Photo 3- Scribing the different radii of each part onto the plates.

Use a stable wood for the dome and dish jigs, so that once you have cut out the spherical shapes into them they will be less likely to distort with moisture variations in the air. I used recycled redwood fencing for mine. This old growth redwood, pulled out of the San Jose, Calif. landfill, is very stable stuff but it had a lot of knots in it. Since I was sinking numerous screws into the jigs to attach parts the knots became a problem occasionally. One screw sheared off in a knot, then I forgot to pull it before resurfacing the jig with a large router bit, which was damaged when it hit the screw. This was the only time in the entire project that I hit metal with any router bit- a good record given how close the expensive cope and stick bits come to screws during use.

Photo 4- Cutting each of the parts out, each at a different angle.

Back to conceptualizing about spheres so we can figure out how to make the pieces that go into the dome and dish blanks. Imagine a four foot diameter sphere with a flat, horizontal plane passing through the middle of it (drawing 3). Now, imagine parallel planes spaced at 3 inches from the first, both above and below it.

Continue adding such planes until there are enough to fill the inside of the sphere. Note that the first plane contacts the sphere at a tangent that is 90o to the plane. But the planes adjacent to the first plane contact at a tangent with an angle less than 90o. The further each plane is from the center plane, the less the angle is of its tangent to the circle. From this we see that each successive layer of the glue up lamination for each of the dome and dish jigs must be cut at a different angle than the last.

Photo 5- Gluing the cutoffs onto other plates made my use of the material more efficient.

As well, each successive layer must be cut at a different radius, because the circle described by that layer has a smaller circle radius to the Y axis, even though that circle has always the same sphere radius as the other circles to the sphere center.

Photo 6- First glue up of the dish jig. The plywood curves upon which the glue up sits keep the parts in correct relation.

I began my jigs by gluing together three pieces of 1x fencing as in photo 1. First I planed the fencing so it would glue well and to make the pieces uniform in thickness. After laminating the groups of three together, I edge glued them together to make large plates as in photo 2. I made a full scale section drawing through each jig to show me the angles and radii required for each as in drawing 3, then began scribing the radii onto the large blanks with shop-made trammel points as in photo 3. Next came band sawing each of the laminations out as in photo 4. For efficiency I glued the off-cuts onto other blanks as in photo 5.

Photo 7- First glue up of a dome jig. Again, the plywood curves keep the parts aligned in the proper spherical relation.

Photo 6 shows the first glue up for the dish jig. It gets glued up in an upside-down position so that the critical inside surface can lie on two special plywood reference jigs during the glue up. Those jigs hold each of the laminations in correct relation to each other so the end result approximates the sphere required. The radius of those jigs is a function of how far apart they are from each other, just as the radius of each lamination is a function of how far they are from the central plane, as explained above.

Photo 8- Successive glue ups produced the larger bowl shape required. Had I tried to glue them all up at once, it would have collapsed, but simply adding two layers to each side at each glue up allowed it to hold together.

Photo 7 shows the first glue up of the dome jig, again upside down so that the surface of the jig that we want to use is the one that contacts the reference jigs. After that dried I glued on two more laminations on each side, after that dried two more went onto each side as in photo 8. I made two domes, one for the router jig and one to use as an assembly table to glue the frames together on.