Pat Warner's Two-Part Dado Jig
In addition to writing four router books, Pat Warner has designed specialty router bits and is currently developing a collection of inexpensive, disposable, single-flute mortising bits.
"Accuracy in routing requires attention to detail—and not much sophistication," Pat notes. This Californian's simple jig helps match dado width to shelf thickness. Using Pat's two-part jig, we set up to cut a dado in less time than it takes to equip a tablesaw with a dado set.
You'll need to assemble two of the jigs, shown above. (We cut two pairs of jigs, one from 3⁄4 "-thick plywood and another using medium-density fiberboard (MDF). If you plan to make dadoes 10" or longer, build additional pairs with the top pieces at least 12" long or more.
Before you begin cutting dadoes, you'll need a sample of the stock that the dado will ultimately hold in your project. Here's one key to a snug dado: Go through each sanding step you plan to follow until your sample piece reaches its finished thickness.
For this example, we're cutting a dado in the side of a cabinet for a shelf. Position the two-part jig where you plan to cut the dado; then snug the sanded shelf scrap between the parts, as shown in photo A. Next, clamp both parts firmly in place at the edges away from the gap to keep the clamps from interfering with the router.
After removing the sample (save this piece for future reference), set your router cutting depth equal to the thickness of the jig parts plus the depth of cut you want. Then rout a dado using a pattern-cutting bit (also sold as a shank-bearing guided trimmer). The bearing of this bit rides along the edge of the jig parts and produces a crisp, square dado, as shown in the drawing below.
"Be sure the cutter isn't larger than the bearing," Pat cautions, "or else you'll tear up the edges of your jig parts."
To avoid tearing out the edge of your workpiece as you finish your cut, as shown below, clamp a sacrificial piece of scrapwood to the edge of your work. Here are other tactics Pat recommends to reduce tear-out:
Good material. Routing straight-grained, properly seasoned hardwood reduces tear-out. Interesting grain patterns, including bird's-eye and quilted maple, have visual appeal, but create more routing challenges.
Well-prepared stock. Wherever possible, eliminate cups and bows in your stock before you get to the routing steps.
Sharpen cutters. Well-honed bits produce less tear-out.
Light cuts. If you cut deeper than 3⁄8 " with one pass, you're apt to introduce tear-out to your project. 18" is ideal.
Feed rate. You'll worsen tear-out with a fast feed rate.
Climb-cutting. You can reduce tear-out with a climb cut (cutting with the rotation of the bit instead of against it). Light cuts and added safety procedures are a must.
Cut end grain first. Because end grain is more apt to tear out, rout it first. Then rout edge grain and clean up any tear-out.