Tricky question. That depends on the way the table is built. If the table is built with a step down area, you can put as thick a material as you want. If the table is flat, the clearance is 6 inches. Now, the thickness depends on the size end mill you insert into the collet. If you use an end mill that has a 2 inch cutting length, then your material thickness is 4 inches without a spoilboard. If you need to mill thick material, I would suggest that you fashion the table accordingly so the table can accomodate any thickness you desire. That's one of the great aspects of the buildyourcnc machines, adaptability.
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What is the Max thickness of wood that can be placed in the 4x8 cnc milling machine?