Expanding Round Table

Construction Blog: Page 6
Making the Substrate Platform

The substrate is where all other components come together for the whole upper section of the table. Because of the critical nature of this component, it is a steel-frame reinforced sheet of 1-inch Appleply. Since the substrate is nearly 6 feet in diameter, it needs to be glued up from standard sheets. The strength of the two joints is paramount.

Jointing the Edges

I've heard that many woodworkers consider a factory edge to be usable as-is, or that it is used as a reference for square. I consider a factory edge neither square, nor usable. However, at 130 pounds a sheet, this is not something I want to drag onto the tablesaw, and at that weight, I still wouldn't be assured a clean cut.

The Festool TS55 guided saw comes in extremely handy at times like this. With the 106-inch guide rail, this hand-held saw actually provides a smoother, straighter cut than my Unisaw can create.


For strong joints, I used the largest size Domino tenon: 10mm x 50mm. This is important because edge-joining laminated plywood is not necessarily a strong joint because half of the laminates are edge-grain and the other half are end-grain. The tenons will make this a very strong joint.

I also decided to try the optional outrigger guides. These worked pretty well considering I didn't follow my own instructions that I wrote in the Owner's Manual. You are supposed to expand the mortise slot when you use the outrigger guides (that's what the little green dial on the top of the joiner is for). However, I decided that I wanted to keep the mortises tight side-to-side, so I plunged them at the standard setting. Surprisingly, only the last two mortises on just one side were off by about 1mm (1/16"), which was easy to tweak.

.Obviously I could have followed the instructions and widened the slots, but I wanted to see how well it would work without.

Test Fitting

Because the tenons are so tight and precise fitting, I have made it a habit to test fit my tenons to make sure I haven't made any mistakes in their placement. What you might be surprised to learn is that when I spun the board around to mortise the other side, I left this board hanging unsupported over the sawhorses. Even without full penetration, the tenons are strong enough and tight enough to support the board without sagging even with that big gap in between.

Cutting the Circle

This might be hard to believe, but I would have actually preferred to cut the circle out on the tablesaw. I thought about it for a while, and I just didn't like the notion of lifting this sheet over to the saw and rigging up a trammel point underneath it. I did this when I cut the wedges out on the original table because the router was tearing out at the joints between wedges, and I got an incredible cut with the tablesaw.

Instead, I decided to use the standard approach of using the router with a trammel. After looking around the shop for an appropriate sized scrap to make the trammel, I saw this piece hanging on the wall in an unused corner of the shop. After taking it down from the wall, I realized it was the original trammel from the first table, and I hadn't used it since then. That's when I recalled that I had used a router to cut the original substrate, and it was only the tabletop that I cut with the tablesaw.

I wasn't sure which router bit I wanted to use. I have a 3-wing straight bit, but it only has a 1/4" shank. My 1/2" bits get so much use that I knew none of them were sharp enough. I would have loved to use my 1-1/2" pattern bit, but I would have had to pre-cut the circle with a jigsaw.

Then I remembered that Carlo Venditto (the CEO of the former Jesada Tools) had given me a down-spiral, solid carbide bit for a magazine article I wrote years ago. The large and aggressive bit had catastrophic results on the small workpiece for the article, so I ended up never using it. However, on a large cut such as this, the aggressiveness coupled with the down-spiral would be perfect, and it was.

Even though the bit could have handled the job in a single pass, I still made several passes to complete the cut. I removed the router from the trammel, but left the trammel on the substrate for drawing layout lines.

Drawing the Layout Lines

While I had the trammel installed on the table, I wanted to start drawing the layout lines that I will need later to locate components and cutouts. They are hard to see here, but there are three primary layout circles drawn. The outer circle locates one screw hole in each of the 16 linear slide pillow blocks (these screw holes are visible in the first drawing above). The other two lines block out the 8 radial cutouts, which are also visible in the above drawing.

Dividing the Circle into 45º Sectors

The precision of the 45º lines on the substrate are the most critical of the project because this will be used to lay out the linear slides later on. This isn't something that could be trusted to simple measuring tools. Instead I used drafting techniques to lay out precise angles.

The first step is to draw a line through the center of the circle. I used the guide rail from my saw because this is the most accurate and long straightedge I have. Then I used a scrap of wood as a large compass to swing a series of arcs whose origin is the intersection of the diameter line and one of the circles I drew previously. When I draw a line through the intersection of these arcs, it bisects the original line at exactly 90º.

Note: Because the thin lines don't show up well in a photograph, I retraced them in my illustration software so you could see them.

The final step is drawing the two lines that bisect the two 90º lines. The concept is the same, except I used a smaller radius arc. I also swung the arcs enough to create two intersections per pair. This means that the bisecting line has to pass through four intersections plus the center of the circle. If it didn't pass through all of these points, then it would have told me that I made a mistake somewhere along the way.

Because of the mathematical nature of this method, I can be assured that the lines are accurately drawn with a precision probably down to a thousandth of a degree. That might seem like I am being anal retentive, but this is one of the hardest lessons I learned from building the first table. Every error I make in laying out the position or shape of the components cascades down the line to other components later. I'll explain this topic more when I cut the pie-shaped wedges.

Up Next: The pie-shaped wedges.