Making the support armThe support arm holds the swirling bowl and the pin board at the top of the machine. The arm connects to both vertical posts.
The arm has two finger joints that connect at 45 degrees. These could be cut with something like the screw advance box joint jig. But I didn't want to make that jig a requirement for building this machine, so instead I cut the joints on the bandsaw using templates.
You can see some extra diagonal lines on the template. These are because I printed my preliminary version of the plans using my BigPrint program. I hadn't yet split the plan into individual pages, so I printed it as one big sheet, with a diagonal grid superimposed to help align the pages when gluing them together. To avoid confusion, the plans I will have for sale won't have these extra lines.
It's important to cut only the cuts shown here at first. If you start cutting away more, you will lose part of the template that you need to make the 45-degree cuts from the other side.
After these are cut, the wood is placed flat on the table again and the rest of the finger joints and outline are cut.
A 45-degree notch needs to be cut out of this bar. It looks a little confusing in this view, because I generated the top template from a transparent view, so the notch is visible from the top as well. I changed the final plans to show a bottom view of this part, which will make this notch appear more logical.
Sufficient bandsaw blade tension ensures that the blade stays vertical even when shaving just a fraction of a millimeter off the side of the wood.
To mount the support arm, holes need to be drilled at a 45 degree angle from the corners of the posts. It would be very difficult to start these holes from the corner, so I'm cutting flat spots into the posts. First by making a cut on either side of where I need the hole to be, then carving a 45 degree chamfer between them with a knife.
It may be better to drill these 45-degree holes before assembling the back frame, but I hadn't yet marked these hole positions on the plans when I built the machine, and I was too lazy to take the whole thing apart again.
Now drilling a pilot hole into the support arm. I'm drilling this through the shank holes drilled previously. I drilled the shank holes freehand, so they weren't that accurate. Drilling the pilot holes through the shank holes ensures that the holes will still line up.
Next, I drill a 5/16" (8 mm) hole to countersink the screw head below the surface. I'm only using 2" (5 cm) long screws, so it helps to sink the head about 1.5 cm below the surface. That leaves enough of the screw threading into the support arm to get a decent hold.
Don't tighten them too much - that support arm needs to be removed again in later steps of the construction.
Next: Marble distribution