Feed Rate vs. Cutting Speed: Understand the Difference
Computer numerical control (CNC) machining has become the world’s most common subtractive manufacturing technology thanks to its precision, flexibility, and efficiency. It relies on computerized controls and machine tools to remove material from a solid workpiece creating a custom-designed part. To achieve perfect results, the machinist must be aware of the feed rate and cutting speed.
Feed rate and cutting speed are critical cutting parameters that determine the efficiency and quality of machining processes. They operate on distinct principles and impact the machining operation in different ways.
If you’re about to use any CNC machine, understanding these two parameters is vital. In this article, we’ll explain the key difference between feed rates and cutting speeds. We’ll explore their significance and how they affect the CNC machining process. Understanding these differences is the key to optimizing your CNC strategy for better results and efficiency.
What Is Feed Rate?
Feed rate refers to the distance the tool advances along the workpiece in a single spindle revolution. For milling operations, the feed rate is expressed in inches per minute (in/m) or millimeters per minute (mm/m). In turning and boring processes, feed rate is measured in inches per revolution (in/rev) or millimeters per revolution (mm/rev). The number of flutes (or teeth) on a cutting tool directly influence the feed rate. More flutes result in a faster feed rate. Other CNC machining parameters that affect the feed rate are the chip load and RPM of the cutter.
Feed rate plays a crucial role in the surface finish of a CNC machined part. Therefore, optimization of feed rates is crucial in any machining process.
When you're setting the feed rate, you need to consider
Your chosen feed rate will directly affect aspects such as tool life, machining efficiency, product quality, and more. To determine the optimum value for a specific project, machinists will need to consider the following factors.
Workpiece Hardness
CNC machining can be used for diverse materials ranging from metals to plastics and wood. Since different materials have varying degrees of hardness, the ideal feed rate will depend on the specific workpiece material.
For example, hard materials such as steel and titanium require a slower feed rate to minimize tool wear and prevent damage to the workpiece. Conversely, softer materials, such as aluminum, can be machined at faster feed rates with no negative effect on the workpiece or machine tool. Higher feed rates improve the productivity of machining operations.
Cutting Tool Material
CNC machine tools are made of different materials, each with unique benefits and applications. The optimal feed rate will depend on the machine tool’s capacity to withstand vibrations and the cutting force.
For instance, tools made from high-performance materials, such as diamond and cubic boron nitride (CBN), offer exceptional hardness and thermal resistance. They are ideal for demanding applications and can handle the vibrations and temperatures associated with high feed rates. In comparison, high-speed steel (HSS) tools cannot match the thermal and wear resistance of such tools.
The Expected Tool Life
The expected tool lifespan plays a key role in determining the optimal feed rate. High feed rates can speed up tool wear resulting in a shorter tool life. Conversely, if the feed rate is too low, the whole cutting process becomes inefficient.
The ideal feed rate value will depend on how long the machinist wants to keep using the tool. Engineers determine the expected tool longevity based on factors such as cost of the tool and produced number of parts.
Surface finish
Lower feed rates typically create a smoother surface finish with minimal defects. Therefore, a common CNC strategy is to use a higher feed rate for a rough cut and then switch to a lower rate for the finishing operation.
What Is Cutting Speed?
Cutting speed is the linear velocity between the cutting tool and the workpiece. In other words, it is the speed at which the cutting edge moves across the workpiece surface. The cutting speed is primarily influenced by the spindle speed (rotational speed of the machining tool) and the cutting tool diameter. It is also called surface speed and is measured in meters per minute (m/ min) or surface feet per minute (SFM). Cutting speed directly affects the material removal rate (MRR). A higher cutting speed translates to a higher MRR, meaning that more material is removed from the material surface per unit time.
Adopting the optimum cutting speed is the basis of efficient operatiion of CNC machines. You can determine the proper surface speed by considering several pertinent factors.
When you setting the rate, you need to consider
Optimal cutting speeds are a vital parameter in efficient machining operations. Here are the different factors that you need to take into account when predicting the optimum cutting speed.
Cut Width
The cut width is also known as the radial depth of cut (RDOC). Simply put, it’s the span along the workpiece’s surface that the cutting tool engages in a single pass. The cut width is an important consideration when determining feeds and speeds. If it is less than half of the cutter diameter, chip thinning may occur. This is a phenomenon that reduces the chip load (the thickness of the chips removed in a single revolution) leading to a longer machining time. A simple way to combat chip thinning is to adopt a higher cutting speed.
Depth of cut
The depth of cut (DOC) refers to how deep a machine tool cuts into a workpiece per pass. In other words, it is the total amount of material removed in one turn of the cutting tool. A large depth of cut reduces the number of passes required during machining processes. However, it also results in a larger cutting resistance and increased cutting temperatures. To mitigate these issues, a lower cutting speed is adopted.
The strength of the cutting tool
Tool strength is a vital consideration when determining the optimal cutting speed. Faster cutting speeds generate more vibrations, high temperatures, and an increased cutting force. Softer cutting tools are susceptible to premature wear and damage if exposed to these conditions. On the other hand, stronger cutting tools are more suited to high cutting speeds. For example, carbide tools will on average have over 3 times the material removal rates of HSS tools. Similarly, polycrystalline diamond (PCD) tools will on average get over 5 times the cutting speed of an HSS tool.
What is the Difference Between Cutting Speed and Feed Rate?
Speeds and feeds are two separate motions in machining. However, given their close relationship, it can be easy to mix them up. Here are some factors that set these two parameters apart.
Cutting Temperature and Tool Life
A key difference between cutting speed and feed rate is the distinct ways they influence the cutting temperature. Excessive heat during machining can cause damage to both cutting tools and the workpiece. The cutting speed has a huge effect on the cutting temperature — higher cutting speeds translate to increased temperatures. On the other hand, the feed rate has a comparatively smaller impact on the cutting temperature and tool life.
Surface Roughness and Scallop Marks
In CNC machining, scallops are a type of surface irregularity. They are otherwise known as feed marks. Scallops occur in the form of tiny ridges that lead to rougher machined surfaces. The feed rate directly impacts the formation of scallops. A high feed rate will promote more feed marks and surface roughness and vice versa. However, the cutting speed has no effect on scallops and, therefore, it doesn’t impact the part’s surface finish.
Directrix and Generatrix
In geometry, a generatrix is defined as a point, line, or surface whose motion along a given path creates a new shape. The path followed by the generatrix is known as the directrix. The primary objective of machining is to create parts with various geometries and quality finishes. Therefore, these two parameters are an essential part of machining operations. Speeds and feeds differ in that the cutting speed generates the generatrix while the feed rate creates the directrix.
Conclusion
Cutting speed and feed rate optimization are crucial for any CNC machining operation. By understanding and adjusting these parameters, machinists can improve tool longevity, productivity, surface finish, and the overall CNC machining outcome. It should be noted that there is no one-size-fits-all solution with speed and feed optimization. The ideal settings will depend on factors such as workpiece material, tool material, surface finish, expected tool life, depth of cut, and more.
Fortunately, you don’t have to worry about the intricacies of machining when you outsource to a trusted and experienced partner like Aria. Our expert machinists and engineers deliver high-quality parts that are ideal for your unique needs. Contact us today for all your CNC machining needs.
Author
Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.