Plywood thickness for CNC furniture: why 3/4" isn't 3/4"
The short answer: plywood sold as 3/4" usually measures about 23/32" (0.719", around 18 mm), and it can be thinner. For CNC joints that fit, measure your own sheets with calipers and enter that number, not the label.
Why the label and the board do not match
Plywood is sold by a nominal thickness. The real thickness depends on the plies, the sanding and the moisture in the wood, and it changes from one sheet to the next and sometimes across the same sheet. Metric plywood sold as 18 mm can also come in a little under. The result is the same: a slot cut for exactly 3/4" is too wide for a 23/32" board.
What goes wrong in a CNC joint
- Slot too narrow for the board: the parts do not go together, or they split the plywood when you force them.
- Slot too wide: the joint wobbles, and glue has to fill a gap that it cannot hold well.
The fix is not a more precise machine. It is the right number in your file.
How to measure it
- Use calipers, not a tape measure.
- Measure in at least three spots on each sheet, away from the edges, and use the most common reading.
- If your readings differ by more than a few hundredths of an inch (about 0.5 mm), buy the sheets for a project from the same batch, or cut a fit test for each sheet.
Set it in your file, then add the right clearance
Every model on this site calculates its slots, tenons and pockets from the thickness you enter. Open the material settings of a model, type the real thickness and the plans update. On top of that, a small clearance makes the joint easy to assemble: about 0.1 to 0.2 mm (0.004" to 0.008") for a snug fit, and 0.3 to 0.5 mm (0.012" to 0.020") for parts you will take apart.
Check it with a fit test
A scrap cut takes five minutes and saves a sheet. The plywood fit test generator makes a small comb with slots at different clearances: cut it in your board with your bit, slide the test tenon in and pick the clearance that fits. Read how in how to run a fit test.
Frequently asked questions
How thick is 3/4 inch plywood really?
Usually about 23/32" (0.719", around 18 mm), and it can be thinner. Measure your own sheets with calipers.
Do I need to measure every sheet?
Measure at least a few spots on each sheet. Sheets from the same batch are usually close, but they can still differ.
What clearance should I use?
About 0.1 to 0.2 mm (0.004" to 0.008") for a snug fit, and 0.3 to 0.5 mm (0.012" to 0.020") if you will assemble and disassemble it. A fit test tells you for your own board.