Hey Jim, hey all,
it would add to precision if you were able to mill the half of the 3" depth from one side and the other after flipping the workpiece around. 3" long bits tend to bend. But I don’t know if you want to have the opening on one side only.
Do you know this video? They mill a rather deep groove into solid wood with an 8 mm (0.325") two-flute upcut bit with 24,000 rpm and 5000 mm/min (= approx. 200 IPM) feedrate and 3 mm plunge (approx. 1/8") (PDF project description), but only half the depth, then they flip the workpiece over by using registration dowels in the wasteboard. The final workpiece still stays attached through small wooden bridges that are removed after milling with a small hand saw.
Bits they used:
- Router bit with positive twist Ø 8 mm | Usable length 42 mm Art. No. 500-06-008P
- Shoulder Routers Ø 8 mm | Radius 4 mm Art. No. 500-06-108H
- Router bit with positive twist Ø 3 mm | Usable length 12 mm Art. No. 500-06-003P
By the way, as motor, they use…