The wooden sculpture finished a few weeks ago needs a base. Marc Adams supplied a granite slab that works well as a base, but ideally the sculpture, designed to show movement, will spin on the base.
The sculpture sits on four points, two on each piece of mesquite. The points are quite widespread, about 5-6" apart on each part of the mesquite base. The two pictures above attempt to indicate the locations of these points of contact with the granite, the brass cap screws close to the contact points.
The question I would like to answer is what is the smallest circle that will stably support the sculpture. A 6" coffee can had no difficulty supporting the sculpture. No location could be found for a 3" cup to provide support. A 4 1/2" plastic container provided a solid support with two caveats. First, the sculpture sits slightly within the container, resting on three points. Second, the sculpture does not sit at the same angle as when resting on the granite, the preferred orientation. The tilt is not completely obvious in the first photo below, but the sculpture is leaning toward the camera.
The instructor of the sculpture class, Christopher White, pointed out the bearings he uses. These bearings are set into the stone base of his sculptures. I do not believe I want to cut/grind a hole in this incredibly tough granite, so my bearing will sit on top of the granite slab. I have some large bearings that might be ideal for supporting the sculpture, the question has now morphed to what kind of base do I want to use. I am leaning toward something triangular and made of brass. It might be ornamented in some fashion. Some idea sketching is in order. Picked up a few ideas from a search for images with three-fold symmetry.
There is an obvious alternative, setting the granite on a bearing or lazy-susan of some sort. So much easier to accomplish than cutting granite or making a triangular support for the sculpture. A bearing was located in my stash of bearings, some from an uncle's estate and even more from calf massagers disassembled upon their demise.
The bearing shown above is 0.391" high, 1.378" ID, and 2.170" OD. The plan is two plates, one with a hole bored to fit the OD and one with a shaft that is the ID in diameter. Both hole and shaft will have a lip, keeping the bearing from rubbing in the wrong places.
Cutting metal will begin with the shaft, 1/2" long with a 1 7/16" diameter lip that is 1/8" long and a 3/8" shaft that is 1.378" in diameter. Ideally the shaft will be a press fit into the bearing.
A scrap of aluminum was located with a hole through the center. This scrap was held in a three jaw chuck and faced. The diameter was reduced to 1 7/16" for over 0.5". The diameter was further reduced to 1.379" for 3/8", thinking I would sneak up on the desired diameter. At this stage the shaft was already a slip fit in the bearing! The shaft was parted off at 0.50" and this new face cleaned up. A bit of chamfering and the lathe work was complete as seen below.
Three screws will hold the shaft to a table, yet to be defined. The holes will sit on a 1.222" diameter circle. #4-40 screws were located that should be appropriate. The plan is to drill and thread holes in the shaft, then transfer these to the table. Holding the shaft in the lathe with the table attached will allow turning it to a concentric circle.
Before that was tackled the other table was completed. Two 5" X 5" tables were cut from a length of 3/16" aluminum plate. One had its edge filed to flat and burrs removed. This table was held in a four jaw chuck on the South Bend lathe after marking the center. The table center was aligned with a pointer in the tailstock. It was center drilled and drilled up to 3/4". It was then bored, opening the hole to 1 3/4".
A stop was set so the boring bar went about 2/3 of the distance into the hole. The hole, up to this stop, was widened to 2.170" to fit the bearing. The bearing is a light press fit in the hole. The completed hole with lip can be seen below with the table still held in the chuck.
One of the other table's cut edges was cleaned up with a file. The second cut edge required the use of the mill to clean up the edge, removing about 0.08" to produce a straight and parallel edge. (Who is the klutz with the hacksaw?) The two tables, bearing and insert are seen in the photo below.
The rotary table was set up on the milling table and centered. The insert was then centered on the table with the help of a dial gauge. Holes were drilled 120° apart with a #43 drill to be tapped #4-40. The insert was not as well centered as thought, but the holes are still acceptable. The threading was started for all three holes in the mill and then moved to the vise. Oops! Broke the tap off in the first hole! The tap could not be removed with pliers.
The part was remade from a length of 1 3/4" round aluminum. The diameter was reduced to the large diameter of the insert followed by the smaller diameter. Parting was begun with a parting tool, but finished with a hack saw. The sawn end was faced providing an insert that is a tight press fit in the bearing, though yet to be pressed into place.
The insert was clamped to the top of the centered rotary table on a scrap of 1/16" aluminum with two strap clamps. The part was centered with the aid of a dial gauge indicating on the smaller OD. It was centered within 0.001". It was moved 0.611" off center. The three holes were center drilled and drilled with a #43 drill.
The smallest transfer punch in my set, 0.093", did not fit through the holes, so they were opened with a #44 drill and it still didn't fit. The punch was sanded a bit and now fits smoothly. Center crossing lines were made on both the insert and the table. Aligning these lines centered the insert on the table. The holes were transfered, then drilled with a #32 drill. The holes were countersunk to fit the screw heads as shown below.
The holes in the insert were tapped #4-40 with plenty of cutting fluid. All went well yielding three tapped holes, which were cleaned up with a chamfer bit and compressed air. The holes did not align sufficiently with the holes in the plate, so the holes in the plate were opened with a 9/16" drill. This proved sufficient and the plate was attached to the insert. The insert was pressed into the bearing with the aid of the vise, resulting in the subassembly shown below.
The bearing was pressed by hand into the other plate seen in the first two photos below. The granite was set on the bearing, third photo and two shots of the complete sculpture and granite base supported by the lazy susan completed the project. At some point the bearing may be glued into the bottom plate. Felt pads on the bottom of the bottom plate will also be added.
