Tips for Drilling Deep Holes

Drilling deep holes in some cases requires drilling to depths of 20 times drill diameter. Drilling to these depths causes concern for chip evacuation and heat build up on the tool, generating excessive wear at the point. Consider the following factors when drilling to these depths.

• Material to be cut and its hardness will determine whether to use high-speed steel M-2 or the cobalt grade M-42. Although M-2 is the most frequently used high speed steel, M-42 is the choice when machining in the Brinell range 296 and above.
• Tool construction must be of a heavy-duty style, with typical web thickness of 40% to 45% of the drill diameter to maintain rigidity over the long flute length.
• Helix angles of 36° to 38° are common to efficiently evacuate chips up the flutes.
• Points are generally 135° heavy-duty and split, sometimes referred to as crankshaft drill points.
• Consider other flute styles including parabolic.
• When calculating OAL consider the reach length, amount of re-sharpening required, bushing or fixture length, and part thickness.
• Minimize excessive overhang.
• Drill points should always be kept sharp.
• Proper lubrication is critical and coolant should be well filtered.
• The most critical machining function is the evacuation of chips, drilling depth, and the critical path of chip evacuation as well as knowing when to withdraw the tool before the chips get hot and anneal the tool.
• For controlling the chip, chose the right feeds and speeds. In general, 50 to 65 SFM is standard. The feed will depend on the tool diameter. If the chip is long and stringy, increase feeds until chip is broken into smaller pieces.