The Difference Between SFM and XY Feedrate

When first learning about how to operate a CNC mill I had a really hard time understanding the difference between SFM and XY federates.  

I probably spent an overall 6 hours googling this concept. I spent plenty of time on forums and websites, but I just couldn’t understand.  So to you, my dear reader, I will attempt to save you hours of time by explaining this in the simplest way possible.  I think it’s best for this explanation that we only focus on end mills

First let’s tackle SFM, what the heck is it?  This confused me so much.  Well here it is, it’s the speed at which the outermost part of the end mill is contacting the surface.  Another way of thinking about it is the speed that the outermost diameter, of the end mill, is rotating at in feet per minute.  That’s it!  Don’t confuse this with the motion of the cnc in the X, Y or Z direction; this is not SFM.  

Let me graphically try and show you this concept

Lets mathematically show this concept with an example: 

For this example, let’s say we have a 1” diameter end mill.  This would mean that the circumference is 3.14.”  This means the outermost part of the end mill travels a distance of 3.14” per rotation.  Ok, now let’s spin this bit at 1000 rpm. How many inches per minute is the outermost part of the bit rotating at?  Well simple multiply the circumference by 1000: 1000*3.14 = 3140 surface inches per minute. We are getting closer, one last step.  we need to convert this into surface feet per minute (SFM).  So simply divide by 12: 3,140/12 = 261.66 surface feet per minute (SFM)   That’s it!  We have successfully calculated the SFM given the RPM and bit diameter!  This example has shown us something very important.  We have shown that there is an unbreakable link between RPM & SFM; one value directly influences the other. If you really wanted you could also do the same thing in reverse, calculate the RPM given the SFM and end mill diameter.

Ok now we are done with SFM.  Let’s move onto XY feedrate.  Fortunately, the XY feedrate is much easier to understand: it’s simply the speed at which the bit itself is translating (or moving) in the XY position.  Basically, if you ever see the cnc machine move in the x, y or z direction this is the feed rate.

I hope this makes some sense, if it didn’t please let me know and I will try to better explain it by updating this article.