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
- Consider other flute styles including
- When calculating OAL consider the
reach length, amount of re-sharpening required,
bushing or fixture length, and part
- 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.
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