Arched Slot Jig

Woodworking magazine articles and books often show jigs featuring arched slots, but I realized that no one teaches how to effectively make them. So here I am with a simple yet effective and safe-to-use jig.

While my jig was designed to rout tight-radius arched slots, larger radii can also be accomplished. I provided two holes for pattern guide bushings (1/2" and 1") so I would be free to use smaller and larger straight router bits according to my needs. I made mine out of HDF laminated floorboard because this manmade material is hard, stable, durable and slick on both sides; however, any good-quality hardwood plywood coated with a slick finish will work as well.

Now, here’s how to make this jig.

Start by removing all tongues and grooves from an 18" long piece of HDF laminated floorboard as wide as the base plate of the plunge router you’ll be using with the jig. Draw nice curves at one end to shape a handle, cut them with a bandsaw or jigsaw, and sand all edges smooth. A spindle sander or a sanding drum would be a perfect choice.

HDF laminated floorboard and sanding drum.

Chuck a 1" Forstner bit in the drill press and drill a centered hole 8" from the tip of the handle. Swap to a 1/2" Forstner bit, drill a second centered hole at 13" and chamfer both sides of the latter using a chamfering drill bit (shown). Those two holes will nest the required pattern guide bushings to be installed in your router.

Drilling a centered hole.

While at the drill press, chuck a 1/4" twist drill bit, drill a centered hole in the edge of a 1" × 1-1/4" (approx.) stick of hardwood and cut it at least 2" long.

Centered hole in 2" long stick of hardwood.

At the router table, using a 1/8" round over bit, ease both sides of the 1" hole and the whole perimeter of the jig on both sides, leaving the end opposite the handle sharp and square.

Hole in HDF laminated floorboard.

Flush to the end opposite the handle, drill, countersink and secure the hardwood block to the jig using two 1" #6 flat-head wood screws, making sure the 1/4" hole is centered on the end of the jig.

Hardwood block secured to jig using two flat-head wood screws.

The jig is completed and ready to use.

Completed jig.

Setting the Jig

Using a compass, draw the path of the required arched slots (two shown) as well as the pivot points directly onto your workpiece, and head to the drill press to drill the 1/4" pivot holes.

Drilling pivot holes.

If your workpiece is small, use double-sided tape to secure it to a sacrificial panel board, which must also be secured to a workstation. If your workpiece is large enough, you may clamp it to the sacrificial panel board. When done, drill through the panel board from the pivot holes of your workpiece. Those holes will receive a pivot pin to rout the slots around.

Double-sided tape used to secure workpiece to panel board.

Place the jig on the workbench, hardwood block facing up, and punch for a 1/4" hole at a distance equal to the radius of the arc you will be cutting in your workpiece from the center of one of the two holes you drilled earlier for the guide bushings. Take into consideration that the wider your slots are designed, the wider-diameter straight router bit you will be using to mill the slots, meaning now is the time to select the guide bushing you will be using later to mill the slots, and measure from the appropriate hole. Back at the drill press, drill and chamfer the upper side of a 1/4" hole meant to accept a 1/4" flat-head machine bolt (shown) that will act as a pivot pin for the jig to spin on to make the slots. Make sure the head of the bolt is below the surface so the router will ride freely and won’t catch on it.

Drilled hole and 1/4" flat-head machine bolt.

Let’s put the jig to good use

Slip the machine bolt into the pivot hole you just drilled on the jig and then thread it through one of the pivot holes of your workpiece and the sacrificial panel board. Slide a removable 12" long 1/4" dowel into the hole of the hardwood block to act as a pointer to locate where to start and stop the slots. Spin the jig around and, looking through the guide bushing hole to locate the starting point of the slot, make a pencil mark on the panel board so you’ll have the perfect sighting guide when you’re routing the slot. Repeat the process to locate and mark for the end point of the slot.

12" long 1/4" dowel in hardwood block.

Install the appropriate guide bushing in the base plate of a plunge router and chuck your straight router bit. Since this is a plunge cut, make sure your router bit is a spiral bit or is designed for such cuts, unless you’re willing to take the time to drill the starting point with your hand-held. Place your router on the jig, making sure the guide bushing is perfectly seated. Spin the router around for a track test and then set the depth of the cut so the router bit will slightly bite into the sacrificial panel board. If your workpiece is thick and/or your slot is wide, it may be wise and the right time to set the turret of your router to make several passes.

Router placed on jig.

Set the pointer close to the starting mark but within the path of the slot before plunging. If the router shifts a bit when plunging, the slot will still be within the expected area. By the way, since this cut is perfectly supported on both sides, you may spin the whole assembly from left to right or right to left to rout the slot safely. No need here to follow the usual and important left to right rule.

After milling each slot, remove the whole assembly, blow the dust off, and make sure the slot is cut all the way through and from end to end. If required, reinstall the jig on the workpiece and make another cleaning pass.

Milled slot.

When the milling process is completed, sand all edges of your slots and cut to final size, if required.

Here it is, a simple yet effective jig to mill what could be a hassle to accomplish. Remember to work safely wearing all your safety gear and have fun manufacturing and using this jig as well as all your other jigs.

Completed project with tape measure.

Cut List

Text and photos by Serge Duclos

Serge Duclos started woodworking 40 years ago after purchasing a house. He soon found it was a way to relax from the stress related to his job as a human resources professional. Since retiring in 2004, Serge continues to enjoy his pastime and to update his bilingual woodworking blog http://atelierdubricoleur.wordpress.com with his projects, as well as his tips and techniques.