Material thickness and clearance: why the joint will not go in
Nearly every joint that will not go together — or that wobbles — comes down to two numbers. They matter more than any drawing trick, and they both live in the “Material” panel.
The real thickness, not the label
Plywood is sold at nominal sizes: 3 mm, 4 mm, 6 mm. Those numbers are indicative. A poplar sheet sold as 3 mm often measures 2.8; a birch sheet sold as 4 mm can be anywhere between 3.8 and 4.2, and vary by a few tenths from one corner of the same sheet to the other.
The program uses the thickness to work out the width of every slot. If you type 3 and the sheet measures 2.8, every joint will be two tenths too wide: the part goes in and moves.
Measure it with a caliper, at three or four points across the sheet, and type the average. It takes ten seconds and solves half your problems before they exist.
If one part of the project is made of something else, do not change the global thickness: select that part and use “Thickness different from the material” in its properties.
Clearance, or how loose you want it
Two parts cut to the exact size will not go together. Wood is not perfectly flat, the cut is not perfectly vertical, and a whisker of resin on the edge is enough to jam everything. Clearance widens every slot by a fraction of a millimetre per side, on purpose.
| Clearance | How it goes in | When to use it |
|---|---|---|
| 0.1 mm | Tight, needs a rubber mallet | Objects never meant to come apart, maximum hold |
| 0.2 mm | By hand, with some effort | Starting point, fine for nearly everything |
| 0.3 mm | Smoothly, with a little play | Parts assembled and taken apart often, or glued |
| 0.4 mm and up | Loose | Only with glue, or for materials that swell |
The test that beats any table
Before cutting a whole project, cut one test joint: two 40 mm rectangles crossed, nothing more. Try them by hand. If they go together the way you want, the numbers are right for that material and you never touch them again. If not, change a tenth and repeat.
Ten minutes of testing saves a whole sheet, and it is the habit that separates cutting with a plan from cutting and hoping.
What about the kerf?
That is the third number, and it does a different job: it corrects the material the laser burns away. Thickness and clearance are about the design, the kerf is about the machine. See the kerf guide.
Try it now
Open the app, type the real thickness of your plywood and set the clearance to 0.2 mm: that is the setup that gets ninety per cent of projects right the first time.
Plyjoin works out the joints for you and gives you a DXF ready for the machine. It runs in your browser, with nothing to install.
Try it now