c69f102b_Calculator_for_CNC_Machinist_s__Notes
[Tip] Drill a pilot hole 2xD deep [Notes] 
On some deep holes, it is advantageous to drill a pilot hole that is 2xD deep. The pilot should be a slightly smaller diameter than the main hole. The pilot is important because the long shank needed for the deep hole makes the tool less rigid. The pilot ensures it gets started properly and is supported by the hole with reduced cutting force before it has to really dig in.
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Notes