Feed Rate for Milling: Guide from Basics to Efficient Applications

In milling, the feed rate for milling is one of the core parameters that determine machining efficiency, workpiece quality and tool life. Whether you are a senior CNC operator or a technician new to milling processing, accurately grasping the calculation logic, influencing factors and optimization methods of milling feed rate is the key to enhancing machining competitiveness. This article will start from basic cognition, gradually disassemble the calculation system of milling feed rate, explain the coping strategies for special scenarios in combination with real machining cases, and provide practical tools and operational suggestions to help you comprehensively solve the core problems related to milling feed rate.

1. Calculation basis of milling feed rate

Milling feed rate refers to the speed at which a tool or workpiece moves in the direction of feed, commonly measured in inches per minute (IPM) or millimeters per minute (mm/min). If the parameters are missing or the error is too large, it will directly lead to the calculation results deviating from the actual demand, causing problems such as fast tool wear, excessive surface roughness of the workpiece and even machining failure.

1.1 Calculate the required core basic parameters

Before calculating the milling feed rate, it is necessary to prepare the following 4 types of key data in advance, which can be obtained from the tool manual, the workpiece material description and the machining process requirements:

• Spindle speed (RPM): the number of rotations of the spindle per minute, derived from the cutting speed (SFM/SMM) and tool diameter, is the core premise of feed rate calculation;
• Number of tool teeth (N): the effective number of cutting teeth of the milling cutter, the number of teeth of different types of milling cutters (such as end mills, face mills, chamfering mills) varies greatly, which directly affects the number of cuts per unit time;
• Feed per tooth (Fz): The distance traveled in the feed direction (unit: inch/tooth or mm/tooth) for each rotation of the tool is determined by the hardness of the workpiece material, the material of the tool and the type of processing (roughing/finishing);
• Processing type and material characteristics: including roughing/finishing, workpiece material hardness (such as 45 steel hardness HB220-250), material toughness (such as the difference in cutting characteristics between aluminum alloy and stainless steel), etc., etc., it is necessary to adjust parameters such as feed per tooth accordingly.

1.2 Basic calculation logic and core formulas

The core calculation logic of milling feed rate is: feed per unit time = spindle speed × number of tool teeth × feed per tooth. Based on this logic, the basic formulas suitable for different scenarios are derived, as shown in the following table:

1.3 Practical case: calculation of feed rate for milling conventional steel parts

A processing plant uses φ10mm carbide end mill (number of teeth N=4) to process No. 45 steel (hardness HB230), in the roughing scenario, it is known that the feed per tooth Fz=0.1mm/tooth, spindle speed RPM=1500r/min, calculate the milling feed rate:

According to the general formula Vf = RPM × N × Fz, the substitution data obtains: Vf = 1500 × 4 × 0.1 = 600mm/min. This calculation result has been verified in practice, which can ensure that the tool wear is normal (the tool wear amount of each 50 workpieces is ≤0.02mm), and the surface roughness of the workpiece is Ra≤3.2μm, which meets the requirements of rough machining technology.

2. Key term analysis: understand the core parameters of milling feed rate

In the calculation and application of milling feed rate, key terms such as SFM, IPM, RPM are often involved, which are the bridge between theoretical calculations and actual machining. Accurately understanding its definitions and interrelationships is fundamental to avoid parameter confusion and ensure accurate calculations.

2.1 SFM and IPM: Feed rate-related core units

• Surface Feet Per Minute (SFM): refers to the linear speed of the cutting edge of the tool and the contact point of the workpiece, which is the core parameter for determining the spindle speed, which is directly related to the tool material and workpiece material. For example, the SFM of steel parts machined by carbide tools is usually 300-500 ft/min, and the SFM of aluminum alloys can be increased to 1000-1500 ft/min (data source: ITMA standard). The formula for this is as follows: SFM = (π × D × RPM) / 12(D is the diameter of the tool, in inches).
• Inches Per Minute (IPM, inches per minute): One of the commonly used units of milling feed rate, which is converted to millimeters per minute (mm/min) as follows: 1 IPM≈ 25.4mm/min. IPM directly reflects the machining progress of the workpiece per unit time, and is a direct parameter for adjusting the machining efficiency, such as: IPM can be appropriately increased for roughing to improve efficiency, and IPM can be reduced to ensure surface quality during finishing.

