Cobalt High Speed Steel Fractional Straight Shank Jobber Length Drill Bits
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These cobalt drill bits are the only thing that will drill into hardened bolts and metals. I have tried standard bits on hardened metal before not knowing what grade the metal was. The standard bit dulls almost immediately so I reach for the cobalt bits. They will sink on through the metal with no problem. The only cons are they are more expensive and they will break easily if you put force perpendicular to the bit instead of linear.