The largest CNC machine we make at this time (12/04/2015) is the GreenBull 6'x12' model. This model has 2 options for Z-axis travel.
The short-z option has a total Z travel of 5.75 inches. From that you would subtract the length of the end mill used and the thickness of the spoil board to determine the maximum material thickness. (Example: If the end mill extends 1.25" from the collet and the spoil board is .75", then the material could be up to 5.75 - 1.25 - 0.75 = 3.75 inches in thickness). This is good for most sheet-type materials.
The long-z option, used mainly for large 3D carvings, requires a custom frame and allows for up to 36" of travel.
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What is the z-axis travel for your largest machine?