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What Does SFM Mean in CNC Machining?
SFM (Surface Feet per Minute) is a critical parameter in CNC machining processes that determines cutting head speed/velocity relative to the raw material. SFM entails surface speed and the unit feet per minute. It matches the spindle speed (RPM) with the rotating component’s diameter to determine the optimal cutting condition.
The principal agenda of SFM is to attain efficient material removal and ensure an extended tool life span. While low SFM can reduce heat generated by machining operations and preserve tool life, high SFM increases production rates but generates extreme heat, resulting in rapid tool wear.
How Is SFM Important to Machining Processes?
SFM is a critical variable that determines the outcomes of machining processes. It significantly impacts the cutting tool’s performance and the final product’s quality. However, SFM helps identify the ideal tool speed for various materials by deciding the appropriate (RPM) for machining cuts. Understanding the appropriate RPM helps machine operators achieve precise and efficient cuts during machining.
Besides, proper SFM helps ensure the appropriate performance of CNC tools. As such, SFM ensures the smooth transitioning of operations and enhanced quality products. While raw materials have varying machinability ratings, SFM significantly determines a material’s machinability and the tools’ hardness. It ensures machinists use the appropriate tools for the workpiece material, minimizing wear and tear.
Furthermore, proper SFM settings help mitigate common issues like the deformation of the workpiece and tool skipping during machining processes. SFM calculation helps machinists prevent cutting tools from skipping across the workpiece, causing defects or damage to the cutting tool. Additionally, incorrect SFM calculation can cause excessive heat that may affect the workpiece. Therefore, proper SFM settings help ensure the workpiece retains its intended form and properties.
Standard Units Used in Measuring SFM
Feet per minute (FM) and millimeters per minute (MM/min) are two primary units product teams use to calculate SFM. Although these units express surface speed, their application is based on the region and the adopted standard measurement system.
Feet per Minute (FPM)
FPM is a common standard unit used in determining SFM. It easily conforms to other imperial measurements. Machine operators working with CNC machines in the U.S. know this measurement unit. However, the feed per minute unit requires conversion before it can be suitable for international use. Also, it is a less intuitive unit for those used to the metric system.
Millimeter per Minute (MM/min)
Millimeter per minute is a standard unit in international and metric-based systems. It aligns with global manufacturing practices and facilitates seamless integration with metric specifications. However, MM/min unit may require conversion before machinists in the U.S. can use it. It is also a less familiar SFM measuring unit to machinists accustomed to the imperial system.
Is SFM Different from RPM?
Although closely related, SFM (Surface Feet per Minute) differs from RPM (Revolution per Minute) because they represent different things. SFM calculates the linear speed of the tool’s cutting edge relative to the workpiece, while RPM measures the spindle’s rotational speed. The cutter diameter determines the relationship between the SFM and RPM. Machinists can convert RPM to SFM using the formula:
SFM=RPM x π x Cutter Diameter (in inches)/12
This formula indicates how surface speed (SFM) is affected by the spindle speed (RPM) and cutter diameter.
How to Calculate SFM in Machining
Machinists can easily determine the best cutting speeds in machining operations by calculating Surface Feet per minute (SFM). Doing so helps to ensure better tool life, efficient material removal, and superior workpiece surface finish. Below is an example of how to calculate SFM in CNC machining:
- Identify key parameters such as Cutting Diameter (D) and Spindle Speed (RPM).
- Use SFM=RPM x π x Cutter Diameter (in inches)/12to calculate SFM.
For instance, let’s calculate SFM using a spindle RPM of 3000 and a cutting diameter of 1.8 inches.
- Spindle speed (n): 1500 RPM
- Diameter (D): 1.5 inches
- Pi (π): 3.14159
- Conversion factor: 12(inches in one foot)
When we insert these values, we have:
SFM = 1500 x 3.14159 x 1.5/ 12 = 589.04
The cutting head speed (SFM) is 589.04 feet per meter
Key Formulas for SFM Calculation
The following equation helps to change SFM between different units and determine the best spindle speed:
Converting SFM to MM/min
The formula to convert surface per minute (SFM) to surface millimeters per minute (MM/min) is:
Surface MM/Minute = Surface feet/minute x 0.03048
Calculating Cutting Head Speed
First, you have to determine the cutting tool or spindle speed. It involves four key numbers, including:
- n (RPM) – Spindle speed
- D (inches) –Workpiece diameter (turning operations) or Cutter diameter (milling operations)
- Pi (π) -3.14159
- The number 12- inches in one foot
Using the formula Vc = n × π × D / 12, you can determine the cutting head if the spindle speed (n) is 3000 RPM and Diameter (d) is 1.8 inches.
Vc = 3000 × 3.14159 × 1.8 / 12 = 1,413.71
Therefore, the cutting head speed (Surface Feet per Minute) is 1,413.71 ft/min (Feet per Minute)
Changing SFM to RPM
Once you know the desired surface speed/SFM for a specific tool or material, these formulas help decide the required spindle speed (RPM). Below is how to convert SFM to RPM:
- To determine spindle speed for the milling process where the tool diameter is known, use the formula: RPM = SFM / Tool Diameter (D) × π
- RPM = SFM / Workpiece Diameter × π is the formula for calculating workpiece speed where the diameter is known. It converts the surface speed into spindle speed for a rotary workpiece.
