using Oak again, we made a 12" strip to slide into this slot. The dovetail helps secure the strip structurally and then three predrilled countersunk holes with 1/2" screws allow us to move the strip to any distance from the blade.
We then drilled a 1/8 hole and slightly countersunk the backside with a 1/2" forstner to keep the bottom flat. This let us insert a simple roofing nail and keep its head flush with the bottom. This nail is our radius pin. We may modify this in the future, but for now it allows us some customization with its length.
Assembling the Sled
Now we can attach the runner to the plywood sled.
On our particular table saw, the runner slot to the right of the blade is closer to the left. This works out to our advantage as we will see in a moment. We measured the distance from the blade to the right runner and then added a 1/4" so we can cut the sled for zero clearance after the runner is attached.
After attaching it with screws as close to square with the front and back edges as could, we tested how it moved on the slot with the blade down. Now is the time to sand or scrape any extra off until it rides smoothly.
Next we raised the blade and cut through the sled and checked it to square with the front and back. We were pleased with the results so we moved on.
Next we added a thick two-by board to the top of the front edge (closest to you with the sled in the right runner still.) The saw naturally pushes materials into this edge. Its good to make it tall enough to hold onto safely as well. Ours was 3 1/2" tall x 1 1/2" thick.
Run some scrap pieces through the saw to test it for square. There are a few techniques for doing this, but the 5-cut method is probably the simplest. Once we are happy with the result we move on.
Using the Sled for Radius
We can use our sled as a sled to cut the ends of larger pieces to the right, but we can turn the sled and put it on the left side and now the sled sits securely on the table with the center pin back from the blade.
We were making 2 foot diameter table tops, so we slid the pin rail so the pin was 12" from the blade and then screwed it in. The multiple screws means eventually you may run into holes that interfere with your needed radius setting. But hey! This was a prototype, and new holes could be drilled in the future.
Next we found center on our piece and predrilled a 1/8" hole partially into the bottom side (the depth of our pin). We marked a line at 12 1/4" and used a jig saw to cut away all the extra. This is very important for a number of reasons. First, it saves time, but it will also make the cutting process safer as there will be very little material that can grab onto the blade.
We dropped that hole onto our pin and hammered the piece down until it was (almost) all the way down. (Use a block to protect your piece).
As you can see in that photo, we still took the piece stationary through the blade to cut off as much extra as possible. Stay to the left side of the blade to avoid any kicking on the small cutoffs. Careful sliding back past a running blade, this is a kickback danger! You can turn off the blade, but that adds a lot of time for a large piece, so I suggest you lift the whole sled up and over the blade once you are past.
Once the circle is trimmed, move it up until it starts to touch the blade. You will want to spin the entire piece 360 degrees at this position, only moving a bit further with each full rotation. You can make this even safer by only raising the blade by these small amounts as well. But we kept the blade up and just spun it freehand. While rotating against the blade, you should never be cutting off more than the kerf of the blade! If you start to, back out, and turn that area, or retrim it stationary. Eventually you will have the blade up to the height of the piece and the piece rotating fully without cutting any more.
You have a perfectly square edged perfect circle now!