Setting the Table

Our first attempt at building the ShopBot’s work table surface worked but as we learned how the robot operates we found some improvements to make.

The Start

Our original table design was the first layer of 3/4in “cabinet” grade plywood from Home Depot attached with carriage bolts. The second layer of 3/4in plywood was attached to the first with drywall screws. The third layer, the spoilboard, was 1/2in MDF. All layers were 48in x 96in. The frame of our ShopBot allows for interior dimensions of 61.5in x 120in.

Make It Better

The 48in x 96in sheets left a gap between the table surface and the rails. Tools, dust, scraps, and everything else then feel through the gaps and ended up under the table in the hardest places to reach. The gap did provide edges that could be clamped to but we’ve found some other holddown techniques.

We’ve learned more about wood and other materials since we started and have found a replacement for home improvement level plywood. We’ve upgraded to Baltic Birch plywood. It is available in a European 60in x 60in format at 3/4in from 11 ply. The 60in width closes most of the gap between the surface and frame.

MDF (Medium Density Fiberboard) cuts like butter, holds an edge, easy to paint, and is inexpensive. But for spoilboard, it is a bad choice. Changes in humidity change the surface’s dimensions because we didn’t seal it. The material crumbles under abrasion and easily delaminates. The material we should have used is MDO (Medium Density Overlay - Outdoor) that is waterproofed with resin and much more durable. We sourced some at our favorite sheet goods supplier, Roddis Lumber, for our new spoilboard and increased it to 3/4in.

Get It Done

First things first

The first layer of plywood, the support layer, needed pockets cut on the edges to fit around the table legs. Without the ShopBot, we had to revert to hand tools and human errors. Rectangular cut-outs were a job for a jigsaw.

Testing the plywood fit.

The advantage of using 96in plywood is that it spans all three sets of table legs and cross braces. 60in plywood puts the seam of the two edges unsupported in the middle of the table. To join the pieces of plywood we looked at biscuit joints we could cut with our trim router, but the guys at Woodcraft recommended against them because they don’t provide strength and are more for alignment. They pointed out joint connectors that are used for joining countertops.

We tried the joint connectors but they are designed for material thicker than 3/4in. As a backup, we’d grabbed supplies to to try dowels.

Doweling

We had a bag of 1/4in x 1-1/4in dowels, a drill bit and depth collar, and a small dowelling jig.

Glueing dowels in one edge.

Ratcheted straps hold panels in place while the glue dries overnight.

Layer 2 of Support Surface

The first layer is bolted to the ShopBot frame. This layer of Baltic Birch is screwed into the first layer. We cut one of the 60x60 sheets in half so the seams don’t line up with those in the first layer. Dowels were added but only every 4 inches and left to dry overnight.

For the second layer, we laid out a grid on full inch increments, 6in x 6in, to avoid the bolts in the first layer that are on 1/2in measurements. More screws are placed around the edges of the layer. We used Spax #8x 1-1/2in construction screws

After all the screws are installed, the surface was sanded.

To seal the raw wood, we apply Minwax clear satin polyurethane.

We let the polyurethane dry overnight.

Third Layer - Alignment

This layer is a little different than other people do it. We are optimizing for working with full sheets of plywood and table setup flexibility. Most people don’t do this layer and would be installing their spoilboard. We’re using this layer to align our spoilboard and provide an escape plan. We will attach our spoilboard to this layer so we could remove them both in the future instead of having to redo the upper support layer. We are sacrificing 3/4in of Z-axis travel but that isn’t important for working with sheet goods. This layer is 3/4in MDO, a moisture resistant version of fiberboard.

To check the layer alignment, we use the SuperZero fixture. We place the layer 6.75in from the X-axis rail and 8.5in from the front edge of the Support table. This allows the bit to travel at least an inch wider and longer than the spoilboard edges.

Before attaching the layer we quickly test fit the Woodpecker Super Track we’ll use for holddown.

We attach the MDO with stainless exterior screws and countersink them so the next layer will lay as fast as possible to get the best glue adhesion. This will let us completely machine the spoilboard off with out hitting any the screws below it.

First layer of MDO is installed.

We will be securing the spoilboard will Epoxy resin so we need to rough up the surface of the MDO with a sander.

To ensure the spoilboard alignment, we install some extra 80/20 brackets we had around the edges.

For our first exposure to epoxy, we use System Three General Purpose Resin with the #2 Medium Hardener that was recommended at Woodcraft. This should give us about 30 minutes to work with it.

We use a long handle paint roller to apply the epoxy.

The bottom of spoilboard is also coated with epoxy.

Once the spoilboard is placed on the table and aligned, we stack on weights to make sure the spoilboard stays in place while the epoxy sets. We let it sit over the weekend to cure undisturbed.

After the resins cured for 48+ hours, we removed the weights and checked the spoilboard. That spoilboard is not going anywhere unless it leaves as sawdust. The final step is surfacing the table. We zero an large bit for the job.

Using a 1.5” diameter bit, the entire spoilboard height is reduced by 0.08in giving a us a perfectly level surface.

The job takes about 20 minutes to cut the entire surface in concentric rectangles. You can see tool marks from the passes but you cannot feel them.

Next Steps?

1. Install T-Tracks for holddown
2. Cleanup Wiring
3. Lubricate Gears, Wheels, & Tracks