My inspiration for this project came from a wooden
chain I made years ago using a handheld plunge router
and plans from Patrick Spielman’s New Router Handbook
(1993). Since making the chain involved routing
a bunch of the same parts, it seemed like a good project
for a CNC. Patrick used a one-link-at-a-time routing
jig setup that partly relied on the router’s base to
hold the links steady during routing—something not
possible with a CNC. So making the links on the CNC
became a good exercise in designing CNC small parts
jigging. I had three goals in mind while developing the
jigs. I wanted them to be simple to make, easy to use,
and sufficiently durable. Aft er trying various methods
involving clamps and hold-downs, I settled on a combination
of recessed cutouts, wedges and small turnbuckles.
But the real secret to these jigs came from the
CNC’s ability to easily and precisely create matching
parts that fit snugly together like puzzle pieces.

Click any image to view a larger version.

Jig 1: Routing Inside the Links

Rather than make this jig out of separate pieces of wood, I found it easier to rout it as a recess in a piece of MDF. I used three
wedges to hold the workpiece in place while routing. The bottom wedge forced the workpiece tight against the upper end of the
jig, while the two side wedges pushed the workpiece to the left. The wedges were also created with the CNC, so matching the
tapers in the recess to the angle of the wedges was a breeze. The wedges have a 1 in 20 taper, which made them easy to secure
and remove with a couple stiff mallet taps. The wedges proved very secure, as they never once vibrated loose during routing. I
also routed an undercut around the bottom of the recess with a T-slot router bit. The undercut insured that the workpiece didn’t
hang up on any stray wood chips at the bottom of the jig.

Jig 2: Routing Outside the Links

Creating the second jig required the most experimenting. I initially created it as a recess similar to jig 1, but the MDF proved
too weak and the center posts easily broke off. A piece of Baltic birch plywood glued to the top of the MDF created a much
stronger jig. I made the center posts .05" shorter than the thickness of the work piece. This made it easy to apply pressure with
the turnbuckles. The center posts were also .05" smaller in diameter than the inside of the links. This slight gap was needed so the
workpiece could slide onto the posts without binding, but was still snug enough to prevent the workpiece from shifting during
routing. A couple workpieces were slightly warped, which made them hard to slip on to the center posts. A few taps from a mallet
solved that problem. Once the links were routed into separate parts, they were easily removed.

Rounding Over the Outside

Jig 2 served a dual purpose. Once all the links were routed into
separate parts, I switched to an ovolo bit and rounded over the
outside corners. The links were flipped over to do the other side.
The snug fit on the center posts and the turnbuckles helped to
hold the link securely in place for this step.

Only Three Bits are Required

The 1/2" dia. straight bit did the heavy work of removing
stock from both the jigs and the chain links. The slotting bit
created the undercut at the bottom of jigs 1 and 2 to prevent
stray wood chips from getting in the way. The 1/4" radius
ovolo bit gave the links their round shape.

Jig 3: Rounding Over the Inside

Similar to the first jig, the third jig used wedges in a recess to hold the parts in place. This jig really took advantage of the CNC’s
ability to cut parts that fit together like a glove. The recessed pockets for the links were cut the same size as the link, without any
clearance gap. This created a very snug fit and required a couple extra mallet taps on the wedges to make sure the links were fully
seated. This snug fit insured that the links didn’t move or vibrate while I rounded over the inside corners. After the first side was
done, I flipped the links over to rout the other side.

Note: Although CNC’s are capable of precise machining, you should always test your setups and adjust the dimensions of your
jigs, parts and tool paths to accommodate slight variations in materials and bit diameters.

A Quick Sanding

Sanding each link took a minute or two per link, but
removing the machine marks at this stage was easier than
doing it after assembly. Next time I’ll use a flap sander or
inflatable drum sander and save my finger tips.

Break Every Other Link

A quick hit with a mallet was all it took to crack the links
in half. I used quartersawn boards for this project because
when broken, they tend to create flatter joints than
plainsawn wood. The flat joints made reassembly easier.

Glue and Clamp Back Together

I assembled the chain by adding two unbroken links to each broken link. Then I assembled these three link sections with
more broken links until the chain was complete. Because the links were broken on the grain, the glue joints were nearly or
completely invisible. Using a light application of glue and removing the squeeze out while it was still soft made cleanup
sanding easy. After it was done, I dipped the chain in an oil finish a couple of times and rubbed it dry with a cloth.

The Dimensions

The six links started out as a board measuring 5/8" x 2" x 20". The .6" spacing between the links provided the necessary
clearance for the 1/2" dia. straight bit and the bottom end of the ovolo bit. The six links produced 11-1/2" of finished chain.
Drawings for the three jigs can be downloaded at AmericanWoodworker.com/CNC.

This story originally appeared in American Woodworker June/July 2012, issue #160.