This is a long and very nerdy post about laser cutter kerf. Kerf is the amount of material that is removed or lost in the cutting process, and, if not compensated for in the design plans, will result in loose fitting parts.
I work at a digital fabrication co-working space called MTRL KYOTO where a big part of my job so far has been to familiarize myself with the tools we currently have on hand. So far I've been focusing mainly on working with the laser cutter, a RayJet. I made this sign for the coffee/bar area using 6 mm Shina plywood, and a little box to hold stickers using some sort of 1.5 mm plywood.
As a side note: I prefer the slightly thicker plywoods to the 1.5 mm stuff. They tend to stay perfectly flat, where as the thin stuff can warp a little, which causes the position of parts of the material to be out of alignment with the laser's focus.
I did a little reading about compensating for laser kerf when making parts that are supposed to fit together snugly. Some pages were saying that laser kerf, depending on the machine and the material, can range from .08 mm to .32 mm. In other places I read that the standard was .0254 mm, so I made some test parts with a .0254 mm compensation and they didn't fit at all. At first I started very gradually increasing the compensation, but after about 2 hours and parts still not fitting together, I started taking bigger steps up. For our machine, with the 1.5 mm ply, the optimal kerf compensation turned out to be exactly .1 mm. Once i settled on that number, parts started popping into place without having to force them, and they held together without any glue. Factoring in kerf compensation usually isn't too hard, but when you're laying out box joint teeth like on the little box above, it can be a little confusing. I made this diagram to illustrate.
(Edit: Shortly after writing this I discovered the offset feature in Illustrator, which made me feel incredibly stupid for having wasted so much time doing that manually...)
The tricky part is getting the teeth to each be 5.1 mm while making sure that the spacing between the teeth is still exactly 5 mm. I did this in Illustrator by first making rectangles the size I want them to eventually be, say 100 x 50 mm. Then I made a 5 x 1.5 mm rectangle (the width of the stock is 1.5 mm) which represented the size of the teeth I wanted. I then spaced the teeth along all sides of the inside of the rectangle. This gives me my correct spacing. Then I widened the big rectangle .1 mm in each direction. Then I made the "cut size" tooth, which was a 5.1 x 1.6 mm rectangle, and I spaced each of those along the outside of the big rectangle so that each "cut size" tooth was centered over the actual size teeth. This is kind of difficult to explain so I'll post some screen shots of the process.
So here is the basic shape of one side of the box. I put 5 x 1.5 mm spacers around the inside perimeter of the shape.
Next I enlarged the big rectangle by .2 mm in both width and length. .1 mm will be lost on each side so that's why I added .2 mm total. Be sure you are enlarging relative to the center of the object. You should have something that looks like this.
Then I made a rectangle that was 5.2 x 1.6 mm (+.2 in length but only +.1 in width because it's only being cut on one side) and copied that shape, each time lining it up to the center of the original spacers.
Once I got all the "cut size" teeth arranged, I deleted the spacers and combined the teeth with the big rectangle using the pathfinder tool so it's all one complete vector.
So with this method, once all your pieces are cut out they should fit snugly together. Every laser cutter is different though, so you'll want to do some tests to figure out what the optimal kerf compensation is for the machine you're using.