2.2 The association logic between RPM and each parameter

Spindle speed (RPM) is the key intermediate parameter connecting SFM and feed rate, and its core function is to convert linear speed (SFM) into the rotation frequency of the tool, and then calculate the feed rate in combination with the number of teeth and the feed per tooth. The correlation logic of the three can be summarized as follows: SFM determines RPM, and RPM affects the feed rate.

Example: Using φ1 inch high-speed steel milling cutter to process mild steel, it is known that SFM=100ft/min, according to the conversion formula of SFM to RPM, RPM = (SFM × 12) / (π × D) = (100 × 12) / (3.14 × 1) ≈ 382r/min. If the number of tool teeth N=4 and the feed per tooth Fz = 0.005 inches/tooth, the feed rate IPM = 382 × 4 × 0.005 ≈ 7.64 IPM.

3. Special factor consideration: calculation and application of chip thinning factor

In actual milling operations, when the cutting width (WOC) is less than the tool diameter (D), there will be a phenomenon of “radial chip thinning” – the actual chip thickness is less than the theoretical feed per tooth, resulting in reduced tool cutting load, insufficient machining efficiency, and even tool vibration. Therefore, the feed rate needs to be corrected by calculating the chip thinning factor to ensure machining stability and efficiency.

3.1 Calculation of radial chip thinning factor (RCTF).

The radial chip thinning factor is the core parameter of the corrected feed rate, and its calculation is based on the ratio of the cutting width (WOC) to the tool diameter (D), and the specific formula and description are as follows:

Formula: RCTF = √(1 – (WOC/D)²) (When WOC ≤ D/2, RCTF = 1 – (WOC/D)²/2, error ≤5%)

Parameter description:

• RCTF: radial chip thinning factor, the value range is 0-1, the smaller the WOC, the closer the RCTF is to 0, the larger the feed rate to be corrected;
• WOC (Width of Cut): The cutting width, that is, the radial length of contact between the tool and the workpiece (unit: mm/inch);
• D: Tool diameter (unit: mm/inch).

Case: Using a φ20mm end mill to process steel parts, cutting width WOC=5mm, calculate RCTF:

Substitution formula: RCTF = √(1 – (5/20)²) = √(1 – 0.0625) = √0.9375 ≈ 0.968. If the original calculated feed rate Vf=500mm/min, the corrected feed rate Vf (corrected) = 500 × 0.968 ≈ 484mm/min, which can avoid tool vibration caused by chip thinning.

3.2 Chip Thinning Factor (ACFT) of Chamfer/Feed Milling Cutter

The cutting edge of special tools such as chamfer mills and forming milling cutters is beveled or curved, and the chip thinning phenomenon is more complex, so it needs to be corrected by a special chamfer chip thinning factor (ACFT). The calculation of ACFT requires a combination of chamfer angle (α), depth of cut (DOC) and tool diameter, which can be used as follows simplified formula (for common 45° chamfer mills):

Formula: ACFT = sinα × (D / (2 × DOC)) (α is the chamfer angle, DOC is the chamfer depth)

Practical advice: For chamfer mills with non-standard angles, it is recommended to directly refer to the ACFT parameter table provided by the tool manufacturer or use specialized calculation tools (such as the online calculator on the tool brand’s official website) to avoid excessive manual calculation errors. For example, for a brand of 45°φ16mm chamfer mill, when the chamfer depth DOC=2mm, the manufacturer recommends ACFT=0.85, if the original feed rate is 400mm/min, the corrected feed rate = 400×0.85=340mm/min.

4. Examples of milling feed rate calculation in different machining scenarios

Different workpiece materials, processing equipment (ordinary milling machines/CNC milling machines) and processing types (roughing/finishing) have different requirements for milling feed rates. The following combines two types of typical scenarios to provide detailed calculation steps and parameter selection suggestions to help you quickly adapt to actual processing needs.

