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What Does SFM (Surface Feet per Minute) Mean?
Surface Feet per Minute is the term machinists use to describe tool head speed. Unlike Revolutions per Minute (RPM), which measures the spindle’s rotational speed, this critical unit of measurement measures the linear speed at which the edges of the cutting tool move across the blank’s surface in a minute. SFM is often indicated in ft/min (feet per minute). The SFM calculation is an essential determinant of optimal cutting speed for different CNC machining materials and processes.
SFM involves the combination of surface speed and the unit feet per minute. It correlates with the spindle speed (RPM) and the rotating workpiece diameter, defining the ideal cutting conditions. Machining experts rely on SFM calculations to optimize cutting efficiency, achieve efficient material removal, and extend the tool’s lifespan.
How Does SFM Impact Machining Processes
Surface feet per minute influences machining operations in different aspects, including:
Influence on Cutting
The appropriate surface speed allows optimal material removal rates and increases machining productivity. The optimal SFM determines how much the cutter should engage the workpiece surface to achieve proper chip load and lower cycle times. Properly applied SFM can ensure maximum production output with tight dimensional accuracy.
Impact on Tool Life
Choosing an appropriate surface speed setting significantly affects the longevity of cutting tools. Maintaining proper SFM values helps preserve the integrity of the cutting tool and reduces replacement frequency because extremely high SFM generates excess heat, accelerating tool wear.
Surface Finish Quality
The cutting head speed directly influences the surface quality of a workpiece. Thermal deformation occurs due to excessive speeds. Conversely, tool rubbing and rough surfaces happen at low speeds.
Hence, an equilibrium between the cutting speed and other parameters like depth of cut, feed rate, and tool geometry guarantees smooth surface finishes since it ensures good chip formation and minimizes tool chatter.
Heat Generation
The set surface speed primarily impacts the thermal conditions of the tool-workpiece interface. High speeds generate excess heat, resulting in material deformation due to thermal expansion. Conversely, low speeds result in heat buildup. Applying the correct SFM values with proper cooling can effectively manage thermal-related complications.
Standard Uses of Surface Feet per Minute in Machining Processes
Milling
The cutting tool rotates while the blank remains stationary in a milling machine. There are several cutting edges on a typical endmill, which come in varying diameters. You can mill the surface of workpieces faster when you input the correct cutting speed. For instance, a higher SFM value helps to increase the productivity rate and reduce the total cycle time for milling the shafts.
Drilling
The correct SFM value will help prevent the drill bit from untimely wear when drilling into harder materials like steel shafts. Drilling critical hole sizes for fitting components in parts like engine blocks requires choosing the correct speed. It helps to ensure the hole is dimensionally accurate and smooth.
Turning
The workpiece rotates while the cutter maintains a fixed position on a lathe. The cutting tool overheats and causes poor surface finish if the speed is extremely high in turning operations. Similarly, extremely low SFM causes the workpiece to rotate slowly, producing a rougher surface finish. Therefore, applying the proper cutting speed for optimal tool performance and superior surface finish is essential.
Grinding
In grinding, choosing the appropriate SFM values helps increase the grinding wheel’s life span. For example, using the correct SFM helps reduce wear on the grinding wheel in cylindrical grinding of steel shafts. It helps to maintain its sharpness and effectiveness for an extended duration.
Calculating SFM in Machining Processes
The formula for SFM consists of variables such as cutter or workpiece diameter and the rotational speed in revolutions per minute.
SFM= (π x D x RPM )/12
Where:
- π (pi)= 3.1416
- D = Cutting tool or workpiece diameter (in inches)
- RPM = Revolution per minute of the workpiece or cutter
- 12 = Conversion factor to convert inches per minute to feet per minute
For instance, if you are using a 3-inch diameter end mill spinning at 900 RPM, the SFM is:
SFM= 3.1416 x 3 x 900/ 12
SFM = 8484.32/12 = 706.86 ft/min
Therefore, the cutting speed is approximately 707 SFM
Standard Tools for Accurate SFM Calculation
There are various essential tools machinists leverage to achieve accurate SFM calculations for specific machining processes:
Surface Speed Calculators
These SFM calculating tools allow machinists to input machining parameters, including cutter diameter and spindle speed (RPM), to calculate the ideal SFM values:
- Machinist’s Calculator: A tailored tool with unit conversion features that help simplify SFM and feed rate calculations.
- Online SFM Calculators: These web-based tools provide instant and accurate results and are easily accessible for quick calculations.
- CNC Machine Software: An onboard software in CNC machines calculates SFM according to the machining parameters, streamlining the setup process.
- CAM Software: CAM features comprise features for calculating proper SFM values, ensuring overall machining process optimization.
Recommended Tools and Software
These are reliable software solutions designed to provide detailed machining calculations and improve cutting conditions:
- G-Wizard: A standard SFM calculator with various features for calculating SFM, speeds, and feed rates, including a material and tool database for precise and reliable recommendations.
- FSWizard: An advanced tool that supports SFM, cutting speed, and feed rates across several cutting tools and materials.
- HSMAdvisor: It is a software programmed to calculate SFM and feed rates to optimize cutting conditions and tool life.
- Machinist’s Calculator Pro: An advanced-level calculator that produces accurate and reliable SFM values and feed rates for experts.
