In CNC machining, feed rate, and cutting speed are two critical parameters that directly impact machining efficiency, part quality, and tool life. Choosing the right feed rate and cutting speed can not only accelerate machining processes and shorten production time but also improve surface quality and extend tool life. A deep understanding of these parameters and their application in actual machining is essential for optimizing CNC processes.
In the following text, Yonglihao Machinery will further explore how to select the optimal feed rate and cutting speed based on specific machining needs.
Table of Contents
What is the Feed Rate?
In CNC machining, feed rate (also known as feed speed) is the measure of the distance a tool moves along the workpiece surface per unit of time, usually expressed in millimeters per minute (mm/min) or inches per minute (rpm). The feed rate is a critical parameter in the machining process, directly affecting machining efficiency, tool wear, machining quality, and overall machining time. The selection of the feed rate needs to be precisely adjusted based on the specific machining task, material properties, tool type, and surface quality requirements.
Choosing the Optimal Feed Rate
When selecting the optimal feed rate, several factors need to be considered:
Workpiece Quality and Surface Requirements: The dimensional tolerance, shape complexity, and surface finish requirements of the workpiece will influence the choice of feed rate. A lower feed rate is usually needed to reduce surface roughness and ensure machining precision, especially for high-quality machining results.
Workpiece Material and Material Properties: The hardness, toughness, thermal conductivity, and other characteristics of different materials (such as aluminum alloys, stainless steel, etc.) directly affect the choice of feed rate. Harder materials typically require lower feed rates to avoid tool damage, while materials with poor thermal conductivity require even lower feed rates to prevent overheating and tool annealing. Adjusting the feed rate according to material properties is essential to ensure efficient cutting and prevent premature tool wear.
Tool Material and Type: Each tool material (such as carbide, high-speed steel, etc.) and type (such as end mills, turning tools, etc.) has specific feed rate requirements. The hardness and wear resistance of the tool determines its performance at different feed rates.
Cutting Tool Geometry: The geometry of the tool (such as tip radius, rake angle, relief angle, etc.) influences the optimal feed rate. Special geometries may require adjustments in feed rate to achieve the best cutting performance.
Other Factors Affecting the Optimum Feed Rate
Type of Machining Operation: Different machining operations (such as CNC milling, CNC turning, and drilling) require different feed rates. Milling typically requires a higher feed rate, while precision drilling may need a lower feed rate to ensure accuracy.
Cutting Width: A larger cutting width increases the cutting force, necessitating a lower feed rate to avoid tool overload.
Tool Durability and Lifespan: An excessively high feed rate increases tool load, accelerating tool wear. Proper adjustment of the feed rate can extend tool life while improving machining efficiency. Balancing productivity and tool lifespan is crucial in optimizing feed rates.
Productivity: A higher feed rate usually increases machining speed, reducing cycle time and thus enhancing productivity. However, an excessively high feed rate may lead to reduced surface quality and increased tool wear, so it is important to balance quality requirements and cost control while enhancing productivity.
Machine Tool Capability: The maximum feed rate of a machine tool depends on its design and drive system capabilities. Exceeding the machine’s capacity may cause vibration, tool chatter, or even machining failure.
What is Cutting Speed?
Cutting speed is a critical parameter that measures the speed at which the cutting edge of a tool moves relative to the surface of the workpiece. In CNC machining, cutting speed is typically expressed in meters per minute (m/min) or feet per minute (ft/min). It not only affects machining efficiency but also has a significant impact on machining quality, tool wear, and machining costs. Proper cutting speed can optimize the machining process, increase production efficiency, and extend the tool’s lifespan.
Choosing the Optimal Cutting Speed
Selecting the optimal cutting speed requires considering multiple factors to balance machining quality and efficiency. The main influencing factors include:
| Material | Turning | Drilling | Reaming | End Milling(Roughing) | |
| Aluminum | 400-1000 | 250-600 | 100-300 | 600 | |
| Brass | 225-300 | 150-300 | 130-200 | ||
| Bronze | 150-225 | 100-250 | 75-180 | Medium: 250
Hard: 125 |
|
| Cast Iron | Soft | 100-150 | 75-150 | 60-100 | 60 |
| Medium | 75-120 | 70-110 | 35-65 | ||
| Hard | 50-90 | 60-100 | 20-55 | 50 | |
| Copper | 100-200 | 60-100 | 40-60 | ||
| Magnesium | 600-1200 | 300-650 | 150-350 | ||
| Stainless Steel | Free machining | 100-150 | 65-100 | 35-85 | 304: 55
17-4PH: 35 |
| Other grades | 40-85 | 15-50 | 15-30 | ||
| Carbon and Alloy Steel | Free machining | 125-200 | 100-145 | 60-100 | Low C: 75
4140: 50 4340: 50 |
| Less than 0.3% C | 75-175 | 70-120 | 50-90 | ||
| 0.3% to 0.6% C | 65-120 | 55-90 | 45-70 | ||
| More than 0.6% C | 60-80 | 40-60 | 40-50 | ||
| Titanium | 25-55 | 30-60 | 10-20 | Ti-6AL-4V: 25 | |
Workpiece Hardness: The hardness of the workpiece material directly influences the choice of cutting speed. Harder materials require lower cutting speeds to reduce tool wear and extend tool life. Conversely, excessively high cutting speeds can accelerate tool wear and degrade the surface quality of the workpiece.