4.1 Scenario 1: Calculation of parameters for milling steel (ordinary milling machine)

Steel (such as 45 gauge steel, stainless steel) has high hardness and high cutting resistance, and the selection of feed rate requires balancing efficiency and tool life. Taking machining No. 45 steel (HB220) as an example, the specific steps are as follows:

1. Determine the tool parameters: choose φ12mm carbide end mill, the number of teeth N=3, the tool manufacturer recommends SFM=400ft/min (corresponding cutting speed V=121.92m/min);
2. Calculate spindle speed RPM: According to the formula RPM = (1000×V) / (π×D), substitute the data to get RPM = (1000×121.92) / (3.14×12) ≈ 3200r/min;
3. Select the feed rate per tooth Fz: In the roughing scenario, the recommended Fz value of the cemented carbide tool corresponding to No. 45 steel is 0.1-0.15mm/tooth, where Fz=0.12mm/tooth is taken;
4. Calculate the milling feed rate Vf: Vf = RPM×N×Fz = 3200×3×0.12 = 1152mm/min;
5. Correction and verification: If the actual cutting width WOC=3mm(≤D/2), the RCTF ≈ is calculated to be 0.968, and the corrected Vf=1152×0.968≈1115mm/min. Actual machining verification: the tool cutting is stable, and the surface roughness of the workpiece Ra=2.8μm, which meets the roughing requirements.

4.2 Scenario 2: Speed and feed calculation of CNC milling

CNC milling machines have the characteristics of high degree of automation and high machining accuracy requirements, and the feed rate calculation needs to be combined with the characteristics of the machine tool system (such as FANUC, Siemens system) and programming requirements. Taking the FANUC system machining aluminum alloy (6061-T6) as an example, the steps are as follows:

1. Determine the core parameters: φ16mm tungsten steel end mill (N=4), the recommended SFM value of aluminum alloy 6061-T6 is 1200ft/min (V=365.76m/min), and the finishing Fz = 0.08mm/tooth;
2. Calculate RPM: RPM = (1000×365.76) / (3.14×16) ≈ 7200r/min;
3. Calculate the feed rate Vf: Vf = 7200×4×0.08 = 2304mm/min (IPM≈90.7in/min);
4. CNC system adaptation: In the programming of the FANUC system, the Vf value needs to be written to the program (such as G94 F2304) and set the S7200 (spindle speed);
5. System automatic correction: Some high-end CNC systems support the “adaptive feed control” function, which can automatically adjust the VF according to the real-time cutting load (detected by machine tool sensors) to avoid machining problems caused by uneven material hardness. Actual processing data: The flatness of the workpiece after finishing ≤ 0.01mm/m, and the surface roughness Ra=0.8μm, which meets the requirements of precision machining.

5. Practical tips for efficient milling: optimization strategies for feed rates

After mastering the calculation method of milling feed rate, through reasonable optimization strategies, the machining efficiency can be further improved and the cost can be reduced. The following are 4 core optimization techniques based on industry experience, combined with actual cases to illustrate their application effects.

5.1 Adjust the feed rate in stages: differentiated settings for roughing and finishing

The core goal of roughing is to quickly remove the allowance, and the strategy of “high feed, large cutting depth” can be adopted. The core goal of finishing is to ensure quality, and a “low feed, small depth of cut” strategy is required. For example, when processing No. 45 steel box parts, Vf=1000mm/min, DOC=5mm in the roughing stage, Vf=500mm/min, DOC=0.5mm in the finishing stage, compared with uniform machining, the efficiency is increased by more than 30%, and the surface quality of the workpiece meets the standard.

5.2 Utilize specialized computing tools to improve efficiency and accuracy

Manual feed rate calculations are prone to errors, especially when involving complex scenarios such as chip thinning factors and special tools, it is recommended to use professional calculation tools:

• Online tools: such as Machinery’s Handbook Online, “Speeds and Feeds Calculator” on the official website of the tool brand (such as Sandvik, Kennametal’s online calculator);
• Offline software: such as G-Wizard Calculator, CutData, etc., which can accurately match and calculate according to the machine model, tool material, and workpiece material.

Case: A processing plant uses the Sandvik online calculator to calculate the milling feed rate of stainless steel (304), and after inputting the tool parameters and machining conditions, the calculator directly outputs the optimal Vf=850mm/min and RPM=4500r/min, which increases the tool life by 25% and the machining efficiency by 18% in actual processing.

5.3 Dynamically adjust parameters based on material properties

The cutting characteristics of different materials vary greatly, and the feed rate parameters need to be adjusted accordingly, the following are the feed rate adjustment suggestions for common materials (based on φ10mm carbide end mills):

6. Yigu Technology’s views

As an enterprise focusing on the research and development of precision machining technology and equipment, Yigu Technology believes that the precise control of Feed Rate for Milling is the core starting point for achieving “efficient, high-quality, and low-cost” milling processing, and its value lies not only in the accuracy of parameter calculation, but also in the dynamic optimization ability of combined processing scenarios. At present, the processing industry is developing in the direction of intelligence and precision, and traditional manual calculation and empirical adjustment are difficult to meet the needs of high-end manufacturing.

