barkster, several things affect your Z axis cut depth.
1. Gantry height clearance over your table. If you have 6" clearance, you can only cut 6" thick parts.
I have seen some designs where the part is clamped to the side of the table and you can skirt this issue. Then its the height from the floor to the bottom of the gantry.
2. Z axis stroke. If you want to cut 6" deep, then you should have at least 7 or 8" of stroke, to compensate for various router bit lengths and some rapid travel clearance.
3. Part geometry. This is very important! If you want to cut a 1/4" wide, 6" deep slot in a part, you are out of luck with a CNC router, maybe a CNC wire machine, but router bits aren't made that long and that thin. But, if your part has a slope like the inside of a cereal bowl, you can cut very deep with a short cutter. The limit becomes your Z axis stroke and your gantry clearance.
Simply draw a side view of your cutter, collet, and router in CAD. Use this as an overlay over a cross section of your finished part and you can look for interferences.
Post a drawing of your part and you might get some good suggestions on creative ways to mill it.
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