Hey, Nova Scotia Mike,
Smaller drills require slower in-feed rates.
I.E. a 1/16 dia. drill should only be fed at .001" per revolution; 1/8 @
.003; 1/4 @ .005; etc.
Your bolts are probably medium carbon steel (SAE 1035 - 1040).
Cutting Speed (CS) for these grades are between 175 - 225 Surface feet per
minute (SFPM).
The diameter of the drill determines the RPMs to use:
RPM = 4(CS) / Drill Diameter
I.E. 4 * 200 / 1/16
= 800 / .0625
= 12, 800 RPM!
I sincerely doubt you were approaching that speed, (unless you have access
to a dentist's tool!), so it's no wonder your drill fractured.
Use a larger drill, preferably one sized to the minor thread diameter of the
bolt. You can turn it slower & feed it faster. Just keep it flooded with
coolant to dissipate the heat from friction & to clear the chips out of the
hole.
Use WD - 40 for coolant, not cutting oil. Use the little red tube (that
comes taped to the side of the can) in the nozzle. It will achieve both
goals.
Then you can clear the threads with a tap, again using WD- 40...
HTH,
Bob Smith (DAKSY2K on AIM)
(9 fingered ex-machinist)
Averill Park, NY
2K DAK SY CC Sport + V6 4X4 5 Speed
WebPage URL: http://home.nycap.rr.com/daksy/
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