From the perspective of industry practice, the optimization of milling feed rate will show two major trends in the future: one is “data-driven adaptive adjustment”, which collects cutting force, temperature, vibration and other data in real time through machine tool sensors, and automatically optimizes the feed rate and rotation speed in combination with AI algorithms to achieve dynamic balance in the machining process; The second is “collaborative matching of tool-material-feed rate”, that is, according to the characteristics of different tool materials (such as PCD tools, CBN tools) and workpiece materials, a standardized feed rate parameter library is established to reduce the dependence on operator experience.

Yigu Technology suggests that when improving the ability to control the feed rate, enterprises should give priority to the three dimensions of “basic parameter standardization, calculation tool specialization, and machining process data”, and at the same time strengthen the technical collaboration with tool manufacturers and machine tool manufacturers, and establish an exclusive parameter optimization system based on their own processing needs, so as to finally achieve the double improvement of processing efficiency and quality.

7. FAQ about the feed rate for milling

Q1: What is the impact of the lack of the tool tooth (N) parameter when calculating the milling feed rate? How to solve it?

A1: Lack of tool teeth will cause complete distortion of the feed rate calculation result, and if it is machined according to the wrong value, there may be problems such as tool vibration, poor surface quality of the workpiece or tool damage. Solution: (1) Check the outer packaging box or product manual of the tool, and the number of teeth will be marked on the standard milling cutter; (2) If it is a non-standard tool, measure the number of cutting edges of the tool by caliper to determine the number of teeth; (3) Temporary emergency can refer to the conventional number of teeth of the same type and diameter tool (for example, the common number of teeth of φ10mm end mills is 2, 3, and 4), but it needs to be verified before machining.

Q2: How should the feed rate be adjusted when milling the same material with different hardness?

A2: The higher the hardness of the material, the greater the cutting resistance, and the feed rate needs to be reduced to protect the tool. Taking No. 45 steel as an example, when the hardness is increased from HB200 to HB300, the feed rate per tooth (Fz) should be reduced by 20%-30%, and the corresponding feed rate should be reduced simultaneously. For example: HB200 Fz=0.15mm/tooth, Vf=1440mm/min; For HB300, Fz needs to be adjusted to 0.105-0.12mm/tooth, Vf=960-1152mm/min.

Q3: What is the principle of linkage adjustment between feed rate (F) and rotation speed (S) in CNC milling machine?

A3: The core principle is to “keep the chip thickness within a reasonable range”, that is, when adjusting the speed, the feed rate should be adjusted synchronously to avoid chips being too thick or too thin. Generally follow the principle of “speed increase, feed rate increases synchronously; the ratio is the product of the two (Vf=RPM×N×Fz) remains stable. For example, if the original RPM = 3000r/min, Vf = 1000mm/min, if the RPM is increased to 4000r/min, the Vf needs to be increased to 1333mm/min simultaneously (keeping Fz unchanged).

Q4: Is the Radial Chip Thinning Factor (RCTF) only suitable for finish milling? Is roughing a consideration?

A4: Not only fine milling needs to be considered, but also radial chip thinning needs to be considered if the cutting width (WOC) is less than 1/2 of the tool diameter in rough milling. If RCTF is ignored during rough milling, the actual cutting load will be too low, the machining efficiency will not be fully utilized, and vibration may occur due to unstable tool cutting. It is recommended that when WOC ≤D/2 during rough milling, the feed rate can be corrected according to RCTF to improve the machining efficiency by 10%-15%.

Q5: What are the special requirements for adjusting the feed rate when using high-speed milling?

A5: The feed rate adjustment of high-speed milling (usually SFM>1000ft/min, suitable for aluminum alloy, copper and other materials) needs to meet two requirements: (1) High feed rate matches high rotation speed to ensure reasonable chip thickness and avoid empty cutting or excessive cutting force of the tool; (2) The feed rate should be controlled within the rated range of the machine tool feed system (for example, the maximum feed rate of most CNC milling machines is 10000mm/min). For example, when milling aluminum alloy 6061-T6 at high speed, φ10mm end mill RPM=7200r/min, Fz=0.2mm/tooth, Vf=7200×4×0.2=5760mm/min, it is necessary to confirm that the machine feed system can meet this parameter.