How do I cut dados and rabbets that actually fit my plywood?

Why dados miss

Sheet goods are not the thickness on the label, and they are not even one thickness. A professional woodworking knowledge base notes variation of several thousandths of an inch between sheets and within a single sheet. A dado drawn at the nominal size is either loose or will not go together.

Sizing the dado

1. Cut or pull the actual mating parts. Measure thickness with calipers in several places and write down the thickest reading.
2. Add clearance. The same knowledge base suggests about 0.010 inch over the thickest reading for glue space. A tooling maker's joinery article uses about 0.1 mm for housing joints. Those are close to the same ballpark at the tight end, so start near the smaller figure and test.
3. The target fit, in that knowledge base's words, is snug enough to drive together with light hand pressure. Too tight scrapes the glue off. Too loose starves the bond.

Toolpath choice

• Dado wider than the bit: draw a rectangle at the sized width and use the Pocket toolpath. Offset clearing with a profile pass last leaves clean walls. Use a bit smaller than the dado so the width comes from the vector, not from the bit label.
• Dado exactly one bit wide: a Profile toolpath set to On an open line works, but the width is then whatever the bit really measures, with no way to adjust. Most people avoid this for fitted joints.
• Rabbet on an edge: draw the rectangle so it extends past the edge of the part, then pocket it. If the rectangle stops at the edge, the bit radius leaves a rounded lip there.
• Through or stopped: a stopped dado ends in two rounded corners. Either round the shelf corner, notch the shelf, or add T-bone fillets at the stopped end.

Order and depth

1. Cut dados and rabbets before profiling the part out of the sheet, while it is still held firmly.
2. Cut one test dado in an offcut of the same sheet and try the real shelf in it.
3. If it is tight, widen the rectangle by a few thousandths and rerun. Keep notes per sheet batch.

Pro tips

• Common shop advice, not from the official documentation: upcut spirals can chip the face veneer of plywood, and a downcut or compression bit often gives a cleaner dado edge. Test on scrap.
• Enter the measured bit diameter in the toolpath with Edit, since bits are rarely their nominal size.
• See the entry on the Pocket toolpath for clearing options and the entry on dog-bone versus T-bone fillets.