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You might need to order .875 hex stock, which is slightly oversized, but will make a good press fit into the wheel. As for centering the hole, the best option is actually to put it in a lathe and drill out the hole that way. The second best option is to mount the piece vertically and use a milling machine. Then use a zeroing tool to locate the center of the hex stock. Then you can drill it out quite easily.
But it sounds like you don't have either... So my recommendation is to mark very carefully the center of the hex and use a centerpunch to ensure the drill bit will not drift. Then mount the hex in a drilling press, and drill through. You'll probably need to start with a small drill (1/8)" and then work up to the final size (1/4" and then 3/8"). If you have a reamer, it would be a good idea to drill just under 3/8" and then use the 3/8" reamer to do the final cut. The key to making this work is mounting the hex perfectly horizontal. Use a right angle of some sort to do this.
You'll probably get some no-good pieces using this method. But some will be good. If the hole is centered on both sides of the hex, it should be good. If it is only off by 0.005" or less, then I do not think a slight wobble will affect performance too much.
- Patrick
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Systems Engineer - Kiva Systems, Woburn MA
Alumni, Former Mechanical Team Leader - Cornell University Robocup - 1999, 2000, 2002, 2003 World Champions
Founder - Team 639 - Ithaca High School / Cornell University
Alumni - Team 190 - Mass Academy / WPI
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