For What Its Worth

Drilling out hard materials

In some of the 1-2nd gen disk brake conversion kits  The original steering arms are used but the 7/16 holes have to be opened to 1/2 inch.

I don't have a drill press so I took them to my local machine shop (Hopefully Santa can think drill press this year).  He called a while later and said he had burned up his new 1/2 drill Tin coated $$$ drill bit.  They were too @#$ hard. 

When I stopped by to pick up my unfinished parts I looked at the drill.  He had it set to 920 RPM.    In short this is too fast for hard material.  I suggested the lowest 460 RPM on the belts, but he wasn't interested in trying again.

I took them home, stuck them in a vise, pumped the holes full of grease and used my 3/8 (10mm) hand drill with a cheap high speed steel drill.   As long as I kept the RPM down it cut fine, but stalled out the drill a lot.  I had to free run the drill from time to time to cool it off.   The grease in the holes is a coolant not lubricant.  When the tip got hot the grease would melt and flow and help cool the cut down.   Made it through 2 1/2 of the 4 holes before I got carried away with the RPM and melted the tip off the drill.  Ground a new tip on the grinder and finished in a few minutes.   Carbide tip drills work well as long as the part is held firm and you don't chatter/crack the tip.  I used a carbide tip cement drill to finish the one hole started at the shop, but it didn't have enough support for the tip and chipped/broke right at the end.

When cutting hard metals remember, heat kills and speed makes heat. You don't have to have expensive tools, just patience. 

Happy Motoring, VHubbard.  May 2009

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