What Is SFM in Machining – SFM Definition, Units, Formula, Calculation & SFM to RPM

Too low or too high SFM settings will have a negative impact on the quality of machined parts and cutting tools. How to set the best SFM in machining? In this article, we’ll dive into the SFM meaning and definition, as well as its units, formula, calculation, chart, and conversion between RPM.

What Is SFM in Machining – SFM Meaning and Definition

In CNC machining, SFM stands for surface feet per minute and defines the relative linear velocity between the cutting edge of a tool and the workpiece. SFM is a common unit of measurement for cutting speed in machining and is primarily used in the United States, while other countries typically use meters per minute. In milling operations, the workpiece remains stationary while the spindle rotates the milling cutter; the spindle’s rotational speed, measured in RPM (Revolutions per Minute), is converted into cutting speed based on the diameter at which the cutter contacts the workpiece. Conversely, in turning operations, SFM is calculated differently as it relates to the rotational speed of the workpiece itself.

 

SFM is a combination of surface speed and the unit feet per minute, a proper SFM is critical to ensure production efficiency and tool life.

 

Why is SFM important? If the SFM is too high, it can lead to excessive heat generation, rapid tool wear, degraded surface finish, and potential material deformation due to thermal expansion. Conversely, if the SFM is too low, it can result in poor material removal rates, increased tool pressure potentially causing breakage, heat buildup due to rubbing rather than efficient cutting, and surface finish issues stemming from inadequate cutting action or tool chatter. Therefore, maintaining an optimal SFM is vital for achieving efficient material removal, prolonged tool life, desired surface quality, and dimensional accuracy in machined components.

 

What is surface speed? Surface speed, sometimes referred to as cutting speed, is the speed at which a cutting tool’s edge moves across the workpiece during machining operations. It is a critical parameter that helps determine the optimal spindle speed (RPM) for different cutter types, materials, and diameters, whether on a lathe or milling machine. The cutting edges are not concerned with the machine type or number of flutes; they only know they are slicing into the workpiece like a razor. Surface speed simply measures the linear velocity of the cutter’s movement relative to the workpiece, typically expressed in units like feet per minute (SFM) or meters/millimeters per minute. Each specific cutting tool has an ideal surface speed range for each material being machined to maximize tool life and achieve optimal cutting conditions.

 

SFM Units

Feet per Minute (FPM) and Millimeters per Minute (MM/min) are the two main units used to measure Surface Feet per Minute (SFM) in machining.

– Feet per Minute (FPM): a unit commonly employed in the United States, allowing for seamless integration with other imperial measurements and familiarity among machinists working with CNC machines in the country. However, its usage requires conversion for international applications, and it may be less intuitive for those accustomed to the metric system.

– Millimeters per Minute (MM/min): standard unit in international and metric-based systems, aligning with global manufacturing practices and facilitating easier integration with metric specifications. While MM/min may require conversion for use in the U.S., it is more widely recognized and utilized in the global machining industry.

Two units can also be converted to each other, the formula is 1 ft/min = 304.8 mm/min.

How to Calculate SFM in Machining – SFM Formula and Calculation

1) Formula:

SFM = (π × D × RPM) / 12

Where:

SFM = Surface Feet per Minute (the cutting speed)

D = Diameter of the cutting tool or stock (in inches)

RPM = Rotational speed of the spindle (revolutions per minute)

π = Constant pi (approximately 3.14159)

12 = Conversion factor (to convert inches to feet)

2) Steps to Calculate SFM:

Step 1: Identify the cutting tool diameter (D) in inches.

Step 2: Determine the spindle speed or RPM (revolutions per minute) at which the tool will rotate.

Step 3: Substitute the values of D and RPM in the formula: SFM = (π × D × RPM) / 12

Step 4: Calculate the value of π × D × RPM

Step 5: Divide the result from step 4 by 12 to convert from inches to feet.

Step 6: The resulting value is the Surface Feet per Minute (SFM), which represents the cutting speed or the rate at which the cutting tool’s surface moves across the workpiece.

3) Note:

To calculate SFM for milling, the D refers to the cutting tool diameter, in turning, use the workpiece diameter, and in drilling, use the drill diameter.

SFM to RPM Conversion Formula in Different Machining Processes

The basic SFM to RPM formula is RPM = SFM/Circumference, considering the diameter and unit difference, the actual calculation is as below in different processes.

– Milling: RPM = 12 x SFM/Tool Diameter x π

– Turning: RPM = 12 x SFM/Workpiece Diameter x π

– Drilling: RPM = 12 x SFM/Drill Diameter x π

RPM to SFM Conversion Formula in Different Machining Processes

SFM = RPM x Circumference

– Milling: SFM= RPM x Tool Diameter x π / 12

– Turning: SFM= RPM x Workpiece Diameter x π / 12

– Drilling: SFM = RPM x Drill Diameter x π / 12

Machining Material SFM Chart

If you are not sure about the surface feet per minute for different materials, here is a chart with typical SFM values for common materials used in machining. The cutting speed of different grades of the same material will vary, machining material hardness, machinability, thermal properties, tool geometry, tool material, and tool coating will affect the final SFM.

Material	SFM
Aluminum	600 to 1000
Mild Steel	100
Tool Steel	30 to 50
Stainless Steel	50 to 100
Titanium	50 to 100
Brass	300 to 600
Copper	200 to 400
Cast Iron	50 to 150
Plastic	300 to 600
Inconel & other superalloys	20 to 50