You cut a box. It fits. Then you buy new plywood and nothing goes together.
That's normal. A "3 mm" sheet is often anything from 2.7 to 3.2 mm. The slots in your file were drawn for one number, and the new sheet is a different number.
Most box files "fix" this by selling a 3 mm file, a 4 mm file and a 6 mm file. That still breaks the moment your sheet is 3.2 mm.
Same file, different sheet: the slot has to follow the real thickness.
What you need
- A laser cutter
- One sheet of plywood or MDF
- Calipers (a cheap digital pair is fine)
- A free FlexCut3D account
Step 1: Measure your sheet
Measure the sheet in three places with calipers and take the average. Write it down in cm, for example 0.31.
Step 2: Open the box sample
In FlexCut3D, open Projects and pick Box Sample. It's a box with a hinged lid and finger joints, ready to change.
Step 3: Set the thickness
Move the Thickness slider to the number you measured. The joints follow it, so the tabs and slots now match your real sheet.
Step 4: Set the size
Drag the Width, Length, Base Height and Lid Height sliders. The panels resize and the joints stay where they belong.
Step 5: Set the kerf
Open the 2D panel. Set the work area to your laser bed and enter your kerf (how much material the beam burns away). Not sure what yours is? Here's how to measure your laser's kerf.
Step 6: Lay out and export
Press Auto Layout so the parts pack onto the sheet. Then press Export SVG and open the file in your laser software.
Step 7: Cut one test corner first
Cut two panels that meet at one corner and push them together. Snug is good. Too tight: lower the kerf a little. Too loose: raise it a little. Then cut the full box. Cheaper than finding out on panel six.
Why this works
The thickness isn't baked into the drawing. It's a slider. Change the sheet, change the number, and every slot follows.
Try it
Open the box sample and set your own sheet: app.flexcut3d.com
Want to understand the joints themselves? Read finger joints for laser cutting.