In other words, don't mix up your G0 , G1 , G90 , and G91 .

Much sadness will ensue.

God, I hate how G commands are sticky in Mach 3. Much sadness indeed.

Can the CNC machine computer not simulate the program and see if it violates some safety rules like cutting into the table or removing too much material at a time?

It's not that easy. You need accurate models of your machine, the tools and the stock material. Your simulation has to be linked to the real machine movements and be ahead of time so you can estop the machine before the crash happens.

But that still doesn't help in case of insufficient workholding or breaking tools.

It seems like the machine itself could implement some safety checks, like if a motor is commanded to go outside of its range of motion you trigger a stop with an error code and let the operator sort it out.

Or go even more oldschool and have physical safeties on the machine that trip if your cutting tool attempts to exit the work area. I mean these aren't cheap machines, it seems like they could be engineered to be just a bit safer and error resistant.

I'm not a CNC machine operator, but I have written a lot of code for motion control systems. Typically, the machine can protect itself in that it knows where the limits are and how not to exceed them.

The problem usually comes up by crashing into something that is inside its known workspace. e.g., cutting into a vise instead of the object that's held in the vise, or an incorrect offset causing a crash into the table. The machine may know that it can only travel up or down by 12.500 inches, but it has to hold cutting tools that may be of unknown length and that can cause it to hit things (like its own table).

The machines do some sanity checking. Most of the issues come from the machine being improperly informed about what is on the table(height or shape) or the length of the tool in the spindle. The tool length can be tested if you have the hardware, but it's more difficult to scan the whole table for stuff that might get in the way.

If an axe could detect your thumb was in the way, it would no longer be useful.

There's a balance. Sometimes it really is up to the operator to maintain safety. Sometimes the machine can handle it. IMHO, other than reliably finding zero, everything else is up the the operator. The tool doesn't REALLY know how big it is or how fast it's going (just what it was told), and even if it did know, some operator would screw it up assuming that the tool was smart enough to adjust itself.

gotta disagree. I don't know anybody who works in a machine shop and doesn't want sawstop enabled, unless they need to disable it for a specific cut (wet wood).

Real CNC machines have limit switches, they monitor the load on the axis and spindle servos etc etc. But that doesn't prevent running your cutter into a fixture at positioning speed and ruining the spindle bearings.

My tin arse Cnc mill I made has stop switches. Probably the cheapest part of the whole thing and saves it grinding itself when you balls the code up :D

Do those stop switches prevent crashing the mill into a fixture? If so, how?

Do you manually monitor the entire job cycle?

High end CNC controls have chuck barrier software built in that makes this less likely. It needs to be skillfully set up. High end shops may also have good 3D rendering software that can be used to prove out toolpaths. The programmer will model his machines and fixtures accurately so he can then prevent crashes. Average CNC controls at least have line output graphics that prevent non-subtle mistakes. Finally, the operator will run the first part peering into the machine, with non-cutting rapid traverse moves artificially slowed down, and with her finger poised over the feed hold button.