I want to do something right, so I'm doing it myself.
I learned how to program an RPM ("Speeds") calculator in Excel 2010 so I can derive my own speeds chart. It's handy because I can add any materials I like at any time as long as I can provide that material's SFM*.
(For those of you new to the game of rotary cutting tools, the "Speed" measures how fast the spinning element spins; the "Feed" measures the speed of pushing the cutting tool into the material. Necessary for clean, safe, repeatable cuts!)
I made this at Techshop Detroit (www.techshop.ws) and it's proven to be USEFUL!
*Going to explain that in a bit.
Step 1: The Formula for Speeds
You can find the SFM in a chart produced by your tool manufacturer; I used one from the Machinery's Handbook. I found that while this SFM number is "scientific", it DOES depend on the material your cutter is made of, too, and it DOES depend on what the chart-makers thought was "efficient." I thought the Handbook represented a solid authority.
So we can pull material SFM and cutter diameter together to find the RPM for spinning that cutter to produce a good cut.
The formula is:
RPM = (SFM *12in/ft) / (pi * Cutter Diameter)
Cutter diameter is going to be in inches, and SFM is Surface FEET per minute, so we multiply the SFM by 12 to get the units the same.
For this situation we're using mild steel as our material, and we're cutting with a pretty standard .5in bit made of high speed steel. The SFM for this scenario is 80. So:
RPM = (80ft/min *12in/ft) / (pi * .5in)
IF YOU'RE GLAZING OVER, just think of this as a recipe. You need the SFM for your material and you need to know the size of your bit. I'm setting you up so you just have to know the basics, then plug in values and have Excel do the calculations.