Router

Mortises by the rule of thirds

A plunge router with an edge guide cuts a mortise faster and straighter than a chisel, and the size is not a free choice. A mortise about a third of the stock thickness leaves enough wood either side of it that the joint fails somewhere else first — too thin and the tenon breaks, too thick and the cheeks around it split.

That third rounds conveniently onto router bits: three-quarter stock wants a quarter-inch mortise, which is a quarter-inch spiral bit, and inch-and-a-half stock wants a half.

Feed against the cutter, which means left to right

A router bit spins clockwise looking down at it. Feeding the router so the cutter pushes back against your advance keeps it in the cut and under control; feeding the other way lets it climb along the work and take off in a direction you did not choose.

On an outside edge that means moving the router left to right. In a router table, where the cutter points up and the work moves instead, it is the reverse: right to left past the bit. The rule people remember is that the cutter should always be trying to push the work away from the fence, never pull it along.

Templates are what it is really for

The deeper trick is that a router with a bearing-guided bit or a guide bushing does not cut a shape — it copies one. Make the template once out of MDF, refine it until it is exactly right, and every part off it is identical. That is how curved parts, repeated joinery and interchangeable components get made without a CNC.

A box joint is the same idea reduced to a spacing problem: a jig indexes the cut, and every finger lands where the one before it put it. The fingers have to divide the board evenly or the joint ends with a sliver, which is why the width gets adjusted rather than the count.

Depth of cut, in passes

The mistake that breaks bits and burns work is taking the full depth in one go. A quarter-inch spiral bit clearing a half-inch mortise in one pass is asking a quarter inch of carbide to remove twice its own diameter of hardwood, and it will either deflect, burn or snap.

Take it in passes of roughly the bit's diameter or less, and let the last pass be a light one — the finish cut removing a shaving rather than a chunk is what leaves a surface that needs no cleanup.

What this page does not tell you

• —The rule of thirds is a woodworking convention rather than an engineering result. It is a good default and not a calculation about the load your joint carries.
• —Blind mortise depth here stops well short of the far face so it is not broken through. A through mortise is a different layout and the calculator handles it separately.
• —Nothing on this page is a safety instruction, and a router is loud, fast and unforgiving of a loose grip. Read the manual that came with yours.