Changing RPM to SFM
These formulas help you determine SFM (Surface Feet per Minute) when you already know the spindle speed (RPM) but need to determine the surface speed (SFM).
- To derive the SFM from a given RPM and tool diameter for CNC milling operation, use the formula:
SFM = RPM × (Tool Diameter × π / 12)
- Use SFM = RPM × (Workpiece Diameter × π / 12) to calculate SFM for a turning operation relative to the spindle speed and diameter of the workpiece.
Why Are These Formulas Necessary?
These formulas serve different purposes. Therefore, understanding and incorporating each in your CNC machining operation can offer extensive benefits, including:
- Adaptability: Since various workpieces and tool sizes are involved in machining operations, formulas for processes turning and milling help ensure accurate setups of different CNC machines.
- Conversion and Calculation: These formulas aid the conversion to ensure all variables align for optimal machining performance, whether you begin with known SFM values from material specifications or RPM values from machine capabilities.
- Precision and Efficiency: These formulas aid the conversion to ensure all variables align for optimal machining performance, whether you begin with known SFM values from material specifications or RPM values from machine capabilities.
- Improved Tool Life and Surface Finish: Correct SFM and RPM settings can prevent heat generation during cutting. As such, tools can last longer while you achieve a high-quality surface finish on the machined workpiece.
Common Mistakes to Avoid in SFM Calculation
You might make significant errors in SFM calculation, which may affect the outcomes of your project if you aren’t careful enough. Here are some of the common pitfalls machinists and engineers encounter when calculating SFM and how best to avoid them:
Incorrect Units Conversion
Machinists may fail to convert units correctly or confuse inches and feet when using the SFM formula. However, it would be best to use a machining software or calculator that can automatically convert the units to avoid human error. Also, it would help always to ensure diameters are indicated in inches and speeds are in revolutions per minute (RPM)
Failure to Account for Tool Wear
Not accounting for tool wear can reduce the cutting performance and influence the SFM needed. Following the tool manufacturer’s guidelines on tool life and wear rates would be best. You should also inspect the cutting tool regularly for wear and adjust the SFM accordingly.
Misinterpreting Manufacturer’s Recommendations
Taking recommended SFM values from tool manufacturers without evaluating the specific setup or material is a common mistake in SFM calculation. It would help to take manufacturers’ recommendations as a starting point and optimize SFM and RPM values/settings according to cutting conditions to ensure overall machining efficiency.
Using Incorrect or Outdated Tool Data
It is wrong to use inaccurate data or rely on outdated information that doesn’t match the specific tool in use. Ensure the cutting tool data is verified and maintain an up-to-date database of your machine tools’ specifications. You can also document cutting data of successful previous operations.
Overlooking Material-specific SFM Values
When calculating SFM, it is wrong to use generic values that don’t account for the machined material since different machining materials require varying SFM due to parameters like thermal properties, hardness, and tensile strength. Experts advise using SFM values of the material being machined.
Essential Tools and Techniques for Accurate SFM Calculation
Machinists generally rely on two main tools and methods to ensure precise SFM calculations. These include:
Surface Speed Calculators
In the modern machining industry, surface speed calculators are invaluable tools that help prevent human error and ensure desired outcomes. Machinists can input spindle speed (rpm) and cutter diameter (D) in these devices and software applications to determine the best SFM for machining operations. Typical SFM calculation tools and software include:
- Machinist’s Calculator: It is a dedicated tool for machinists that simplifies the SFM calculation process and other machining parameters. A machinist’s calculator converts units and calculates feed rates.
- CAM Software: Computer-aided manufacturing software has features for calculating SFM and other essential machining parameters. These computer programs can create toolpaths and improve machining processes.
- Online SFM Calculators: Most websites offer free calculators for SFM values. They are often easily accessible, and their results are instant.
- CNC Machine Software: Various CNC machines possess built-in software that can calculate SFM values according to the programmed spindle speed and cutting/workpiece diameter. This CNC software is capable of streamlining the setup process.
Recommended SFM Calculation Tools and Software
- FSWizard: It is a versatile tool that offers comprehensive machining calculations, including feed rate, cutting speed, and SFM. FSWizard supports different cutter types and materials.
- G-Wizard: This tool offers extensive features for SFM, speeds, and feet rate calculation. It is a popular and commonly used tool among machinists since it entails a database of material properties and cutting tools to proffer accurate recommendations.
- HSMAdvisor: This machining software calculates SFM, feeds, and speeds to optimize cutting conditions and tool life. The HSMAdvisor provides recommendations according to the cutting tool and material properties involved.
- Machinist’s Calculator Pro: It is a sophisticated calculator that machinists use to calculate SFM, speed, and feed rate. It is specially built for professionals who seek precise and reliable machining data.
