Jigs

A router T-square: grooves across a panel

A simple T-shaped fence guides a router square across a panel and shows exactly where each shelf groove will land.

Author
Published
Reading time
5 min

Grooves across a panel, often called dados or housings, are the classic way to hold shelves in a bookcase or cabinet side. The shelf sits in the groove along its whole width, so it can't drop or rock. A router with a straight bit cuts them cleanly, as long as it runs along a straight fence set square to the edge and in exactly the right place. A router T-square does both: it's a fence with a crossbar at one end, and once it's made it takes seconds to set up for each groove.

How it's built

It's nothing more than a straight fence with a crossbar fixed across one end. Push the crossbar against the edge of the panel and the fence falls square to that edge on its own. Make the fence longer than the widest panel you expect to groove; plywood or any straight strip of solid wood will do for both parts.

Set the crossbar with a square and fix it with a few brads or screws. Leave it long enough on the router side that the bit will pass over it (you'll see why in a moment).

Next, clamp the T-square to a bit of scrap, set the router to cut a couple of millimetres deep and run it along the fence and straight through the crossbar. What's left in the crossbar is a notch as wide as the bit, sitting right on the bit's path. Line that notch up with your pencil marks later on, and the groove can only go where you drew it.

A router T-square on a panel, seen from above, its parts numbered. 2: a crossbar lies against the far edge of the panel. 1: a straight fence runs from it across the panel at a right angle. 5: the round base of the router runs along the fence, with the bit at its centre. 3: a shallow notch the width of the bit, hatched, is routed across the crossbar beside the fence and lined up with two marks on the edge of the panel. 4: the groove, hatched where the router has already cut it and dashed ahead of the router, runs from the notch across the panel. An arrow shows the direction the router moves
1: fence, 2: crossbar, 3: notch in the crossbar, 4: groove, 5: router. Line the notch up with the marks on the panel edge, and the groove lands right under it. Bit width drawn larger than scale.

That only holds for the bit and router that cut the notch. Change either one and you'll want a second T-square with its own notch.

Routing the groove

Mark the groove with two pencil lines on the panel edge. Hook the crossbar over the edge, slide the T-square along until the sides of the notch sit on the lines, and clamp it down (at both ends, if it's a long one).

Take no more than about 3 mm of depth per pass, which is the usual good practice. A deeper groove just means lowering the bit and going again. Keep the base pressed against the fence all the way and don't drag the router back through the groove, since that's when it tends to wander off the fence. At the end of the cut, switch off and let the bit stop before you lift it out.

Watch the direction too. A router bit turns clockwise when you look down on it. Fed against that rotation, it tugs the router a little to the left, so with the fence on your left the tug keeps the base tight to it. Push from the wrong end and the router creeps away from the fence; you'll see it in a groove that wobbles.

Two views from above of a router running along a fence on a panel, with an arrow showing it pushed up the page and a curved arrow showing the bit turning clockwise. Left, marked correct: the fence is on the left of the router, and the bit's turning pulls the base against it. Right, marked wrong: the fence is on the right, and the router has drifted away from it, leaving a gap
The bit turns clockwise and pulls the router to the left. With the fence on your left, the base presses against it by itself.

For a groove that stops short of the far edge, pencil a mark where the base ended up on the first pass and stop there every time after.

A groove that fits the shelf exactly

Plywood sold as 18 mm is often a shade under that, and in an 18 mm groove the shelf rocks. You won't find a bit for every odd thickness, so cut the groove with a narrower one in two passes. Don't measure the width at all; take it straight from the shelf. A tape or calipers always add a little error of their own, and the shelf itself can't.

Stand the shelf on the panel edge where the groove goes and draw along both its faces. For the first pass, put one side of the notch on one line and rout. Then slide the T-square over until the other side of the notch is just shy of the second line, and rout again. (Because the bit is narrower than the shelf, the passes overlap in the middle.)

A section across a groove in a cabinet side, with the shelf that will go into it above, exactly as wide as the groove. The groove was cut in two passes with a bit narrower than the shelf: the first pass outlined dashed in ink, the second, after the fence was moved over by the arrowed distance, dashed in accent colour. Dimension lines under the groove mark the width of the bit and the extra width the second pass adds
The first pass at one line, the second after moving the T-square over. The shelf itself sets the width of the groove.

Now try the shelf. It'll probably still be tight. Good. Nudge the T-square over a hair, take off a whisker more and try again, creeping up on the fit until the shelf slides in snugly without a mallet. Going slowly costs you a few extra passes, while going too far costs a loose shelf or a new panel. It's worth running through the whole thing once on a scrap before you touch the real cabinet side.

Sources

All posts