Monday, October 5, 2015

Auto-Regulator: Circular logic, part 1

The top of the Auto-Regulator is curved, and tapers in thickness. Most of the methods I've seen for cutting a radius involve driving a screw through the center, and using that as a pivot point. And that works well enough for many things. But in this case, I'm working not only with two different radii, but with two different centers. And that starts to get sticky. 

Because of all of the curves I have to cut into the blank, I need to start with 12/4 stock. The end result will be an arch with a 7.5" inner radius, and an outer radius of 11.5". Using the screw through the center technique, I'd want some extra material to contain said center hole. To use the simple, drill a screw into it method of curve cutting, I'd need to drill a center hole through a 12/4 beam that would have to be at least 12-13" wide enough to also contain the center. The finished part would be 15" long, and could be contained within a 7" wide board. There's enough waste involved in cutting an arch, without adding 6" of width to a 15" long blank of 12/4 walnut, just for the sake of locating the center. 13" wide 12/4 walnut isn't exactly rare, but it's not exactly lying around, either. And it's certainly not cheap.

A side note: Not only would the above method be incredibly wasteful, but a 6" x 15" blank would also fit squarely into that sweet spot on the Venn diagram of 'I can't throw this out, it's too big,' and 'I'll probably never actually use this.' And that's a recipe for hoarding scraps.

Added to that, I had this hair-brained idea of making a rough cut on the band saw, and then moving directly to the router table with the same jig, for a finish pass.

And so it was that I set out to untangle radiused cuts (inside and outside radii) on pieces that aren't wide enough to contain the pivot point... and a few other things.


Most of the radius cutting that Ive seen involves driving a screw through the geometrical center, into a precisely placed hole on the jig that's the proper distance from the cutting action. But the screws are usually short, it's almost impossible sometimes to see where you're supposed to be driving it into... and it never really felt like an accurate method. Not only that, it doesn't allow for subsequent fine tuning: I can always take 1/16" off of a straight part on the table saw. But I can't do that with the wood screw technique for curve cutting. Especially not if the relevant dimension is the concave side of the cut, because the process involves cutting away the center of the curve. In that case, you just have to make a new blank, and cut a new piece.

So I simplified things. Or, it could be argued, made them more complicated, to make them more simple.

The base mounts to the band saw table. And on top of that is a sliding dovetailed piece, to adjust for different radii, with two options for pivot centers: a removable pin that sticks up, to be used with a 1/4" hole, and a 1/4" brass shelf pin sleeve that serves as a bushing for a center that will be seen shortly. The pin started out as a 1/4-20 bolt that was only partially threaded. I cut it off, chucked it up in a hand drill, and domed the end with a bench grinder.

Making the cut on the band saw is as simple as I assumed it would be.

As I said, I also wanted to be able to make finishing passes on the router table. The jig below is a sliding mount for two more centers, as on the band saw. The idea is that I should be able to set up whatever jig I'm going to use, and move directly from the band saw to the router table.

Part of the problem I had with cutting an inside radius with the traditional screw-center method is that you only get one shot. After that, if the radius isn't quite big enough, you no longer have a center to work from, because you've just removed the concave part, from the center that was your reference point. I came up with the idea of a scissoring pair of arms to mount to a blank, and the only problem I had then, was that I'd cut right through the arms when I used it. So, I came up with this:

Again, 1/4" bolts cut off into pins. The pins slide through the jig, into 1/4" holes that are laid out and precisely drilled in the blank. The dog-leg shape is there to give clearance for the blade to exit the material without cutting into the arms of the jig. And, the pin being used for the pivot point slides into the brass bushing in the band saw jig, or the router table jig.So the whole pivoting assembly can be simply lifted off of the band saw table, and dropped onto the router table for a finish pass.

Coming up: laying out the mounting holes, using the jig to make a bending form, and a few other things...

To be continued.