Skip to content 
What Is Feed Rate?
The feed rate is the distance a cutting tool covers during one spindle in revolution or the velocity at which the workpiece advances the milling cutter or vice versa. It can also be called the cutting tool engagement speed for milling operations. Machinists often measure it in millimeters/minute or inches/minute (mpm or ipm). Feed rate can be measured in millimeters/revolution or inches/revolution (mpr or ipr) for boring or turning operations.
The tool material (Cermet, Ceramic, HSS cutting tool, etc.), the blank material (Stainless Steel, Mild Steel, Aluminum, Wood, etc.), and other cutting parameters like CNC machine characteristics and surface finish will determine feed rate variation. The feed rate determines the machined product’s physical appeal; hence, the feed rate’s optimization is essential in CNC machining processes. Machinists calculate the feed rate by considering the number of flutes or teeth on a CNC cutter and calculating each tooth’s feed rate.
Although the same factors affect the cutting speed and feed rate, their effects are less pronounced. The feed rate is paramount to the final aesthetic appeal of the finished parts. Thus, feed rate optimization is critical in CNC machining processes.
Here are different factors machinists must consider to determine the optimum feed rate value:
Cut Width
Cut width or radial depth of cut (RDOC) is the span along the surface of the workpiece that the CNC tool engages in a single pass. Typically, chip thinning occurs when any cut width is less than half the diameter. It is a common manufacturing defect where there is a reduced chip load or material the cutter removes in one revolution. Since chip thinning could result in extended lead time, it is essential to prevent it. Besides, increasing the feed rate will help mitigate the effects of chip thinning, increase productivity, and extend the tool’s lifespan. However, you can adopt a higher cutting speed to resolve chip thinning.
Productivity
The primary objective of machining operations is to maximize the material removal rate without sacrificing tool life and the quality of finished parts. However, there are cases where you can increase the feed rate for higher productivity and cycle time at the expense of superior surface quality. Nevertheless, you can maintain a balanced speed and feed rate to achieve cost-effective production.
Feed Rate Limit
Typical CNC machine tools possess a feed-by-feed rod within the minimum and maximum feed rate limits. Beyond the limit is impermissible for these machine tools, and only limited feed rate options within the permissible limit can be applied for conventional lathe machines. As such, maintain the permissible feed rate based on the machine’s capability and as the tool manufacturer specifies.
Surface Finish
To attain a superior surface finish, it is advisable to employ lower feed rates for workpiece finishing while you consider a coarse feed rate for the rough cut. For instance, you can adopt 0.01 to 0.05 mm/rev for finishing operations and 0.1 to 0.3 mm/rev for roughing operations. Hence, the required surface roughness is calculated, and the feed rate is calibrated to meet the specifications.
Cutting Tool Geometry
Aside from the feed rate, the cutting tool geometry can affect a machined part’s surface finish. If the geometry permits, a higher value for the tool geometry would be advisable. CNC tools with more cutting edges shear less material per pass. Hence, they can handle higher feed rates. As such, ensure the tool’s geometry is utilized to attain an optimal feed rate.
Machine Tool Capability
Higher feed rates result in high cutting force and high vibrations experienced during machining. Each CNC machine tool has operational limits and capabilities based on its rigidity, power, and torque. Hence, it would be best to choose the feed rate based on the absorption and transmission of high forces and vibration of the machine tool.
What Is Cutting Speed?
Cutting speed describes the velocity between the workpiece’s surface and the CNC cutter. Machining experts define cutting speed as how fast the workpiece moves past the CNC tool edge. In other cases, it can described as the feet per minute or linear distance of meters per minute that the cutting tool engages the workpiece surface.
Cutting or surface speed is generally measured in ft/min (feet per minute) or m/min(meters per minute). Cutting speed is critical in determining other CNC machining parameters, including power consumption, cutting temperature tool life, etc. The values of cutting speeds of a milling machine vary based on different materials, including plastics, low-carbon steel, high-carbon steel, and aluminum. Machinists must operate other machine tools, such as knurling and threading tools, and lower cutting speed.
Ensuring optimum rotational speed in CNC machining processes is essential to attain the best results. However, it is feasible to determine the optimum cutting speed for a specific CNC machining process by examining other factors. These factors may include:
Cutting Tool Materials
Machinists use various lathe tools for different CNC machining. Each tool has different hardness properties since they are created from varying material types. The cutting speed used in the CNC machining process will significantly affect the cutting tool material. The machinist can use a high cutting speed with slight effects if the cutting material possesses high strength. However, softer cutting tool materials will likely wear out quickly with higher cutting speeds, leading to shorter tool life.
Workpiece Hardness
The hardness of the workpiece being cut is critical when determining the optimal cutting speed for a cutting process. Hardness refers to the resistance of a workpiece to deformation caused by indentation, scratching, and abrasion. The softer the material, the faster the cutting speed, and vice versa. For instance, you may require a faster cutting speed for CNC materials like aluminum, unlike steel, which may require a slower cutting speed since it is a harder metal.
The Expected Tool Life
Another factor determining optimal cutting speeds is how long the machinist wants the CNC cutting tool to last. This often includes evaluating variables like the tool’s cost and cost compared to the quantity of parts fabricated. While higher cutting speeds might be feasible for use, provided these variables are favorable, softer cutting tool materials will lead to premature tool wear.
Importance of Feed Rate and Cutting Speed in CNC Machining Operation
In CNC machining, speeds and feeds are paramount since they determine the rate at which the workpiece material is sheared and the amount of material removed. Besides, the speed and feed in machining significantly affect the tool’s life.
