Cutting to deep

I set the depth of cut to 1/8" it cuts 1/2" or better, I have tested the z axes it goes up and down like it should 1" is 1" but when cutting it goes crazy. what is going on and how do I fix it?

share your file so we can look at it

If your z hits its max this can happen, the machine thinks it is higher than it should be and plunges deeper. Watch the z carefully when the carve starts and see if it maxes out.