Simple guide for proper machining RPM

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Here's a simple rule-of-thumb guide I dreamed up long ago to determining a good starting point for choosing proper RPMs for different diameters. It's good for working on the fly and I use it all the time. This comes up frequently in my "travels" so I thought I'd post it here and hope it serves to be useful.For a drill press, the diameter refers to the drill bit and that's where I'll leave it for now since the drill press (or hand drill) is one of the most basic, common metalworking tools, though the same rule applies to any kind of metal cutting, generally speaking.First some background.It's important to drive cutters at the right RPM for two basic reasons. 1) too slow wastes time, and it can affect finish too 2) too fast can burn up the tool, cause it to become hot and wear prematurely, or even harden the material you're working on which can make it impossible to finish drilling the hole you started without switching to a carbide (expensive) drill bit or something. This is a common problem in stainless steel for instance, but it's not a big deal if you have the correct RPM (and back it off now & then to let things cool). Along with proper RPM, the cutter needs to be sharp too, but that's a whole 'nuther writeup.OK, so we agree that proper RPM is important enough to get right. Note that an "optimum" RPM can vary depending on several variables. Even the steel you get from the same supplier can vary a bit from batch to batch and this can change its machinability at a particular speed, so the following is just a good starting place, and even at that the math isn't perfect, so you can obviously adjust as you go.Now we need to establish one more thing. This will be easy if you're used to reading a micrometer or calipers. It won't be too difficult if you aren't well-versed in this though so maybe just read it twice.What I'm speaking of is decimal equivalents. For the purposes of this explanation, you can get by with just three easy ones.1 inch   = 1.0001/2 inch = .5001/4 inch = .250note that I wrote it to three decimal places, which is the very common tolerance in the machining world.OK, now just remember that 1/2" or .500" is the pivot point.So the numbers above refer to drill bit diameters. Keep that in mind.Now put three more numbers into your head. It's the same three numbers above just with the decimal point removed.1.000 = 1000.500  = 500.250  = 250OK easy enough. Now remember that 1/2 or .500 was the pivot point. You now just flip the order of the list around and line them up at .500/500 making a new list:1.000 - 250.500  - 500.250  - 1000The way I wrote it here, the diameters (to three places) are on the left, and the RPM on the right. This is a fairly good starting point for High-Speed steel tooling for cutting mild (yer basic) steel. This is based on a conservative (safe & slow) surface speed of about 65 feet per minute. If you don't know what that means, it's not really important but feel free to look it up and do a little research.From here, you just extrapolate outward and in between to get other sizes, i.e.:.750" = halfway between .500' and 1.0" so 375rpm.125" = half the diameter thus twice the speed of .250" so 2000rpm2" = twice the diameter thus half the speed of 1" so 125rpm, etc.The material of both the cutter and what you are cutting can change the RPM drastically. Here's a couple more seat-of-the-pants numbers regarding RPM:If using carbide tooling, multiply by 3If cutting aluminum or brass, multiply by three(example - if cutting aluminum with carbide cutter, multiply by 9 - 3x3)If cutting wood or plastics, you can go about as fast as you wantIf material is harder, like stainless steel or some hard steel, divide by 2 and see how things go - adjust depending on how it works, but that should be close.For titanium, feeds and speeds are about like stainless steel (unless it's hardened, in which you must have it annealed first. If you're machining titanium, you likely know this already)There are extensive tables and recommended RPMs in machining books and tooling catalogs that go all over the place. It's a huge pile of info like a logarithm table that nobody commits to memory. It's all easy to find if you need to, but sometimes (usually?) it's inconvenient to have to go look it up.This way you can just figure a quick place to start and poke a dang hole.So that was a lot of writing for the short list you need to remember, which I'll repeat here:DIA.  - RPM1.000 - 250.500  - 500.250  - 1000This is a very simple and easy to remember way of figuring a good starting point for proper RPMHappy machiningLast edited by tyrone shewlaces; 05-30-2010 at 04:47 PM.
Reply:I just push them until the chips turn blue then back off a bit.....zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.