Search for CAD software for laser cutting and you get two very different kinds of answer: vector drawing programs like Inkscape, Illustrator and Affinity Designer, and actual CAD, which includes Fusion, FreeCAD and parametric tools built for cutting, like FlexCut3D. Both will hand you a cuttable SVG. Only one of them survives a change of material.
This walks through that difference on a small real part: a phone stand made of four pieces of 3 mm plywood that slot together without glue. Along the way it covers the two numbers that decide whether a tab-and-slot joint holds, which are worth knowing whichever program you use.
Drawing a part versus specifying one
In a vector program you draw a 3.0 mm slot. It is 3.0 mm because you typed 3.0 into a box, and nothing in the file records why. Move to 6 mm ply and every slot, every tab and every panel position is now wrong. The only fix is to find each one and retype it, and the parts you miss are the ones you find out about after cutting.
In CAD you say the slot is material thickness minus kerf. The number is derived from something, so when the something changes, the number follows. That is the entire distinction, and it costs you nothing on the first sheet and everything on the second.
The part
The phone stand is four pieces cut from one small offcut of 3 mm plywood:
two side panels, each cut to the profile of the stand: a foot, an upturned lip at the front, and a back post leaning 20 degrees off vertical
two crossbars that pass through slots in the side panels and set the width of the stand
The phone sits in the channel between the lip and the post, resting on the top edges of the two sides. Nothing is glued. The bars carry tabs, the sides carry slots, and the tabs pass all the way through and finish flush with the outside face.
The two numbers that decide the fit
1. Tab length equals the ACTUAL thickness of the panel it passes through
Not the nominal thickness. 3 mm plywood is almost never 3.00 mm. Put calipers on a sheet and you will usually read somewhere between 2.8 and 3.2, and it changes between deliveries from the same supplier. Cut the tab shorter than the panel and it vanishes inside the slot with nothing to lock against. Cut it longer and it stands proud of the face.
Measure the sheet. Use the number you measured, not the number on the label.
2. Slot width equals thickness MINUS kerf
A laser removes a strip of material as it cuts. On plywood that kerf is usually 0.1 to 0.2 mm, and you can measure your own: cut a 20 mm square, then measure both the square and the hole it came out of. The difference between them is one kerf width.
Here is the part people get wrong. A slot is a hole, and a hole grows by the kerf. A tab is a solid edge, and it shrinks by the kerf. So take the kerf off the slot only. Draw a 3.0 mm slot in 3.1 mm ply and it presses in at exactly 3.1.
Most badly fitting files apply kerf to both, or to neither. Applying it to both gives you a joint that rattles; applying it to neither gives you one you have to hammer.
Doing it parametrically
Everything above still works if you type the numbers by hand. You measure the sheet, subtract the kerf, and edit every slot. It fits. The problem is only that it fits once.
The parametric version says the same thing differently: the slot is as wide as the material, and the tab is as long as the panel it passes through. Neither is a number you typed. They are both the same number, referred to from four places.
In FlexCut3D that number is a slider. The phone stand has three of them, and the material one is the interesting case. Dragging Thickness from 0.2 to 0.9 cm changes four things at the same time:
The panels physically get thicker in 3D. This is real material depth, not a number in a box.
The tabs on the crossbars grow to match, so a tab is always exactly as long as the panel it has to pass through.
All four slots widen to match, so the joint stays a press fit instead of rattling or refusing to go together.
The bars themselves shorten by the same amount at each end, so the outside width of the stand does not drift while the material changes
None of those four is a separate edit. They are one value, read from four places, which is the idea from the top of this article applied to a real part.
The practical version: your supplier sends 5.8 mm instead of 6. You put calipers on it, type 5.8, and cut. There is no round of hunting down every affected number, and no test cut to discover the one you missed.
Which one should you actually use
Not everything is a parametric problem, and it is worth being straight about that.
Vector programs such as Inkscape, Illustrator and Affinity Designer are still the fastest route for artwork. Engraved logos, lettering, decorative panels, anything where the shape is the point and it is only ever cut once. If that is your work, a CAD tool will slow you down.
Full CAD such as Fusion or FreeCAD is more capable than FlexCut3D at almost everything. Curved surfaces, assemblies with moving parts, machining, simulation. If you already know one of them, keep using it. The cost is that it installs, it takes real time to learn, and getting a laser file out is its own step: sketch, export DXF, then set up the cut.
FlexCut3D is narrower on purpose. It does flat parts that slot together, it runs in a browser with nothing to install, and it exports the SVG directly. The sliders are the parametric layer without the rest of a CAD package attached. For boxes, trays, stands, lamps and enclosures cut from sheet, that trade is worth it. For a curved bracket, it is not.
The short version: vector program for art, full CAD when the geometry is genuinely three-dimensional, and a parametric cutting tool when you will make the same design again at a different size or in a different material.
Try it on your own part
The idea transfers whichever program you use. Take a design you have already cut, find every place the material thickness appears in it, and count them. That number is what a change of material costs you today.
To see it working, the Box Hinge model in the FlexCut3D store is free and uses the same tab-and-slot corners described here. Open it, drag its sliders, and watch the joinery hold as the box changes size. FlexCut3D runs in the browser at flexcut3d.com, and the two numbers at the top of this article apply the same way whatever you design in.
Two numbers decide whether the parts this produces actually fit: the real thickness of your sheet and your machine's kerf. Both are covered in how to measure your laser's kerf, and the joint itself is sized in finger joints for laser cutting.