How Do CNC Tools Affect SFM?
The choice of CNC tools employed for machining operations often influences SFM calculation and application. Due to the variation in the capabilities and requirements of cutting tools, CNC tools often influence the optimal SFM for a specific operation. Here is how CNC tools Influence SFM in machining:
Cutting Tool Material
CNC tools are made from different materials, and each affects SFM in machining differently:
- Carbide Tools: These tools can handle higher temperatures and cutting speeds, which allows for higher SFM and faster machining techniques.
- High-Speed Steel (HSS): High-speed steel tools are often employed due to their versatility and longer lifespan. However, they require lower SFM to prevent rapid tool wear.
- Ceramic and Cermet Tools: These cutting tools are built for high-speed machining of hard materials. They can also withstand operating at considerably higher SFM than carbide and high-speed steel tools.
Tool Coatings
TiN, TiAlN, and other Coatings are commonly used to enhance cutting tool performance by reducing friction and increasing resistance to heat. Coated CNC tools can withstand higher SFM, unlike uncoated tools.
Tool Geometry
- Tool Diameter: Generally, larger diameter tools may require lower RPM to attain the same SFM as those with smaller diameter. This influences the overall cutting speed and efficiency.
- Flute Design: The design and number of flutes a cutting tool has can affect the chip load and workpiece’s surface finish. CNC tools with more flute numbers can handle higher SFM, although it may require feed rate adjustments.
Effects of Incorrect SFM in Machining
Incorrect SFM settings can have significant effects on tool life and the quality of machining. Understanding what happens if the SFM is too low or too high will help maintain an efficient and precise machining process:
What Happens if SFM is Too High?
Issues that may arise when you set the SFM too high include:
- Tool Wear: When the SFM is programmed to high, the tool’s cutting edges can become dull faster, resulting in faster tool wear and reduced lifespan.
- Excessive Heat Generation: Setting SFM too high increases cutting speed, leading to excessive heat generation. This heat can deteriorate the quality of the tool and workpiece.
- Material Deformation: High SFM causes excessive heat, leading to thermal expansion in the workpiece, resulting in dimensional inaccuracies.
- Surface Finish Degradation: High temperatures resulting from high SFM can impact surface finish quality, making it uneven or rough.
What Happens if SFM is Too Low?
On the other hand, when the SFM is set too low, complications can arise, including:
- Increased Tool Pressure: When you set the SFM too low, the cutting tool may have to work rigorously to cut material, increasing tool pressure and breakage.
- Poor Material Removal: Low SFM compromises cutting efficiency, resulting in slower material removal rates and extended machining time.
- Surface Finish Complications: Setting the SFM too low can cause poor finishes because of inadequate cutting action and likely tool chatter.
- Excessive Heat Buildup: While high SFM causes heat buildup, low SFM can ironically do the same. The tool rubs the workpiece continuously instead of slicing through the material efficiently.
How to Adjust SFM to the Demands of Different Material
SFM settings can vary significantly according to materials. The material’s hardness, machinability, and thermal properties would determine the best SFM value for machining operation. Below is a guide on how to adjust SFM for different materials:
- Aluminum: Aluminum is a soft metal machinable at higher SFM values within 600 to 1000 SFM. This high cutting speed helps ensure efficient material shearing and good surface finish.
- Brass: Like aluminum, higher SFM ranging from 300 to 600 SFM is ideal for machining brass.
- Copper: Copper is from the red metal family, like brass, and can be machined at higher speeds with SFM values between 200 and 400.
- Mild Steel: It would be best to use about 100 SFM when cutting mild steel to ensure optimal cutting performance and prevent rapid too wear.
- Tool Steel: Due to their, tool steel requires low SFM values, typically between 30 to 50
- Stainless Steel: Stainless steel exhibits high hardness and the potential to work-harden. Hence, they generally require a lower SFM between 50 to 100.
- Titanium: Machining titanium typically requires meticulous handling and lower SFM values, between 50 to 100 SFM, to mitigate rapid tool wear and excessive heat.
- Inconel: Superalloys such as Inconel are often difficult to machine and demand very low SFM values ranging from 20 to 50 to avoid heat buildup and excessive tool wear.
- Cast Iron: Depending on the specific material composition, cast iron usually requires SFM values within the range of 50 to 150.
- Plastic: Although plastics vary widely, they generally require SFM values between 300 and 600.
Conclusion
Setting the correct SFM in machining operations ensures optimal tool performance, reduced tool wear, and superior quality finished products. Since different materials and processes require varying cutting speeds, a comprehensive understanding of calculating the best Surface Feet per Minute for your project is essential to achieving desired outcomes.
AT-Machining is your one-stop CNC machine shop for reliable and effective machining capabilities. Our team of professionals can carefully analyze the requirements of your machining project to decide the ideal SFM to attain optimal efficiency and high-quality machining operation. We have experience handling a plethora of materials, and our state-of-the-art CNC machinery offers the flexibility and precision we need to meet the specific requirements of your application. Submit your CAD file now for instant quotes!
