Expanding Round Table

Construction Blog: Page 11
Substrate and Hardware

I’ve been putting off machining the substrate for as long as I could, but now that I have the welded steel support frame, I needed to move forward on this so I could mount the substrate to the frame.

The most significant change in my design from the previous version of this table is that all of the mechanical system is located below the main substrate of the table. Even though the mechanics were only visible when the table was being changed, I wanted a cleaner look to this new version.

To transfer the movement from the mechanics below to the moving wedges above, I use riser blocks that penetrate the substrate. Just thinking about cutting the slots for these moving riser blocks was rather intimidating, because I knew that if I made a mistake there would be no recovery. The whole substrate would have to be thrown away.

Just as I was getting ready to tackle this task, the idea for a modified trammel hit me. These slots are 7/8” offset from the radius, so I built a trammel with a slot that was offset from the trammel point. Then, all I had to do was rotate this around to each of the marked radii and clamp it down. I used the same down-spiral solid carbide router bit I used to cut the circle, but I installed a 5/8” guide bushing in the router to follow the template.

Oh, I just realized that the picture below shows how the slots are offset from the centerline (radii) of the table. You can see that they run parallel to the centerlines, but are offset.

In the center of the table is a precision machined disk that all other components are mounted to. This is the single key piece to the alignment of the entire table. Because I spec’d out this disk with very low tolerances, it cost me a fortune to have it machined. My greatest concern in mounting this disk is that I would lose the center-point-hole in the substrate. I had to make sure that no future operations would hinge on needing this center point.

The first step was to carefully mark and drill the positions of the disk’s mounting holes. These mounting holes were between the radial lines I had drawn previously, so I needed another set of radial lines.

I used a "poor man's drill press" to make sure these were drilled straight. I first took a piece of scrap wood and drilled a hole through it with my drill press, and then I used this piece to guide the hand-drill for drilling these holes. This way I didn't have to haul the drill press back over to the table.

The disk will mount to a flange 1/4 inch below the surface, but I realized that I can cut the center out of the flange later. For now, all I need to do is inset the disk to check it for size. It wasn’t until I was ready to make this cut that I realized that I could still keep the very center of the substrate intact because the disk has a clearance hole in the center (see next picture below).

Using the down-spiral router bit and the 5/8” guide bushing, I made a small trammel with a series of 5/8” guide holes to let me change diameter from the outside to the inside. Each of the holes you see in this picture represents a different trammel radius.

For the first attempt at this, I had the router set to just barely cut the wood surface. This way I could gauge how accurately the disk would fit into the mortise. My first guide hole was slightly farther out that I wanted, so I made a nearby bore that was just a little closer. This new hole turned out to be too close, so I elongated the hole to tweak the final diameter.

For a while I was concerned that I would not be able to get this mortise to the right size, but decided to make a full-depth plunge anyway. Even though my first couple of shallow passes didn’t seem to be the right size, on this full-depth plunge, everything came together better than I had hoped, and the disk fit so snugly that I had to insert a screw into it to remove it from the mortise.

The aluminum disk has a chamfer on the edge, so this was making it difficult to gauge the proper fit until I made a full-depth cut.

The center of this will eventually be cut out, but I don’t want to do this any sooner than I absolutely have to because this will also cut away my center point for the whole substrate. I won't do this until I am ready to mount the gearbox and other hardware into the table.

The fit is absolutely perfect. There is no slop, yet I didn’t have to force it either. As a matter of fact, I think the fit is so close that I may gave to remove the paint/lacquer from the edge for final installation (see pictures below).

The same day that this very expensive center disk arrived from the machine shop, I realized I made a mistake in my design drawings I sent to the machine shop. I hadn’t realized that the bearings for the input and output shafts were different diameters. This is a detail that is very obscure in the manufacturer’s specifications. As a matter of fact, even knowing this now, it still took me a little while to identify this detail in the spec’s.

I probably would have noticed this sooner, but I gave the gearbox to the machine shop shortly after it arrived so they would have a reference for drilling the mounting holes. (The gearbox is the red block in the background.)

The result of this oversight was that the disk didn’t fit over the protruding bearing for the output shaft. I couldn’t make the center bore of the disk larger, so I had to chamfer the edge to clear the bearing.

I bolted the disk (upsidedown) to a piece of scrap wood and cut a center clearance hole in the plywood scrap for the router bit bearing. Then I used a 1/4 inch cove bit to cut a wide, reverse clearance fillet into the disk for the bearing.

I’ve machined aluminum with woodworking tools in the past, but I was a little leery about doing this with a small interior circle. If the router bit had caught, it would have been catastrophic and resulted in an unexpected climb-cut.

For a cut this shallow, I could have done it in a single pass, but made three progressive passes just to be safe. It went easier than I had thought, and it gave me just the right amount of clearance for the bearing.

I painted the disk (and a few other parts) with black spray paint. I took the automotive paint approach of “base and clearcoat.” The spray can paint gave me the color, and I top-coated this with lacquer for durability. The result is a high-build finish that I could never achieve with standard painting techniques.

As I mentioned previously, the high-build finish may make the disk too tight fitting, and I will probably have to scrape the finish from the edge to get the disk to fit.

Out of curiosity, I mounted the gearbox to the center disk and installed the input and output drive shafts. The input drive shaft is the long one, but I’m not going to reveal the purpose of this until later.

You are probably thinking that the little red gearbox is “cute” or “pretty.” What you probably don’t realize is that this cute little gearbox that weighs just a couple of pounds probably cost more than your tablesaw! Yes, you read that right. This little gearbox that is less than 4 inches long, costs more than a contractor-grade tablesaw. Because the output shaft makes only 1/2 revolution to open the table, the gearbox has a 40:1 ratio.

Up Next: Installing the Subframe.