Short answer: Size each slot from the measured thickness of the actual sheet plus about 0.004 to 0.008 in. of clearance for a slip fit, not from the nominal size. Make tabs roughly 3 to 6 times the sheet thickness long, keep slots at least two sheet thicknesses from an edge, and glue slip-fit joints instead of forcing a press fit. Laser-cut slots don't need dogbone reliefs; routed slots do.
Key facts
- Slot width: measured sheet thickness + 0.004 to 0.008 in. for a slip fit.
- Nominal isn't actual: "1/4 in." acrylic is usually made as 6 mm (0.236 in.) and varies across the sheet.
- Tab length: about 3 to 6 times the sheet thickness.
- Edge distance: at least 2 sheet thicknesses between a slot and the part edge.
- Kerf: about 0.006 to 0.010 in.; we compensate when you draw to finished size.
- Dogbones: needed for routed slots, not laser-cut ones.
Start with the real sheet thickness
A slot is only as good as the number you size it from. Acrylic sold as 1/4 in. is usually made to 6 mm (0.236 in.) and often measures somewhere between 0.21 and 0.25 in., depending on the sheet and where you measure. Draw slots at 0.250 in. and your joints will rattle. See actual acrylic sheet thickness for every size.
Measure the sheet with calipers in several places and use the average, or the thickest reading if every slot must accept every tab. Then add clearance:
| Fit | Slot width | Use |
|---|---|---|
| Loose | Thickness + 0.010 to 0.015 in. | Easy assembly, parts that come apart, joints filled with thickened cement |
| Slip | Thickness + 0.004 to 0.008 in. | Most boxes and stands, solvent-glued joints |
| Snug | Thickness + 0 to 0.003 in. | Dry-fit stands; test first |
The laser's kerf (about 0.006 to 0.010 in.) widens slots and shrinks tabs slightly. Draw to finished size and tell us the parts slot together; we compensate and can measure the sheet we cut from. More in acrylic laser kerf.
How long should tabs and fingers be?
- Tab depth (how far it goes into the slot) equals the thickness of the mating part, so the tab ends flush. Some designers make it 0.010 in. short so it never sticks out.
- Tab length along the edge: roughly 3 to 6 times the sheet thickness. For 1/8 in. acrylic, about 3/8 to 3/4 in.; for 1/4 in., about 3/4 to 1-1/2 in.
- Spacing: on a long edge, several medium tabs hold better and stay straighter than one long tab.
- Distance from edges: keep slots at least two sheet thicknesses from the part's edge. Plaskolite's Optix guide asks for the same margin around fastener holes (at least twice the hole size and twice the part thickness), because a thin strip of acrylic next to a cutout is where cracks start.
- Inside corners: a tiny radius (about 0.010 to 0.020 in.) at the ends of slots spreads stress better than a dead-sharp corner, without stopping the tab from seating.
Finger joints (box joints)
- Use an odd number of fingers along each edge so both ends match and the corners look the same.
- Finger width around 3 to 5 times the sheet thickness is a good-looking, strong proportion.
- Finger length equals the mating panel's thickness, so measured thickness matters here too.
- Where three panels meet at a corner, check the drawing carefully: it's easy to leave a corner missing or doubled.
Box generator programs can lay these out for you. Set the material thickness to the measured value, not the nominal one.
Do you need dogbones or relief cuts?
For laser-cut parts, no. The beam is only a few thousandths of an inch wide, so inside corners come out essentially sharp and a square tab seats fully.
For routed parts, yes. A router bit leaves a radius in every inside corner equal to its own radius, often 1/16 to 1/8 in. A square tab won't seat in a slot with rounded corners. Add a dogbone (a small circle at each inside corner) or a T-bone (a relief along one side) so the bit clears the corner. This matters for thick parts we route rather than laser cut.
Should you glue or press fit tabs?
Glue them. Acrylic stretches only about 4% before it breaks (Acrylite lists 4.2% for cast sheet), and Acrylite warns that sustained stress well below its breaking strength can cause crazing; it advises keeping continuous loads under 1,500 psi. A tight press fit holds that kind of stress in the slot wall permanently. A slip fit with solvent cement carries no built-in stress and makes a strong joint. See press-fit tolerances in acrylic and how to glue acrylic.
Caution with laser-cut edges: the Plexiglas fabrication manual says not to flow solvent cement onto laser-cut, flame-polished, or dry-sanded edges, because those surfaces carry the most fabrication stress. Annealing the parts first, using a thickened cement sparingly, or ordering machined edges for the joints all lower the crazing risk.
Water-thin cement (Weld-On 3 or 4, SCIGRIP 4) needs a close fit to wick in. If your slots are on the loose side, a thickened cement such as Weld-On 16 or SCIGRIP 16 fills small gaps better.
T-slot joints for bolting
For an assembly you can take apart, use a T-slot: a slot cut into the edge of one panel with a wider cross slot that traps a square or hex nut, and a matching hole in the other panel. A machine screw passes through the hole into the nut.
- Size the nut pocket to the nut plus a little clearance, and the slot to the screw's clearance diameter.
- Keep the nut pocket at least two sheet thicknesses from the panel's edge.
- Use 1/4 in. or thicker sheet for the panel holding the nut; 1/8 in. is too thin to trap most nuts well.
- Use pan- or round-head screws with washers, not countersunk heads, and tighten gently. Use nylon-insert lock nuts instead of liquid threadlocker, which Plaskolite warns can crack and craze acrylic.
This is the standard joint for enclosures and machine guards (acrylic electronics enclosures).
Test before cutting the whole job
For anything with many joints, cut a test piece first: a comb of slots in 0.002 in. steps around your target width, plus a tab, from the same sheet as the job. When you're ready, upload your drawing and note that the parts slot together.
Questions
What if my parts are cut from different sheets?
Thickness can differ between sheets, so a slot sized for one may not fit a tab from another. Tell us the parts fit together and we'll cut them from the same sheet where possible.
What slot width should I use for 1/8 in. acrylic?
Measure your sheet first. Most 1/8 in. acrylic is made as 3 mm (0.118 in.) and measures about 0.110 to 0.125 in., so a slip-fit slot is typically 0.118 to 0.130 in. Cut a test comb if the fit matters.
Are tab and slot joints strong?
Glued, yes: well-fitted solvent-bonded joints are strong. Dry, they rely on friction and fit, which is fine for stands and organizers but not for anything that carries weight or gets moved often.
Why did my acrylic crack next to a slot?
Usually the slot was too tight, too close to the edge, or the joint met a solvent or cleaner while under stress. Leave at least two sheet thicknesses of material beside slots, use a slip fit, and avoid alcohol and ammonia cleaners.