I just completed drilling 8, 10 gallon tanks for my fish room and none broke in drilling. That being said my dad did break one tightening up the bulkhead, ill take the $11 out of his paycheque. (damn I don't pay him)
We didn't have a fancy setup just a peice of 1 x 4 that was set-up, 2x4 behind for stability and used water for cooling.
As for a new hole over a smaller one ??? but a new guide would make it easy. Andrew I have the 2 bits, one for a one inch bulkhead and one that fits the 3/4 inch bulkhead, so let me know if you want to borrow.
Wendell
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