Cutting Tool Strength: The material, strength, and wear resistance of the tool determine the cutting speed it can withstand. Carbide tools can operate at higher cutting speeds, while high-speed steel tools typically require lower speeds. Insufficient tool strength may lead to tool breakage or rapid wear, affecting the machining outcome.
Tool Lifespan: Although higher cutting speeds can increase machining efficiency, they also increase the thermal load on the tool, accelerating wear and shortening the tool’s lifespan. Selecting an appropriate cutting speed can reduce tool wear rates, extend tool life, and maintain high machining quality and efficiency. In optimizing the machining process, it is crucial to find the optimal balance between tool lifespan and cutting speed.
Why is Speed and Feed Important in Machining?
Speed (cutting speed) and feed rate are two crucial parameters in machining, as they directly affect various aspects of the machining process, including efficiency, surface quality, tool life, and overall machining costs.
Machining Efficiency: Higher cutting speeds can reduce machining time, while an appropriate feed rate ensures rapid material removal, thereby shortening production cycles and increasing output.
Surface Quality: Excessively high cutting speeds may lead to increased surface roughness, while too fast a feed rate can cause tool marks or surface defects. Adjusting speed and feed can achieve the desired surface finish and part accuracy.
Tool Life: Higher cutting speeds and faster feed rates increase the thermal load on the tool, leading to accelerated wear and shorter tool life. By optimizing these two parameters, tool life can be effectively extended, reducing the frequency of tool changes and lowering production costs.
Machining Costs: Improper parameter settings, whether too high or too low, can lead to unnecessary cost increases. Optimizing these two parameters can reduce material waste, decrease tool consumption, and lower energy costs, ultimately improving the machining economy.
Machining Stability: Reasonable speed and feed rate settings help maintain the stability of the machining process, reducing vibration and noise, ensuring smooth operation, and thus enhancing product consistency and reliability.
Differences Between Feed Rate and Cutting Speed
Feed rate and cutting speed have different physical meanings and impacts in CNC machining. The feed rate controls the forward speed of the tool relative to the workpiece, while the cutting speed controls the movement speed of the tool’s cutting edge on the workpiece surface. The feed rate mainly affects machining efficiency and surface quality, while cutting speed has a greater influence on tool life and machining temperature. The following chart of cutting speed and feed rate makes the discrepancy easier to see:
| Parameter | Cutting Speed | Feed Rate |
| Generatrix and Directrix | Directrix is generated by the cutting speed | Generatrix is generated by the feed rate |
| Units of Motion and Short Form | Measured in meters per minute (m/min) or feet per minute (ft/min) and denoted by Vc | Measured in meters per revolution (mpr) or inches per revolution and denoted by s or f |
| Chip Direction | No effect on deviating from the orthogonal chip direction | Affect the actual chip flow direction |
| Cutting Force and Power Consumption | Influence the cutting force and power consumption | Not influence the cutting force and power consumption |
| Surface Roughness and Scallop Marks | Not directly related to the scalloping or marks produced on the machined surface | Directly related to the scalloped marks on the finished surface |
| Cutting Temperature, Tool Life, and Tool Wear
|
Greatly impacted | Less impacted |
Cutting Temperature and Tool Life
Cutting temperature directly affects tool life. High cutting temperatures accelerate tool wear, reducing the tool’s durability. By appropriately adjusting cutting speed and feed rate, cutting temperature can be lowered, thus extending tool life.
Surface Roughness and Scallop Marks
Feed rate and cutting speed have a significant impact on the surface quality of the workpiece. A high feed rate may increase surface roughness and even cause scallop marks. To achieve a smoother surface, it is necessary to reduce the feed rate and optimize the cutting speed.
Lead and Generatrix
Lead and generatrix are critical factors in describing the geometry of the workpiece. Feed rate and cutting speed affect the precision of these geometric shapes, which in turn affects the overall dimensional and shape tolerances of the workpiece.
Physical Differences and Operation
The physical characteristics of feed rate and cutting speed determine their different applications in operation. The feed rate influences the forward speed of the tool, while the cutting speed affects the contact speed between the tool and the workpiece.
Tool Motion
Tool motion is jointly determined by the feed rate and cutting speed, influencing the tool’s trajectory and the final machining outcome during the process.
