Namac New Reader
11/28/19 6:05 a.m.

I have the cog off a coaster break style bike hub. I need to bore 6 holes approx 3/16" in dia. through the cog. This steel is incredibly tough, and I'm having difficulty drilling it. I tried a random steel drill bit, the black kind we all probably have, chucked it in my drill press and it barely made a dent in the steel, and under pressure just tried to wander. I was using a silicone spray lubricant for this operation if that matters. I switched to a TiN bit, which is actually a cheap bit for all its fancy coating, and that fared no better.

What am I missing here? Is there a specialty bit that I need? Different lubricant? I though about creating a wood bit guide, which would prevent the bit wandering, but then how do I lubricate the cutting action?

Not that it matters but because I know someone will ask, I am doing this in order to mount the cog to a disc brake hub, which in turn is part of a fixed gear conversion my son and I are doing to an old mountain bike.

wheelsmithy SuperDork
11/28/19 6:59 a.m.

Pilot hole? 

Buy brand new bit just for this?

Sparkydog Reader
11/28/19 7:02 a.m.

It’s heat treated steel. As hard or harder than your drills. Masonry bits with carbide tips may work but you will probably need 1 for each hole.

A machine shop with a wire EDM will also have a hole popper and they can put the holes in.

ntsqd New Reader
12/12/19 3:45 p.m.

Cheap though not necessarily inexpensive hardware store drill bits probably won't work. Find your local machine shop supplier and buy a Cobalt stub length drill bit from them. Or buy one from either or While you're at it buy real cutting fluid, it WILL make a huge difference.

79rex Reader
1/4/20 10:21 a.m.

Carbide drill and a mill, or bolt a vise on a drill press.  As long as the drill press has a wide range of RPM.  And use coolant.  Go slow, its going to take time.  Gentle pressure let the drill do the work.

robmillion New Reader
4/23/20 1:36 p.m.

this may be a better option, it will be really hard to drill through a coaster brake cog. and they are often offset, possibly right where you would need to drill.  it's 3/32"  but it would work fine with a 1/8" chain if that's what you have.


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