My slot-together parts fit one sheet of plywood but not the next. How do I handle thickness variation?

Why it happens

Fit is one subtraction: finished slot width minus finished tab thickness. A positive result slides, near zero is snug, and negative is a press fit that needs force. The slot width is set by your drawing, bit and machine. The tab thickness is set by the plywood, and plywood sold as one size varies from sheet to sheet and across a sheet. A slot tuned to last month's sheet is wrong for this one.

Test-fit workflow (from a published design guide)

1. Measure the actual sheet with calipers at several points, including near where the slotted parts will be cut. Note the minimum, maximum and typical value.
2. Confirm which cutter is installed and its measured diameter.
3. Draw a coupon: a row of labeled slots stepping in width around the measured thickness, changing only that one variable. A machine maker's tutorial steps in 0.003 inch increments, which is a practical ladder.
4. Cut the coupon with the same bit, feeds, speeds and cut direction you will use for the job.
5. Clean fuzz off every slot the same way, then try an offcut of the same sheet in each slot by hand.
6. Pick the winner. Keep the coupon and write on it the tool, measured stock, file and date.
7. If the parts will be painted or finished before assembly, test a finished sample too.

Applying the result without redrawing

• For slots cut as closed shapes, change the toolpath allowance instead of the vectors. In the 2D Profile Toolpath, Allowance Offset takes a negative number to cut a shape smaller and a positive number to leave it larger, per the official documentation. Preview and measure to confirm which way your slot moves.
• Use Do Separate Last Pass so the final wall cut is a light one. Size is more repeatable when the last pass removes very little.
• If the design was built with document variables for material thickness, change the variable and recalculate.

Pro tips

• Every inside corner of a slot keeps the bit radius. Add dog-bone or T-bone fillets sized to the real cutter radius so square tabs seat fully.
• Buy all the sheets for one project together and test each one.
• See the entries on the slot-together gadget and on dialing in a cross lap joint.