Your assumptions are correct regarding how the limit switches work on a typical “CNC machine” (like a CNC router or a CNC mill). To confirm what you said, the switches will typically be placed in a way that they are activated (“switched”) if the machine attempts to move too far.
OK, there are a lot of ins and outs with this. I’m going to try to give the basics for clarity, but I’m also going to skip some detail for brevity.
The following block of text is only about lost steps, if you know what they are already, skip to the stuff after the line…
The reason a machine like a CNC router may “attempt to move too far” is because of something called “lost steps”. Lost steps happen when the machine controller commands the machine to move somewhere it is physically unable to and are a potential problem for (virtually) any machine that uses stepper motors (like I am assuming is the case for the Glowforge).
Lost steps a big problem for CNC routers as the force required to perform a cut can vary wildly. As you can imagine, a 1/8"-deep cut is going to be considerably easier on the machine than a 2"-deep cut. Similarly, a 1/2"-deep cut made at 5 inches per minute is going to be considerably easier than a 1/2"-deep cut made at 50 IPM. And, of course, cutting through foam is going to be considerably easier than cutting through steel. This variability in difficulty may lead the (human) operator of the machine to try to exceed the capabilities of the machine. If the operator does this, one of two things are likely to happen: the bit will break or the machine will stall. If the bit breaks, the machine will have no way of knowing and will continue its job as if everything is fine. When it stalls is when you get lost steps. Unfortunately, a stepper-driven machine will also have no way of knowing it lost steps, so it will continue along as if everything is fine. Machines are dumb like that. The problem is: although the machine doesn’t know it, after losing steps, the cutting head is physically in a different place than it thinks.
Imagine someone starts a router at the “bottom-left” of the table and attempts to make a diagonal cut to the opposite corner. Now imagine the machine isn’t strong enough to make this cut and ends up stalling. If this happens, the machine will continue to try to move to the opposite corner for as long as it normally would have taken if it was successfully cutting, but since it’s stalled-out, the head isn’t moving away from the bottom-left corner. When it thinks it’s done it thinks the head is at the top-right corner. If, then, it is told to go back to the bottom-left it’ll attempt to move the head farther “down” and farther left than it should. If you don’t have limit switches to stop the machine, it’ll plow into the “physical limits” of the machine. This collision can damage the machine and cause parts to become misaligned.
OK, sorry for the long description of lost steps! I hope that, if it didn’t give you any new information, @marmak3261, it is informative to anyone else who may be curious!
Fortunately, the Glowforge won’t really have to worry about lost steps. This is because the only resistance to movement it will typically experience is the friction between the linear rails and the rotation of the belts. This lack of variability means that motors with the “perfect” amount of power can be chosen, which keeps costs down and can also lead to better performance (generally speaking, fast motors aren’t strong and strong motors aren’t fast).
Since lost steps aren’t really a concern for a laser cutter, the machine can use a concept called “soft limits”. In a nutshell, soft limits basically dictate that a machine with 20" of X travel and 12" of Y travel will never attempt to move to the coordinates X21 Y13. Of course, it will also never attempt to move to X20.001 Y12.001.
Since it can use soft limits, it really only needs limit switches when the machine is homed, which is when it establishes where 0, 0, 0 (XYZ) is. So, instead of six switches, a machine like this will only need three. And those three switches aren’t likely to see a lot of use. I suspect you’ll only need to home a machine like this once per power cycle (and even that may not be all that necessary) and any time you move the head by hand, either accidentally or on purpose.
A quick eBay search found some $4 limit switches that claimed they could be clicked 30 MILLION times. So, if you homed your machine 10 times every single day for the next 80 centuries you might have to start worrying about wearing out your switches. These $4 switches might not be the IDEAL switch… There are also 40¢ switches, $40 switches, and , I’m sure, $400 switches. There exists, somewhere, an ideal limit switch.
Actually, with clever design and programming, it would probably be possible to only have a single switch, and just make sure only one axis is capable of “clicking” it at a time. I’d say taking the time to make that clever design and program is not worth the minuscule savings of eliminating two exceptionally inexpensive switches.