A crucial factor known as cutting temperature determines the differences between the feed rate and cutting speed. Higher cutting temperatures can adversely affect parameters, including surface roughness and tool life. However, since there is an extensive margin for error, the effects of speeds and feeds are not visible on softer materials such as aluminum or resin. Nevertheless, the poor effects of speeds and feeds are noticeable on harder materials like Inconel and titanium due to the limited error range.
Rapid tool breakage usually occurs due to slight differences between the feed rate and speed. Therefore, the feed rate and speeds are mandatory to achieve superior surface roughness on machined parts. The chatter marks will appear on the machined surface if the machine runs at a high spindle speeds and tool rate.
Exploring the Differences between Feed Rate and Cutting Speed
Despite being intertwined in machining operations, feed rate, and cutting speed are two distinct motions in CNC machining. Here are some key differences between these parameters:
Directrix and Generatrix
A generatrix in geometry refers to a surface, point, or line whose motion along a defined path creates a new shape. The directrix is the path the generatrix follows. Machinists denote a directrix by s or f and measure it in mm/rev or mm/min. On the other hand, a generatrix is denoted by Vc while it is measured in m/min or ft./min.
Hence, feed rate and cutting speed parameters are paramount to machining operations. Feeds and speeds differ in machining because cutting speed produces the generatrix while the feed rate produces the directrix.
Surface Roughness and Scallop Marks
Scallops, called feed marks, are surface irregularities in CNC machining. This surface irregularity occurs as tiny ridges that result in rougher machined surfaces. A low feed rate will mitigate feed marks and surface roughness and vice versa.
However, the cutting speed does not impact scallops; hence, it doesn’t affect the surface finish. Meanwhile, the direct involvement of the feed rate influences the scallop marks on a workpiece surface.
Cutting Temperature and Tool Life
The way the feed rate and cutting speed influence cutting temperature is another essential difference between these parameters. The workpiece and cutting tools can be damaged when exposed to excess heat during CNC machining.
The cutting speed substantially impacts the cutting temperature because higher cutting speeds lead to increased temperatures, while slower cutting speeds ensure moderate temperatures. Conversely, the feed rate possesses a comparatively lower impact on the cutting temperature and CNC tool life.
How to Determine Feed Rate vs Cutting Speed in Machining
Defining parameters such as feed rates and cutting speeds is paramount for optimal machining conditions. The chart above provides essential parameters to determine the feed rate units and cutting speeds of different machining operations. The spindle speed is the primary requirement for determining cutting speed and feed.
You have to follow two methods to achieve the final feed – the first method is to determine the feed per tooth, while the second method involves determining the feed rate of the tool using the feed per tooth.
Below are other essential considerations for determining feeds and speeds in machining:
Spindle Speed Limit
Depending on the raw material, you must consider the milling tool diameter and surface feet per minute (SFM) to determine the cutter speed in RPM. However, the calculated speed may be unfeasible, especially with smaller tooling and certain materials.
Therefore, the machinists would have to run the tool with the available machine’s maximum speed while maintaining the required chip load for the diameter. Consequently, you can achieve optimal parameters at the machine’s top speed.
Interaction of Cutting Velocity and Feed Rate
The cutting tool compresses the surface of the workpiece when machining and shears a thin layer of material as a chip. The relative velocity between the CNC tool and the workpiece is required to transfer the intended compressive force. The cutting velocity produces the primary relative velocity, which helps envisage the material removal.
Another synchronous motion (feed motion) must be provided to the CNC tool or workpiece along the required direction to envisage the material removed from the total workpiece surface. The feed rate, cutting tools’ simultaneous actions, and feed rate will fulfill the basic requirements of the machining process.
Non-Linear Path
The feed rate generally involves a linear motion (i.e., the distance covered in a line). However, there are certain situations when the feed rates are regarded as being in an arc or circular interpolation path (inner or outer diameter). There is an increase in the angle of engagement on a tool, which results in a non-linear path as the depth of the cut increases. The tool’s engagement is higher for internal corners than external corners.
Conclusion
Feed rate and cutting speed are paramount variables in optimizing the efficiency and quality of your CNC machining process. Comprehending these parameters helps machinists adjust them to attain optimized tool longevity, desired surface finish on machined parts, improved productivity, and overall CNC machining results. When it comes to CNC machine speed and feed rate optimization, there is no one-size-fits-all. Thus, variables such as depth of cut, surface finish, tool material, expected tool life, and workpiece material type determine the ideal configuration.
You need not worry about machining intricacies such as feed rate and cutting speed when you partner with AT-Machining. As your trusted and experienced manufacturing partner, our expert teams leverage our manufacturing capabilities and state-of-the-art CNC facilities to deliver high-quality parts that meet your design requirements and standards. Don’t hesitate to contact us to speak to our professionals about your CNC machining needs!
FAQs
Does Cutting Force Influence the Surface Roughness of Finished Parts?
Cutting force is a crucial determinant of a finished part’s quality. Hence, excessive cutting force can result in tool chatter, deflection, and vibration, adversely affecting the overall quality of the fabricated products, surface finish, and dimensional accuracy.
How Do Feed Rate and Cutting Force Affect Cutting Tool?
Generally, an increase in feed rate for all cutting speeds and depths of cut causes an increase in cutting force. Besides, cutting force increases as the tool wears since a worn cutting tool has less efficient teeth (cutting edge). Hence, it would be best to mitigate excessive tool wear and adjust feed accordingly to ensure consistent cutting force and extended tool life.
What Happens if the Feed Rate Is Too Low or Too High?
Feeding the material too slowly during CNC machining can cause rubbing instead of cutting, resulting in a poor tool life. Conversely, an extremely high feed rate or higher speed than the maximum RPM results in overheating or cutter breakage due to the excessive friction generated during machining. As such, it is essential to maintain optimal feed rates during various machining operations for the best tool life and surface roughness.
