CNC Two-sided Machining

Skilled woodturners can easily match a bowl’s inside and outside surfaces by eye and touch. Creating two-sided shapes on the CNC, however, requires a different strategy: To ensure that both sides align, the top and bottom shapes must be positioned carefully during both the design and the machining steps. I’ll demonstrate the process using the lid of the box shown here. (The bottom of the box is machined using the same setup even though the bottom has a different shape and wall thickness.) To set this box apart from a typical round turning, I made it as an oval with a weave pattern on top. In fact this process can be used with almost any shape—one of the many benefits of CNC machining.

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Step 1: Work from the Center

Layout each side of your design on a pair of centerlines. Then locate index holes at opposite ends of the long centerline, outside the machined area. Here, I’ve started with the lid’s bottom side (Side 1). Its concave oval shape measures 1/2" deep x 4-1/2" wide x 7-1/2" long. This lid is designed to be made from 3/4" thick stock."

Step 2: Layout Side 2

The size of side 2 determines the thickness of the project. To create the 1/4" thickness for this lid, I added 1/4" to the measurements of Side 1. Thus, the model for the top side of the lid (Side 2) measures 3/4" thick x 5" wide x 8" long. In addition, I added a 1/2" wide flange around the lid to provide clearance for the small router bit used during the finish routing phase (see Step 7).

Step 3: Rout Index Holes in the Deck

Use a straight bit to rout a pair of index holes 1/2" deep into the deck of your CNC or use an auxiliary deck board. For this project, I fastened a piece of 3/4" MDF to the aluminum deck of the CNC Shark Pro Plus I was using. The centerlines on the deck served as the X-Y (left and right) reference point for setting the router bit’s location.

Step 4: Rout Index Holes in Side 1

Fasten your work piece to the deck and rout 1/2" deep index holes into it. This establishes Side 1 of your project—the underside of the lid in this case. The index pins I used were 1" long so the 1/2" deep holes worked just fine. The workpiece does not have to be perfectly centered with the centerlines on the deck for this step, because the index holes and pins will correctly align the blank when it’s turned over to rout Side 2.

Step 5: Rout Side 1

To create the lid's concave underside surface, I used a 1/4" ballnose bit for both the roughing and finishing passes, but I routed across the grain on the roughing passes and with the grain on the finishing passes. The finishing pass left only light mill marks that were easily removed with 120 grit sand paper.

Step 6: Insert the Index Pins

Remove the blank and insert index pins in the deck. Then flip over the blank and install it on the pins. Here I used 1" x .30" dia. aluminum dowels for index pins, but I’ve also used nylon dowels (both are available at most hardware stores). Pieces of a round pencil also work quite well. They’re cheap and easy to replace when eaten by the dust collector.

Step 7: Rout Side 2

Machine side 2 similar to Side 1. Start with roughing passes across the grain, using the 1/4" ballnose bit. The flange around the lid is routed during the roughing phase. To achieve more detail in the lid’s weave pattern, I switched to a 1/8" dia. ballnose bit for the finishing pass (shown above). The finishing pass stays on the weave, and the flange provides clearance to keep the small bit from contacting the tall shoulder.

Step 8: Cutout and Complete

Rout around the oval and through the blank using a 1/4" straight bit. Leave tabs to keep the lid attached. Remove the blank and finish cutting out the lid on the bandsaw. Then use a disc sander to smooth the lid’s edge. Clean up the edges of the weave pattern by hand with a V-Parting carving chisel. The 3D model for this project can be downloaded at AmericanWoodworker.com/CNC.

This story originally appeared in American Woodworker August/September 2012, issue #161.

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