Calculating the RPM for your machines

hi everyone today I'd like to talk about the RPM of machines and specifically how to calculate what your RPM should be there's a lot of people out there and some of whom have been in the trade for a long time that believe that any rpm works for any material and that's not the case your material and the diameter of your material impact what your RPM should be so I've got two pieces of aluminum here one of them is larger than the other one and when we talk about rpm the critical thing and figuring out what your RPM should be is what is called surface feet per minute and that is your cutting speed so what that is is on the lathe it would be how many feet of circumference of the material is moving past your cutter in one minute or on the mill it would be how many feet of cutter is moving past your material the larger of the two pieces would have to run at a slower rpm because it has a larger circumference so you have more feet of material moving past the cutter in one minute whereas our smaller piece if we put our lathe bit there this would have to move at a faster rpm because you've got less circumference and less surface feed now depending on what your material is as well your cutting speed is going to change if you have a soft easy to machine material like aluminum you can run that at a higher cutting speed than you could if you had a hard steel these are the numbers I recommend to my students at Parkland College and these are based on using a high speed steel tool so either a high speed steel lathe tool or an end mill or a high speed steel drill mild steel is basically anything that's not hardened so your 1018 steel your cheese steel from the hardware store black pipe tubing things like that tool steel is anything that is already hardened or hardened a bull so I would use this for anything from 4140 which was a very tough steel up to any of the drill rods die steels and like that mystery steel kind of goes hand-in-hand with that because if you just pull something out of the scrap bin and you're not entirely sure what the alloy is it could very easily be a hardened steel and you don't want to burn up your tool by running it at mild steel speeds when it's actually a hardened steel the aluminum and brass speed is very conservative and the reason for that is if you're running something super fast then it decreases your reaction time in case there's a problem so these are the speeds that I use for my students they're at Parkland College and I try to get them to be as conservative as possible with their speeds partly so their tool lasts the entire semester but also so they don't crash the machines now you may not have to machine a ton of bronze but I actually machine it quite a lot and bronze is a lot harder than brass it looks like brass but it's really not it is a beautiful material to machine I mean you have to work really hard to get a bad surface finish with bronze but it's a lot harder than brass and then stainless steel I like to run at 40 stainless is not necessarily hard it's actually just that it doesn't conduct heat very well so if you let the tool rub or you run it too fast it has a tendency to harden locally in the spots where the tool is rubbing so for that reason you really want to run stainless quite a bit slower than even tool steel now these are actually very conservative numbers which is by design if you're running at these speeds you're not going to destroy your tool with heat as I mentioned before these numbers are based on using a high speed steel tool so if you were using a carbide tool you can actually double or triple these depending on whether or not you're using coolant another thing that you might find is you might look up cutting speeds for different materials and find charts that are wildly higher than these on the internet and those are usually found from manufacturers of cutting tools well those are production oriented numbers so those are actually based on a 15-minute tool life meaning that at that speed that corner of that insert should last 15 minutes before have to flip it to a fresh corner obviously in the home shop world and in non production shops that's not really consideration whereas tool life might be so how do we turn our sfm chart into the RPM of the machine well we have to use math here's our equation our revolutions per minute will equal our surface feet per minute and that's dependent on your material times 3.8 - that's a constant I'll talk about that in a second and then divide that by the diameter the diameter is going to be the size of the cutter or the hole your boring on the mill and it will be the size of the stock usually on the lathe unless you're drilling a hole that's the big exception there if you're drilling or boring a hole it's going to be the size of the hole because you're concerned about how many feet are moving past the drill or the boring bar in a minute the outside is irrelevant so where does three point eight two come from well three point eight two is actually 12 divided by pi so what we're doing is converting our surface feed into surface inches because the diameter that we're going to be dividing by is in inches now if you're going to be programming a CNC machine you use this formula because you can specify the exact rpm of the spindle in the program so you want to definitely use three point eight two however if you're working with manual machines you can simplify your math and just make it sfm times four divided by diameter the four is an approximation of three point eight two and in the case of manual machines you're very rarely ever going to be able to choose the calculated rpm you're always going to end up choosing one that's close to it but it's not going to be exactly right so four actually gets us in the ballpark and it's a whole lot easier to remember so for my students they're at Parkland College this is the formula that I want you guys to use now in those cases where your calculated rpm is one thing and you have the exact option you want to follow prices right rules that means you're going to choose the closest one without going over your calculated rpm so if you calculate your rpm at 1440 rpm and you have a choice between 11:15 and 1750 you want to choose the lower of the two you want to choose the 1115 because if you run it at 1750 you're actually machining at a much higher sfm and you might burn up your tool so let's do some samples let's say that we're using a 3/8 diameter end mill and a piece of aluminum in the mill the cutting speed of aluminum according to my chart there is 150 sfm time is 4 divided by our diameter which I said was 3/8 so that would be 0.375 got to use decimal inches here we do 150 times 4 that's 600 divided by 0.375 and we end up with 1600 rpm now of course you would then go ahead and choose whichever speed option you have that's close to that but without going over so how's about if we're turning a piece of mild steel in the lathe and it's 5/16 in diameter so mild Steel's SFM is 90 times 4/5 sixteenths which is 0.3 1 2 5 so if we do that on our calculator 90 times 4 equals that divided by 0.3 1 2 5 and we get 1152 rpm now what if we were drilling a hole in the end of a big piece so let's say we had a quarter inch hole in the end of a six-inch diameter piece of stainless steel so stainless steels sfm is 40 times and then which of those two diameters are we going to use it's a quarter inch hole but it's a six inch piece of stainless if we're drilling a hole we need to use the size of the drill because that's what our material is moving past so in this case it has to be 0.25 down here 40 times 4 divided by 0.25 and we get 640 rpm now don't worry metric folks I haven't forgotten you there is a formula for you guys to your cutting speed is expressed in meters per minute and the diameter is going to be given in millimeters so your RPM is going to be equal to 1,000 times the meters per minute divided by pi times the diameter so 1,000 times the meters per minute is basically giving you millimeters per minute and remember your diameter is going to be based in millimeters so how does that actually affect your cutting speed you can take my sfm numbers from before and divide them by 3.3 to get your meters per minute it's a nice approximation gets you pretty close so these are those numbers converted into metric for you so 27 for mild steel 15 for tool steel and mystery steel 45 for aluminum and brass 30 for bronze and stainless steel is 12 so can the RPM be too low the answer is definitely yes I mean especially with a carbide tool they aren't as sharp as high speed steel tools so if your RPM is too low it will tend to kind of plow off material and tear it more than cut it the big area where the RPM can be too low though is with small drills if you're drilling at too low of an RPM especially with small drills the drill can get overloaded because you're feeding proportionately faster to the RPM and the drill tends to snap small drills are already fragile and easily broken and the breaking problem is just compounded by the fact that most manual machines can't run fast enough to actually cut at the proper SFM for those small drills I hope this video helps and if you have any questions or comments please leave them down below thanks for watching

Channel: Stuart de Haro

Views: 191,272

Rating: 4.8536587 out of 5

Keywords: Machining, Machine Shop, Machinist, Milling Machine, Metal Lathe, Machine tools, calculating RPM, SFM

Id: SZ4QL64ZSyQ

Length: 11min 22sec (682 seconds)

Published: Sun Sep 03 2017