SFPM explained?

[NShiflet]

I'm training new lathe operators and need practical examples that explain surface feet per minute?

[Glacern]

For a lathe, SFM = the number of feet per minute the outer diameter of the workpiece travels.

So if you've got a 4" diameter bar spinning at 100RPM,
SFM = circumference in feet * RPM
= 2 * pi * r * RPM
= 2 * 3 * 1/6 foot * 100 RPM
= 100 SFM

I rounded down pi, which is what I usually do for ease of calculation. That little .14 doesn't really matter.

If you were calculating SFM on a mill, same formula but you use the cutter radius instead of the workpiece radius.

[bborb]

If you touch a 1 inch grinding wheel thats spinning at 60 RPM, just over 3 inches of grinding surface travels across your finger in one second.
If you touch a 24 inch grinding wheel spinning at the same RPM, over 75 inches of grinding surface travels across your finger in one second. Now if you slow the 24 inch wheel way down to just over 2 RPM, then you're back to 3 inches of grinding surface traveling across your finger in one second.

The moral is, the larger "something" gets, the slower you want to spin it to keep the effects the same.

[timlkallam]

SFM = ( RPM x DIA ) / 3.82
RPM = ( SFM x 3.82 ) / DIA

[Geof]

Do you want a non-machine tool example? On a car travelling at 60mph the SFPM on the tire relative to the vehicle is 5280.

[NShiflet]

Thanks Geof,
Yes I have a lot of technical information so I was looking for non-machine examples to give more simple everyday ways to help understand.

[Geof]

The car can also be used to explain 'relative to'. Relative to the road the periphery of the wheel is stationary.

Another peripheral speed example is the Merry-Go-Round at the Fair; how fast do you need to run to keep up with Johnny when it is a certain diameter going at a certain rpm.

On the technical side but removed from lathes are chain and belt drives; the pulleys or sprockets have the same SFPM, they have to or the belt/chain is slipping, but different rpm.

[HuFlungDung]

Originally Posted by Geof Do you want a non-machine tool example? On a car travelling at 60mph the SFPM on the tire relative to the vehicle is 5280.

Or, if the muffler is dragging beneath the vehicle, the road surface is grinding the muffler at 5280 feet/min at a vehicle speed of 60 mph

[Geof]

Originally Posted by HuFlungDung Or, if the muffler is dragging beneath the vehicle, the road surface is grinding the muffler at 5280 feet/min at a vehicle speed of 60 mph

Which is about correct for grinding; 6" dia at 3500 rpm.