What Is the Impact of Too High or Too Low SFM on Surface Finish and Tool Life?
Cutting tools generate more heat based on how fast the cutting edges engage the material. CNC tools and workpieces are ruined quickly when the SFM is too high, causing complications like tool half-life, poor finish, and high machining costs. Although cutting tools are commonly made from material designed to withstand heat, extreme heat can cause trouble.
A cutting tool will fail to resist heat when it exceeds its limits, and the temperature is too high. The tool material softens due to heat-induced thermal expansion, causing the edges of cutters to become dull and creating dimensional inaccuracies in a workpiece. Dull cutting edges often generate more friction and heat in machining.
Cutting becomes slow when the SFM is too low, increasing the machining time. Furthermore, complications such as rubbing occur because low SFM causes heat buildup during machining. It makes the tool hot and compromises the surface quality of the workpiece.
Although slowing the spindle (RPMs) helps reduce the heat in the cut and extend the tool life, the logical question is, how can we determine the ideal spindle speed with SFM calculation?
It would help to note that every cutting tool has a recommended surface speed, depending on the material being cut. Hard or toughest materials like titanium or steel require slower surface speeds, while soft materials like plastic or wood can accommodate relatively high surface speeds.
Recommended SFM Settings for Different Machining Materials
Machining materials have varying hardness, heat resistance, and machinability properties. Even though tool coating and coolant help to mitigate heat generation in machining, choosing the right SFM setting helps to achieve better results. Hence, the SFM values can be adjusted to meet the specifications of the cutting tool and the workpiece material selected.
Here is a general guide on how to choose the appropriate SFM for your project:
- Aluminum: It is a metal with excellent heat conductivity and relatively low hardness, making it compatible with cutting speeds of 250 to 400 SFM for HSS tools and 600 to 1000 carbide tools.
- Brass: Brass is compatible with cutting speeds of 300 to 600 SFM for HSS tools and 800 to 1200 for carbide tools.
- Cast Iron: Cast iron can handle cutting speeds of 60 to 120 SFM for HSS tools and 400 to 800 for carbide tools. However, the specific cast iron type and its hardness may influence the recommended SFM value.
- Mild Steel: Mild steel can handle 70 to 150 SFM cutting speeds for HSS tools and 300 to 600 SFM for carbide tools.
- Inconel (Nickel-Based Alloys): SFM values for machining Inconel range from 50 to 200 SFM for carbide tools and 10 to 30 SFM for HSS tools.
- Copper: 100-300 SFM for HSS tools and 500 to 1000 SFM for carbide tools.
- Plastic: 300 – 800 SFM for HSS tools and 800 to 2000 SFM for carbide tools is the recommended SFM range for plastics such as Acrylic, PMMA, Nylon, etc.
- Titanium: The recommended SFM range from machining titanium alloys is 30 to 70 SFM for HSS tools and 100 to 300 for carbide tools.
Factors that Influence SFM Values Selection
Various factors influence the choice of the correct SFM values for machining operations. They include:
Type of Machining Operation
Experts use different machining techniques, including milling, turning, drilling, and grinding. These operations often require varying speeds based on the tool geometry, including flutes and tool engagement.
Factors such as feed rate, tool diameter, and number of flutes will decide the ideal cutting speed for milling operations. Meanwhile, the drill diameter and depth of cut influence cutting speed in drilling operations.
Tool Material
The ability of a cutter to withstand wear and heat hinges on its material. High-speed steel (HSS) tools work well with lower SFM, making them suitable for general-purpose machining. Tools made with ceramic and Cubic Boron Nitride are compatible with ultra-high-speed applications, especially in hard materials like titanium. Carbide tools exhibit better heat and wear resistance, making them suitable for higher SFM.
Workpiece Material
Since each machining material has varying properties, different SFM settings are required. Hard materials like hardened steel, stainless steel, or titanium require lower cutting speeds to prevent thermal complications like rapid tool wear. In contrast, soft materials like plastics, aluminum, or brass use higher cutting speeds to achieve better results.
Machine Tool Capabilities
The capabilities of your machine are another critical determinant of the achievable SFM settings. The spindle speed limits of your machine determine the highest cutting speed you can choose for each machining operation. Also, the machine’s rigidity influences vibration and cutting stability in machining. Furthermore, the power and feed mechanisms of the CNC machine can impact the speed and feed combinations for specific machining tasks.
Coolant and Lubrication Effects
Adequate use of coolant improves SFM by reducing heat, which could cause workpiece or tool distortion. Similarly, proper use of coolant improves chip evacuation and ensures smoother cuts, less friction, and a better surface finish.
Conclusion
Surface feet per minute is a core phenomenon in machining because it determines cutting head speed that meets the needs and properties of different cutting tools and materials. Choosing the appropriate SFM for the workpiece or cutting tool ensures better results, including reduced production costs, enhanced efficiency, and quality surface finish.
AT-Machining is a renowned CNC machining services provider with years of maintaining top standards in delivering quality precision machined components. We leverage our advanced technology and efficient equipment to meet the dynamic specifications of our clients. Our professionals can carefully analyze the requirements of your projects to determine the right SFM for optimal results. Contact us to discuss your next project with an expert!
