How to run a plywood fit test before cutting the whole project
The short answer: cut a small comb with slots at different clearances in a scrap of the same board, with the same bit. The tightest slot where the test tenon slides in by hand and does not wobble is the clearance to use in your designs.
Why it is worth five minutes
Two sheets of plywood with the same label can differ by a few hundredths of an inch, and every bit and machine cuts a little differently. A joint that fits perfectly on one board can be tight or loose on the next. A scrap test turns a guess into a number before you spend a sheet. It also tells you the right clearance for how you want to assemble: snug for glued parts, looser for parts you will take apart.
Step by step
- Measure the thickness of your board with calipers (see plywood thickness for CNC furniture).
- Open the plywood fit test generator, enter the board thickness and your bit diameter, and keep the default clearances to start with.
- Download the DXF and cut it in a scrap of the same board, with the same bit, feed and depth you will use for the real job.
- Slide the test tenon into each slot and mortise, from the tightest to the loosest.
- Choose the tightest one that goes in by hand with a small push and has no wobble. For parts you will take apart, choose the next one looser.
- Use that clearance in the material settings of your model.
How to read the result
- Needs a mallet: too tight. It will split the plywood or the glue will not spread.
- Slides in with a small push, no play: the one you want.
- Moves when you wiggle it: too loose. Glue will not hold the gap well.
A clearance above about 0.6 mm (1/32") is already loose. The usual range is 0.1 to 0.3 mm (0.004" to 0.012").
When to repeat it
Repeat the test when you change the board, the batch of plywood or the bit, and after you sharpen or replace a bit.
Why the test is worth it
The test comb cuts in a few minutes on a scrap piece. Compare that with a ruined sheet of plywood plus an hour or two of machine time, which a small shop would bill at roughly $45 per hour. A cheap test is the best insurance against a loose or tight joint. See the cost of a CNC furniture project.
Frequently asked questions
How much scrap do I need?
A piece of about 16 × 5 inches (400 × 120 mm) is enough for the default five tests. The tool tells you the exact size.
Which clearance should I pick?
The tightest one that goes in by hand with a small push and does not wobble. For parts you will take apart, choose one step looser.
Do I repeat it for every sheet?
Repeat it when you change the board, the bit or the batch of plywood.