What is SFM? [ Surprisingly simple to convert SFM to RPM ]
What is SFM? How can we convert SFM to RPM?
You don’t have to be fooling around with machining or CNC long to come across the abbreviation “SFM” and not long after you’ll see converting SFM to RPM is a standard part of Feeds and Speeds calculations. In this article I’ll explain this useful concept.
Let’s get one thing out of the way quickly, SFM is an acronym for “Surface Feet per Minute”. It’s a unit of measurement for a concept in machining called “Surface Speed.” Of course there are also metric units for Surface Speed. We use Surface Meters per Minute in metric.
Now why is that important?
Surface Speed and SFM are used to help determine the best spindle rpm for machining cuts. That sounds pretty useful, right?
Here’s the thing–cutters come in many variants. On a lathe, the work spins and the cutter remains stationary. Typically, there is only one cutting edge. On a milling machine, the cutter spins and the work remains stationary. Not only that, but there are usually multiple cutting edges or flutes on a typical endmill. The goal of surface speed is to provide a single quantity that can help determine the best spindle rpm for every cutter type, no matter whether it’s on a lathe or mill.
Sounds like a tall order, but it’s really pretty simple.
Those cutting edges don’t know whether they’re on a lathe or a mill. They don’t know if there are other cutting edges either. All they know is they are slicing into the workpiece, like dragging a razor over your skin when you shave. Here’s a simple diagram:
Surface speed is simply the speed the cutter moves across the workpiece. Pretty easy, right? And now you can see why you might have a unit like SFM: the cutter is moving at that number of feet per minute. The metric measure of surface speed can be either meters or millimeters per minute (or second), but it’s the exact same concept.
What’s the impact of too much surface speed?
Well, the diagram does mention the notion of rubbing two blocks to start a fire. The faster you move the cutting edge through the material it’s cutting, the more heat it generates. Cutting tools are made of materials that are designed to resist heat. Things can get quite hot before trouble starts, and that’s perfectly okay. But, there are limits. If there is too much heat, and temperatures rise too high, the cutting tool can no longer resist. It softens, which causes the edge to dull. When the edge dulls, it creates even more friction and heat. Pretty soon we have a vicious cycle and our tool is ruined.
What about the opposite? Can we have too little surface speed?
This is a logical question to ask. We can have too slow a feedrate and that’s very bad for tools because it causes rubbing, which makes the tool too hot, and we just talked about where that leads. Not good!
But, as it turns out, there is no real penalty for slowing the rpms. In fact, it’s one of the most beneficial things you can do to extend tool life. Slowing things down via rpm will reduce the amount of heat in the cut, which will help the tool to last longer.
How can we use SFM to find spindle rpm?
First thing, is every cutter has a recommended surface speed that is usually based on the material you’ll be cutting. Soft materials like wood or plastic can tolerate relatively high surface speeds. Hard materials require slower surface speeds. The very toughest materials may force you to use very low SFM’s indeed.
To find the recommended surface speed, you’ve got a few choices. If you have a Feeds and Speeds Calculator like our G-Wizard, it will have some default recommendations. Just select a tool and material and you’ve got it:
For mild steel and a carbide endmill, G-Wizard suggest 333 SFM…
You can also find tables of SFM’s in places like Machinery’s Handbook or our free online feedrate calculator. Lastly, manufacturers of cutters nearly always published recommended Surface Speeds for their cutters.
Once you’ve got a suggested surface speed, it’s relatively easy to convert it to spindle rpms. The simple machining formula you’ll use is:
Spindle RPM = SFM / circumference
Where the circumference is that of the workpiece on a lathe or the cutter on a mill. Given that simple formula, now you know why small diameter tools such as drill bits have to be spun faster than larger diameter tools. Their circumference is smaller, so the rpm goes up.
There are various reasons to use fancier calculations. For example, if you’re drilling a deep hole, it is often helpful to slow down the rpms a touch. But, for the most part, you’ve just learned everything you need to know about Surface Speed, SFM, and calculating spindle rpms.
Now I know you’re wondering. Given how easy it is, why would you need a fancy feeds and speeds calculator like G-Wizard?
First, not everything is as simple to calculate as spindle rpms. Second, I’ve already mentioned fancier calculations can be beneficial to your tool life. But probably the most important thing is that these variables don’t exist in isolation. Each one impacts the others. For example, your machine is limited to a certain amount of power based on the size of its motor. All sorts of things, including spindle rpm, go into determining how much power is used in a cut.
What should be done if the cut exceeds your spindle’s available power?
We could choose to adjust a lot of different variables. We could throw up our hands and just tell our user that particular scenario is impossible. But, the right answer is to adjust the variables in some optimum order that gets the user as close as possible to their desired result. We just learned we can reduce rpms and all is well–tool life improves!
But, if you have a high speed spindle, perhaps for a CNC Router, you can only make it run so slow. For many machines, slowing down too much also reduces available power. Can you see all the interactions that take place between all these variables?
Dealing with that complexity is the role of a good Feeds and Speeds Calculator. If you’ve never played with one, get the free 30-day trial for our G-Wizard Calculator. It’s fascinating to see how all these variables interact with one another. Getting a little help mastering those interactions will make your CNC work go so much faster and easier too.
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