Drilling nickel and cast iron tools & technique

admin

I have a cast iron part that I filled a 3/8" diameter hole, 3/8" deep with a mild steel plug. The mild steel plug was plug welded in so that the steel has about 1/8" of weld on top. The cast iron was pre heated and post cooled to prevent cracking. The process was TIG with 59% nickel filler rod that is said to be machineable.I need to drill a new 5/16" hole through the nickel and mild steel plug. Normally I drill using a drill press. I think I am going to have a hard time drilling the nickel. What is the best tool to drill the 5/16" hole that will be tapped afterwards using a 5/16-24" tap? I plan on buying a few extra taps to do the hole.Should I buy a 1/8" carbide drill bit, 17/64" carbide drill bit and use the drill press?I have access to and bridgeport. Should I buy end mills to do the drilling? Would it make the hole that much better? If so, what type of end mill tools should I get for the job?As you can see, I am a bridgeport newbie. I tried a cheap HSS 1/8" on a test weld and it pretty much just dulled the drill bit. I only have one shot to get this drilled right so buying tools and using the bridgeport is worth it for me.Last edited by weld; 08-22-2011 at 11:06 AM.
Reply:I decided that the drill press with some Precision Dormer cobalt twist bits should  do it.
Reply:5/16-24 tap drill is 'I' letter size .2720, using smaller drill (27/64=.2656) will kill tapsYou may need to go larger than .2720--for tapping-drilling and then tapping welded cast iron or steel can be a krap shoot.Higher the nickel content the more machinable-but the weldingstill creates hard spots-that may or may not be drillable withcobalt and/or carbide. Nickel is soft, gummy and likes to tear.The hard spots can be composed of carbide formations, etc.--which is 'why?' they can be unholy to machine. Sometimesone's lucky, sometimes one's-not.Drilling is one thing--tapping with  hard spots can be difficultto impossible, even with high end, hardened coated taps.Hopefully, your plug weld has a minimum of mixing with the CI,which should help.Slow speed, lube is critical to the drilling. Part needs to be rigidly clamped in the drill press.Suggest next time--use larger steel plug to the surface, then doa double bevel groove weld around the plug OD to retain it, soyou don't have the hassle of drilling welded CI or nickel.Last edited by dave powelson; 08-22-2011 at 03:18 PM.Reason: addition and additionBlackbird
Reply:Your drill selecton looks good, use a good quality coated tap and invest in a bottle of Moly-Dee tapping fluid. It is made for high compression tapping but works well with gummy materials also.LarryMiller XMT 304 CC/CVSyncrowave 180 SDLincoln PowerMig 255XTTermalDynamics 52Lincoln 305GComlpete machine shop to back it up