Woodworking Tip of the Day: Router Circle Jig

Round holes are where a nice boat starts lookin' homemade….. A wobbly jigsaw circle around a deck plate or an inspection port shows from across the yard, and no amount of sanding turns it round again. A router on a simple trammel arm cuts a dead-round hole or disc every time, right to the size you set, in about the time it takes to find the jigsaw blades. The one thing you have to get right is which edge of the bit you measure to.

Why it matters: On the Chamberlain Marblehead gunning dory I've got round inspection ports to cut, and the same jig handles the radiused corners on hatch openings in decks and bulkheads, coaming corners, and lightening holes. A hatch or port flange covers a bad cut only so far, and a round port wants a round hole so it seals evenly all the way around. A trammel is just a compass for your router: one end holds the router, the other end has a pivot pin that drops into a hole at the center of the circle. Fine Woodworking, WOOD, Woodsmith and This Old House all publish versions of it, and the bigger router makers' manuals cover the feed direction. On the GC side it cuts speaker holes, round tabletops and arched trim just the same.

Materials

• An arm for the jig: a strip of ¼″ plywood or hardboard. Mine's about 6″ × 24″, which is just an example. Make it long enough for the biggest radius you'll cut and wide enough for your router's base. This Old House uses a board about ½″ thick.
• Machine screws to mount the router to the arm, in place of its subbase (use the subbase as a drilling template)
• A pivot pin. WOOD Magazine's jig uses a ¼″ dowel that drops into a ¼″ hole, ¼″ deep, and Fine Woodworking's Yeung Chan uses the same ¼″ hole. The Jasper circle guides come with a ⅛″ pin. This Old House's quick version uses a finish nail, because a screw can bind as you turn.
• A sacrificial backer: rigid foam insulation, MDF or scrap plywood (WOOD and Woodsmith both use foam)
• Double-sided tape, and a couple of short screws

Tools

• A plunge router. Woodsmith prefers one for this jig because it gives you better control starting and ending the cut.
• A ¼″ upcut spiral bit (the one WOOD's jig is built around, and Woodcraft says spiral carbide upcut bits work particularly well) or any straight bit. Measure your bit with calipers instead of trusting the label.
• Drill with a ¼″ brad-point bit (or whatever matches your pin)
• Tape measure or steel rule, square, pencil, awl
• Calipers if you have them
• Shop vac, safety glasses, hearing protection and a dust mask

Steps

1. Build the arm. Unplug the router. Take off the subbase and use it as a template to drill and countersink the mounting holes in one end of the arm, with the bit hole dead center on the router. Mount the router, chuck the bit, and plunge it through to make the bit hole. Then draw a centerline down the arm from the bit. Pivot holes go on that line.
2. Know which edge to measure to. This is the whole tip. The bit cuts a kerf as wide as the bit, and one side of that kerf is your finished edge.
   • Cutting a hole (you keep the panel, the disc is waste): measure from the pivot to the far edge of the bit. That distance is the radius of the hole.
   • Cutting a disc (you keep the disc): measure from the pivot to the near edge of the bit. That's the radius of the disc.
   If your jig is set off the center of the bit instead, it's the same thing in arithmetic: hole radius minus half the bit, disc radius plus half the bit. Example: for a 6″ hole with a ¼″ bit, that's 3″ from the pivot to the far edge, which is 2⅞″ to the center of the bit. For a 6″ disc it's 3″ to the near edge, or 3⅛″ to the center. Woodsmith calibrates its jig off the inside edge of the bit for exactly this reason, and the Jasper guides' scale reads true for holes and tells you to add twice the bit diameter to the setting for a disc. Get it backwards and the radius is off by a full bit width, so the diameter is off by two: with a ¼″ bit, your 6″ hole comes out 5½″, or your 6″ disc comes out 6½″. On a deck plate, too small is the good mistake, because you can open it up.
3. Mark the center and drill the pivot hole. For a hole, the center sits in the waste disc, so drill right through the face. For a disc you want to show off, WOOD and Woodsmith both drill the pivot hole in the back face so it never shows. Drill straight and snug. A sloppy pivot hole makes an egg, not a circle.
4. Set the radius and test it on scrap. Drill the pivot hole in the arm at your measured radius (or set your sliding pin block if you built the fancy version). Then cut a test circle in scrap the same thickness and measure it. Ten minutes on scrap beats an oversized hole in a deck you just glassed.
5. Back it up and pin down the waste. Lay the work on foam or scrap so the bit has somewhere to go besides your bench. Here's the part people miss: on the last pass the disc breaks free. If you're cutting a hole, that disc is carrying your pivot. If it shifts, you've lost your center, and if it spins, it can grab the bit and gouge the edge you're keeping. So before the last pass, tape the waste to the backer with double-sided tape or drive a screw through the waste into the backer. Infinity Tools tapes the work to the backer. Another way to go is to stop a hair short of breaking through and finish with a knife or a flush-trim bit.
6. Rout in shallow passes. Plunge, go once around, raise the bit, vacuum the chips, go deeper. WOOD says ⅛″ to ¼″ per pass. Fine Woodworking and This Old House start with a first pass of ⅛″ or less, and This Old House keeps going down another ⅛″ at a time. Bosch and DeWalt both say several light passes instead of one heavy one. Example: ½″ dory-bulkhead ply at ⅛″ a pass is four trips around. Last one just kisses the backer.
7. Mind the direction, especially on the clean-up pass. Seen from above, the bit spins clockwise. DeWalt's manuals put it simply: counterclockwise on an outside cut, clockwise on an inside cut. Bosch says the same thing. Cutting a disc, the edge you keep is an outside edge, so go counterclockwise. Cutting a hole, the edge you keep is the inside of the opening, so go clockwise. In a full-width slot both walls are cutting, so it matters most once you've opened things up and you're taking a light clean-up pass. Fine Woodworking's Chan and Woodsmith both jigsaw most of the waste off and finish with a light router pass. DeWalt warns against climb cutting, and if you have to do it, take very small bites.

For hatch openings with radiused corners: treat each corner as a hole. Lay out the opening, mark each corner's center one radius in from both sides (it's in the waste), set the jig to the corner radius to the far edge of the bit, and rout each corner. Then join the straights with the router along a clamped straightedge, or jigsaw close and flush-trim. Example in the plan: a 16″ × 12″ opening with 3″ corners. For a round inspection port, cut to the cutout size on the maker's template, not the nominal size printed on the package.

MOAM's shop note: The most common mistake is measuring to the wrong edge of the bit. Write "HOLE = FAR EDGE, DISC = NEAR EDGE" right on the arm with a marker and you'll never think about it again. The second is letting the waste disc go free on the last pass, so tape it or screw it down first. Pro trick: on marine ply, I seal the fresh-cut edge with a coat or two of epoxy before the port or hatch goes in. That's my own practice, not a number from a book. A raw plywood edge in a deck is where the water goes looking first.

Sources: Yeung Chan, "Cutting Circles with a Router" (Fine Woodworking #177) and "5 Smart Router Jigs" (Fine Woodworking); WOOD Magazine, "Router Jigs" and "Simple circle-routing jig runs rings around other trammels"; Woodsmith, "Router Jig for Cutting Circles"; This Old House, "How to Cut Circles With a Router"; Woodcraft, "Circle-Cutting Jig"; Infinity Tools, "How To Cut Perfect Circles using your Router" (Jasper circle guides); DeWalt DW616 and DWP611 router instruction manuals; Bosch 1615EVS and GOF13-25 router manuals.

MOAM Out.