Life of a drill bit

[dg2010]

Just wondering what others use for drilling 316 plates.

I have been told to use cobalt but a drill site recommends TCT. Presumably for a small number of holes cobalt will do but how long do they last, and also if wanting a 3/8" holes what increment drills do you use.

Thanks

[BioFarmer93]

I obviously dumb thumbed something.. I posted several hours ago but it never made it. Anyway, cobalt bits are fine. Invest in a Drill Doctor, it will pay for itself in very short order. Use a prick punch to precisely and firmly mark the hole center- one good hit only. Clamp your plate onto the drill press work table and set the belt to the lowest speed pulleys. Use lots and lots of synthetic ATF as a lubricant for the bit but especially to keep the heat down- I say use lots because it works as a coolant to keep from work-hardening the SS. Work hardening of the metal just makes it that much more difficult to drill. Use high pressure (force/loading).. I know, counter intuitive, right? Slow speed high pressure makes much less heat than high speed medium pressure. The metal should come off in little spirals, not micro sized chips (dust). Start with a new or freshly sharpened bit, keep it slow, lubed, cooled and highly loaded.

[dg2010]

Thanks for the tips. Haven't done any drilling yet as the metal merchant did the drilling for me.

Do you do it in one hit or drill smaller holes first? I see there is mention elsewhere here of using a step drill. They don't seem to sell these in the UK.

[BioFarmer93]

You can drill a small pilot hole first if you want, or you can use the centering (prick) punch method. They both serve the same purpose, to keep the full sized bit from "walking". Ideally,(but most time consuming) is to center punch, pilot drill and then full bore. Home builders rarely go to that trouble any longer with the availability of advanced material bits. Step drills are a convenience, and work OK in mild steels, but unless you have very deep pockets are not usually available in cobalt or carbide for cutting SS. They also are virtually impossible to sharpen properly.

[dg2010]

You can drill a small pilot hole first if you want, or you can use the centering (prick) punch method. They both serve the same purpose, to keep the full sized bit from "walking". Ideally,(but most time consuming) is to center punch, pilot drill and then full bore. Home builders rarely go to that trouble any longer with the availability of advanced material bits. Step drills are a convenience, and work OK in mild steels, but unless you have very deep pockets are not usually available in cobalt or carbide for cutting SS. They also are virtually impossible to sharpen properly.

Thanks for your help. Got a good idea where I am going now.

[Owen_]

I would suggest you save a little cash and go with standard HSS rather than the cobalt. Cobalt tools are good for SS, and can give a better finish, but unless your using a CNC machine the benefits are minimal. The cobalt drills are more brittle (as they are harder) which can be a down side if you slip up a little. The real advantage to the cobalt is that they can take a higher heat without loosing their hardness. This is something that shouldn't come up if your careful. I would recommend getting a few non-cobalt HSS drills, and one or two double sided #2 center-drills. Just go slow and keep things wet (with coolant, oil, or if need be spit)

[dg2010]

I would suggest you save a little cash and go with standard HSS rather than the cobalt. Cobalt tools are good for SS, and can give a better finish, but unless your using a CNC machine the benefits are minimal. The cobalt drills are more brittle (as they are harder) which can be a down side if you slip up a little. The real advantage to the cobalt is that they can take a higher heat without loosing their hardness. This is something that shouldn't come up if your careful. I would recommend getting a few non-cobalt HSS drills, and one or two double sided #2 center-drills. Just go slow and keep things wet (with coolant, oil, or if need be spit)

Thanks for your advice but have already done the job and the cobalt drills worked a treat and were not that expensive. I drilled a 4mm pilot then the 10mm. As suggested above, I used the slowest speed on my pillar drill and used some Dexron as coolant.