Surface feet per minute (SFPM or SFM) is the combination of a
physical quantity
A physical quantity (or simply quantity) is a property of a material or system that can be Quantification (science), quantified by measurement. A physical quantity can be expressed as a ''value'', which is the algebraic multiplication of a ''nu ...
(''surface speed'') and an
imperial and
American customary unit (''feet per minute'' or ''FPM''). It is defined as the number of
linear feet that a location on a rotating component travels in one minute. Its most common use is in the measurement of
cutting speed (surface speed) in
machining
Machining is a manufacturing process where a desired shape or part is created using the controlled removal of material, most often metal, from a larger piece of raw material by cutting. Machining is a form of subtractive manufacturing, which util ...
. It is a
unit of
velocity
Velocity is a measurement of speed in a certain direction of motion. It is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of physical objects. Velocity is a vector (geometry), vector Physical q ...
that describes how fast the cutting edge of the
cutting tool
Cutting is the separation or opening of a physical object, into two or more portions, through the application of an acutely directed force.
Implements commonly used for cutting are the knife and saw, or in medicine and science the scalpel an ...
travels. It correlates directly to the
machinability of the workpiece material and the
hardness
In materials science, hardness (antonym: softness) is a measure of the resistance to plastic deformation, such as an indentation (over an area) or a scratch (linear), induced mechanically either by Pressing (metalworking), pressing or abrasion ...
of the cutting tool material. It relates to
spindle speed via variables such as cutter diameter (for rotating cutters) or workpiece diameter (for
lathe work).
SFM is a combination of ''diameter'' and the ''velocity'' (
RPM) of the material measured in feet-per-minute as the
spindle of a
milling machine
Milling is the process of machining using rotary cutters to remove material by advancing a cutter into a workpiece. This may be done by varying directions on one or several axes, cutter head speed, and pressure. Milling covers a wide variety of ...
or
lathe. 1 SFM equals 0.00508 surface meter per second (meter per second, or m/s, is the
SI unit of speed). The faster the spindle turns, and/or the larger the diameter, the higher the SFM. The goal is to tool a job to run the SFM as high as possible to increase hourly part production. However some materials will run better at specific SFMs. When the SFM is known for a specific material (''ex 303 annealed stainless steel = 120 SFM for high speed steel tooling''), a formula can be used to determine spindle speed for live tools or spindle speeds for turning materials.
In a milling machine, the ''tool'' diameter is used instead of the ''stock'' diameter in the following formulas when the tool is revolving and the stock is stationary.
Spindle speed can be calculated using the following equation:
:
[Brown & Sharpe; ''Brown & Sharpe Automatic Screw Machine Handbook'', Publisher B&S]
SFM can be calculated using the following equation:
:
See also
*
Speeds and feeds
References
{{Reflist
External links
Surface Feet per Minute
Machining
Units of velocity
Metalworking terminology
Velocity