Carbide Drilling Tolerances

[Andrew Blair]

Okay, our team is considering using the Dewalt CIM transmission presented by Dr. Joe. http://www.chiefdelphi.com/media/papers/2738

We are having some amount of trouble contacting our EDM sponsor, and I would like to be able to machine the sun gear for the transmission at our school anyways. The diameter of the hole that needs to be machined is .3120, with a possible tolerance of .0005" smaller. The white paper suggests that if an EDM cannot be obtained, to use carbide tools. We are ready to buy, if its not too outrageous, but I can't find a suitable bit.

At http://www.mcmaster.com/, I searched under "solid carbide drills" and got a wide array of drills, however, none quite right. A 7.9 mm drill is .3110" in diameter, .001 to small. Half a thousandth is the tolerance. A 5/16 drill is .3125, or half a thousand bigger than tolerance.

What should I do? Should I buy the bigger drill and hope it still pressfits, or buy the smaller drill and hope that it'll over size correctly? Or something different? Will the bit even last long enough to justify the purchase? And, since the gear is hardened, if I can go with the carbide drill, what type? I was looking at the straight flute, as it was recomended for hardened metals.

Thanks in advance!