Chip Direction
Feed rate and cutting speed affect the formation and ejection direction of chips. Improper settings may cause chip buildup, hindering the smooth progress of machining.
Cutting Force and Power Consumption
Feed rate and cutting speed directly influence the magnitude of the cutting force, which in turn affects the power consumption of the machine tool. Proper settings can reduce cutting force, decrease energy consumption, and extend tool life.
Determining Feed Rate and Cutting Speed
Factors Affecting Feed Rate and Cutting Speed
When determining feed rate and cutting speed, it is essential to consider multiple factors, including material properties, tool type, machining method, and machine capability, to ensure machining efficiency and quality.
Material Properties and Tool Type: The hardness, toughness, and thermal conductivity of the workpiece material directly influence the choice of feed rate and cutting speed. Generally, harder materials require lower cutting speeds and feed rates to reduce tool wear. The tool material and geometry also determine the appropriate feed rate and cutting speed. For example, carbide tools can withstand higher cutting speeds, while high-speed steel tools are suited to lower speeds.
Machining Type and Surface Requirements: Different machining operations have varying requirements for feed rate and cutting speed. Additionally, the surface finish requirements of the workpiece will impact the selection of these parameters. High-quality surfaces typically require lower feed rates and cutting speeds to minimize surface roughness and defects.
Spindle Speed and Machine Capability: The spindle speed limit directly affects cutting speed, and it is crucial to match the cutting speed with the spindle speed while operating within the machine’s allowable range. Similarly, the machine’s feed capability must align with the set parameters to avoid overload and ensure machining stability.
Interaction Between Cutting Speed and Feed Rate: Cutting speed and feed rate influence each other, and it is necessary to find the optimal balance between them during machining to ensure quality, efficiency, and tool life.
Nonlinear Paths and Operational Adjustments: For complex nonlinear machining paths, it may be necessary to dynamically adjust feed rate and cutting speed based on the path’s variations to ensure uniform cutting and stable machining. In practice, parameters should be continuously adjusted based on machining feedback. If rapid tool wear or poor surface quality is observed, adjustments should be made promptly to optimize machining results.
Calculating Feed Rate and Cutting Speed
Calculating feed rate and cutting speed is a critical step in determining machining parameters. These calculations are typically based on the material type, tool specifications, machining conditions, and the desired outcome. Empirical formulas and data provided by tool manufacturers are commonly used reference tools. Correct calculations ensure that the selected parameters meet machining requirements while balancing efficiency, quality, and tool life. For complex machining operations, special attention should be paid to nonlinear paths or special materials during calculations to ensure that the final results meet expectations.
Conclusion: Maximising CNC machining efficiency
The key to maxing out efficiency in CNC machining is to understand and apply the feed rate and cutting speed. Picking the best feed rate and cutting speed boosts productivity. It also lengthens tool life and ensures high-quality parts. You must adjust these parameters precisely. Consider the workpiece and tool materials, the type of machining, and the machine’s capabilities. This is the basis for efficient and economical machining. So, understanding the impact of these machining parameters is key. Finding the best settings for a task through continuous testing is an effective way to improve CNC machining efficiency. What you should know before performing CNC production machining includes how these key parameters are set and adjusted, and how to select the right machining conditions for your specific task and material.
FAQ
Do feed rates and cutting speeds affect tool life?
Yes, feed rate and cutting speed have a significant effect on tool life. Cutting too fast can cause overheating and rapid tool wear. Feeding too fast can cause tool breakage or premature wear. Properly selecting and optimizing these parameters is key to extending tool life. It ensures machining quality and improves productivity.
Are cutting speed and feed rate the same?
No, they are not the same. Cutting speed refers to the surface speed of the cutting edge of the tool, while feed rate refers to the speed at which the tool moves relative to the workpiece.
What is the feed rate in CNC machining?
Feed rate refers to the speed at which the tool moves relative to the workpiece, typically measured in millimeters per minute or inches per minute.
What happens if my cutting speed is too high?
Excessively high cutting speed can lead to rapid tool wear, generate high temperatures, degrade machining quality, and may even damage the tool or workpiece.
What has the most influence on feed rate and cutting speed?
The hardness of the workpiece and the wear resistance of the tool are the main factors. They have the biggest impact on the feed rate and cutting speed. Hard materials require lower cutting speeds and appropriate feed rates to minimize tool wear. At the same time, carbide and other high-performance tool materials allow higher cutting speeds. They do this without significantly affecting tool life.
Can I use the same feed rate and cutting speed for different materials?
Typically, no. Different materials have varying hardness and characteristics, requiring different feed rates and cutting speeds to avoid tool wear and machining quality issues.
Why must engineers and machinists consider cutting speed and feed rate in CNC machining?
Because these two parameters directly affect machining efficiency, surface quality, tool life, and overall machining costs.