The short answer is that it varies based on material but not by much. The ideal thing I found was to cut a 1 inch square and use calipers to see how far off it is from that 1 inch mark. With that you can use some maths to determine how much bigger you’ll need to make stuff.
Good luck!
Yes - do a search in the forums for kerf and you’ll find a HUGE rabbit hole to go down.
For me personally, I offset .004 -.008 an inch outward in CAD when I’m doing this type of cut. That seems to work the best for me. .004 if I’m mating 2 pieces and .008 if I’m doing an inlay. The laser width seems to be around that for mine.
Also remember - the cut isn’t perfectly straight up and down, it’s actually angled. So if you’re mating 2 pieces together, mirror the cuts so you’ll flip a piece when you put it together and it’ll match incredibly close this way.
Again - forum search for kerf - there are some AWESOME posts by many people on this.
Acrylic is less forgiving than wood; wood compresses and bends where as acrylic not so much.
I use anywhere from .006" to .008". It is important to flip one piece over before inserting it if you are doing a full cut out inlay. If you are engraving a pocket and then inlaying into the pocket, I’m not sure if flipping the inlaid piece would gain you much. If your design isn’t perfectly symmetrical you’ll need to flip it (mirror it) as well.
As the others have said, there’s a wealth of information here on “kerf.” You’ll also learn about how the laser cuts at a minuscule angle, which is why a couple of people have mentioned flipping your inlay pieces over to insert them – you’ll get a tighter fit that way, as the angles will match up. I just wanted to also mention @evansd2 here – he’s done some really great inlay work and searching on his name will bring up some beautiful examples (with helpful accompanying text) of what the Glowforge can help you accomplish. 
Thanx for the info. i just ordered my Glowforge and I think this will save me a lot of headaches. Cheers!
Looks like 1/8" acrylic? Here is a tip to get you started.
For a simple shape where you are plugging a hole, cut the inner piece +.007 - .008 inches bigger.
For a more complex shape where you may have multiple inset pieces touching, adjust each piece by half that amount.
You will need a function like Illustrator’s “offset path.” There is something like it in Inkscape too.
See: Calling kerf compensation wizards: working with multiple cuts in contact with one another?
I 100% second this. @evansd2 stuff is amazing and his posts are awesome when it comes to explaining kerf.
In Inkscape, I found a cool trick probably suggested by @evansd2. I make the piece. set the line width to the full kerf and use Path>Stroke to Pathand get two lines. The inner one gets added to the outer piece and the outer line to the inner piece.
I did that on a very complicated inlay and had to use a rubber hammer to set it.
If you don’t have a set of calipers, I’d highly recommend getting some. There are some very good rules of thumb from what other people use, but there’s nothing like measuring it on your material, on your machine, with your settings.
Also:
What!? Brilliant.
And this is the first I’ve read about flipping pieces because of the slight angle of the cut… makes total sense, but never would have crossed my mind. Damn I love this group.
Even though we do not love the bevel angle produced by the laser in most cases, it is actually an advantage for fitting inlays. The parts are easy to start, and get tighter as you press them home. Of course I am talking about flipping over inlaid pieces to match the angles.
Everyone of these replies were helpful.
I definitely want to learn to kerf also, so sounds like I have some reading to do
I want to thank everyone here for the help and look forward to learning more every time I come one here
Is this the caliper you all are referring to? I have one of these from Harbor Freight but I want to get a little more expensive one so I know that its accurate. Any suggestions?
I like this one, a no-name from Amazon but pretty decent, noticeably better than the Harbor Freight ones, and still cheap:
…I find it more than adequate for laser-type stuff.
I think mine was one like that. I have two others that are plastic and veneer and really cheap. All are able to read accurately to two decimals but digital is easier. More money makes a better tool if it measures 15" instead of 6 or gets a third decimal of accuracy, but otherwise there is not much difference.
If you do not want full on machinist quality like Mitutoyo, then look at Fowler. They are good,but only 60%of the cost.
If you use digital calipers make it your habit to wipe the jaws and check zero each time you use them. Many cheaper units do not have absolute zero, and they can be off due to various things like resetting with dirt on the jaws. If a dimension really matters, check for zero again after you measure as some cheaper calipers can skip, usually by a multiple of .200"
Also, do not get drawn in to the idea that your measurements are actually accurate to half a thousandth of an inch just because the display resolves to that value. It still takes good feel to actually measure within .002".
And do not bang the inside measuring jaws at the points - you can’t repair that damage
If you try this be sure to flip your design of one piece before cutting.
Granted if you’re doing a symmetrical shape like a circle, square, etc then you are fine but if your inlaid piece is an odd shape (many letters, map shapes, etc) you won’t be able to flipmate them unless you flip the design first.
This makes my head a little wonky when I do it, but agreed that it gives great results!
Hah, yeah I know that feeling. There have been times where I was like “no way will this fit”, yet sure enough, it always does.
Understood. Yea, it makes total sense. Makes me crazy that my detailed oriented whacko ScorpioticNeurosis™ brain didn’t think of it.
