Cutter speeds - where is an authoritative table or formula?

WobblyHand

WobblyHand

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I thought I looked in the right table... An old Machinery's Handbook (17th edition). I'm looking at the table "Speeds and Feeds for Milling, Drilling and Turning". It says 700-900 FPM for aluminum using HSS cutters. If I worked out the math right that yields 6100 RPM for a 1/2" mill (800 FPM).

The basic formula should be:
FPM [ft/min] = circumference of bit [ft/rev] x RPM [rev/min], correct? So solving for RPM we get:
RPM[rev/min] = FPM [ft/min]/(pi x D [ft/rev]) = 12 [in/ft] x FPM [ft/min] /(pi x D [in/rev]) ? (I put in the units to make sure the math is ok. It does clutter the equations, sorry for that.)

Could someone set me straight? Is this the right formula? Is the advice just to run aluminum cutters fast, or as fast as your machine will allow? Thanks. The 17th ed. Handbook claims this is conservative, modern practice.

Moments after posting, I see there is a sticky. Doh!
 
I see there is a tooling correction factor which appears to reduce the RPM. Where does this term come from? 3.82 is a funny number.
 
One thing I found helpful was to make my own feed/speed tables.

The first is a table of feed and SFM for different metals, which is basically a range from Roughing (low) to Finishing (high), with an all-purpose number that usually lies in between.

The second is a table of RPM for different SFM and diameter (tool or workpiece) combinations. The diameters will increase by 1/8 or 1/4 until 1", then by 1/2" until 4", then by 1", and so forth. You'll find that the small-diameter RPMs are impossible to achieve, and the large-diameter RPMs tend to converge.

Print it out, hang it by the lathe or the mill. Becomes pretty simple to choose an SFM, from that, select an RPM by interpolating from known diameters. If you have a fixed selection of speeds (e.g. a pulley system), then you can alter the second table to only include the RPMs you have available.
 
Some of the tool companies make a slide rule type of speed/feed calculator out of hard cardboard, so no need for calculations.
In school we used a formula; RPM= cutting speed X 4, divided by the diameter of the moving part, the cutting speed for soft steel being 100 FPM.
 
Take a look at G-Wizard at CNCcookbook.com

speeds and feeds all done in a nice easy app

no table needed.
 
Ok, now I know where 3.82 comes from... That's just 12/pi. 12/pi = 3.81972...
benmychree said:
Some of the tool companies make a slide rule type of speed/feed calculator out of hard cardboard, so no need for calculations.
In school we used a formula; RPM= cutting speed X 4, divided by the diameter of the moving part, the cutting speed for soft steel being 100 FPM.
Click to expand...
And the 4 is just an approximation of the 3.82. Got it. It seems the units for your equation are ft/min for the cutting speed and inches for the diameter. Makes sense now. So my equation is correct, unless there's some kind of efficiency or fudge factor, I've left out.

Still think 6100 RPM is fast. I've only got 3100 RPM available, so that will have to do.
 
Cadillac STS said:
Take a look at G-Wizard at CNCcookbook.com

speeds and feeds all done in a nice easy app

no table needed.
Click to expand...
Need to make an account to do anything on that website. Pass for now. Maybe later. I'll bet it's easy to use. Me, got a calculator and the Handbook.
 
ThinWoodsman said:
Print it out, hang it by the lathe or the mill. Becomes pretty simple to choose an SFM, from that, select an RPM by interpolating from known diameters. If you have a fixed selection of speeds (e.g. a pulley system), then you can alter the second table to only include the RPMs you have available.
Click to expand...
That's a pretty good idea.
 
If you have a smart phone type feed speed in the search of app store and pick which app you like
 
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