Distinction Between Feed Rate and Cutting Speed in CNC Machining

 CNC machining, a subtractive manufacturing method, involves the removal of material chips until the final product is achieved. To commence the process, machinists must be aware of the amount of material removed in one revolution and the speed at which the CNC machine operates. The crucial discrepancy between feed rate and cutting speed comes into play at this stage.

When conceptualizing parts for CNC machining, it is imperative to take these variables into account as they play a pivotal role in optimizing different facets of the CNC machining process. Cutting speed predominantly influences factors such as tool life and power consumption, while feed rate is instrumental in determining machining time and the surface roughness of the finished area. This article aims to draw a comparison between feed rate and cutting speed while elucidating the methodologies for calculating each.

Distinction Between Feed Rate and Cutting Speed in CNC Machining插图

Understanding Cutting Speed

 Cutting speed is commonly defined as the relative velocity between the workpiece’s surface and the cutting tool. Alternatively, it can be perceived as the speed at which the workpiece moves past the cutting tool’s edge. Measurement units for cutting speed include meters per minute (m/min) or feet per minute (ft/min). This parameter significantly impacts other CNC machining parameters like cutting temperature, power consumption, and tool life, setting it apart as a crucial factor differentiating feed rate from cutting speed.

Optimizing Cutting Speed

Achieving the optimal cutting speed is imperative for obtaining the best possible outcome in the CNC machining process. Predicting the ideal cutting speed involves considering various factors, such as workpiece hardness. Material hardness plays a pivotal role in determining cutting speed, with harder materials requiring slower cutting speeds and vice versa. For instance, machining steel necessitates a lower cutting speed compared to aluminum.

Material for Cutting Tools

Various lathe tools are employed in CNC machining, each crafted from diverse materials with distinct hardness properties. The choice of cutting tool material significantly impacts the cutting speed during machining. High-strength cutting materials allow for increased cutting speeds with minimal consequences, while softer materials wear out quickly under higher cutting speeds, resulting in reduced tool life.

Projected Tool Longevity

The desired lifespan of the tool is a crucial factor in determining the appropriate cutting speed. Considerations include the tool’s cost and how it compares to the production quantity. Favorable conditions may warrant the use of higher speeds.

Understanding Feed Rate

 The feed rate represents the distance covered by the cutting tool during one spindle revolution or the velocity at which the cutter advances against the workpiece. It is measured in inches per revolution (ipr) or millimeters per revolution (mpr) for turning and boring, while milling processes use inches per minute (ipm) or millimeters per minute (mpm). Machinists calculate the feed rate by considering the number of flutes or teeth on the cutting tool.

Distinction Between Feed Rate and Cutting Speed in CNC Machining插图1

Feed Rate and Spindle Speed Relationship

 The feed rate influences factors similarly affected by cutting speed, albeit to a lesser extent. However, the feed rate plays a crucial role in achieving the desired surface finish of the machined part. Factors influencing its optimization include cut width, tool type, spindle power, workpiece strength, and threads per inch (TPI) for tapping and threading tools.

Distinguishing Feed Rate from Cutting Speed

Feed rate and cutting speed share closely related definitions, often causing confusion. Some refer to these parameters as speeds and feeds. Practical differences include their effects on cutting temperature and tool life. Feed rate has a lower impact on cutting temperature and tool life compared to cutting speed.

Cutting Temperature and Tool Life

The cutting temperature distinguishes feed rate from cutting speed, as higher temperatures can affect tool life and surface finish. Feed rate has a comparatively lower impact on cutting temperature and tool life than cutting speed.

Surface Roughness and Scallop Marks

Distinction Between Feed Rate and Cutting Speed in CNC Machining插图2

Scallop marks, or feed marks, contribute to surface roughness in CNC-machined parts. The feed rate directly influences scallop marks, with higher feed rates resulting in increased marks and roughness. In contrast, cutting speed does not affect scallop marks and, consequently, does not impact surface finishes.

Directrix and Generatrix

Within the realm of geometry, a generatrix denotes a point or surface that, upon movement along a specified path, gives rise to a new shape. The designated trajectory along which the generatrix traverses is referred to as the directrix. In the field of machining, the fundamental objective is to fabricate geometric surfaces boasting visually pleasing finishes and heightened precision. Consequently, these two variables are indispensable in machining processes. The distinction between cutting speed and feed rate lies in the fact that cutting speed dictates the generatrix, while the feed motion governs the directrix. Additional factors that set apart feed rate from cutting speed encompass:

• Units of measurement.

• Effect on cutting force and power consumption.

• Moreover, the motion responsible for generating cutting speed differs from that which determines feed rate (cutting motion and feed motion, respectively).

Cutting Temperature and Tool LifeIt affects these parameters to a greater extent.It affects these parameters on a smaller scale.
Surface Roughness and Scallop MarksIt does not influence surface roughness.It has a significant influence on the surface roughness of a finished part.
Directrix and GeneratrixCutting speed generates the directrix.The feed rate generates the generatrix.
Units of MotionMachinists measured it in meters per minute (m/min) or feet per minute (ft/min).Machinists measure it in meters per revolution (mpr) or inches per revolution (ipr).
Generating MotionCutting motion generates cutting speed.Feed motion generates feed rate.
Impact on cutting force and power consumptionCutting speed affects the parameters on a larger scale.Feed rate influences these parameters on a smaller level.

How to Calculate Cutting Speed and Feed Rate

Distinction Between Feed Rate and Cutting Speed in CNC Machining插图3

The determination of cutting speed and feed rate involves various parameters, as illustrated in this image. It is important to note that the foundation for calculating both cutting speed and feed rate lies in the spindle speed. Additionally, the calculation of feed rate requires two formulas to be applied before arriving at the final result. Initially, you must ascertain the feed per tooth, and this value is then utilized to determine the feed rate of the cutting tool.

In summary

Optimizing the feed rate and cutting speed plays a crucial role in enhancing the CNC machining process for achieving well-machined parts. However, there is no need to concern yourself with these production-related issues when you choose to outsource to BOYI. With our team of experienced machinists and CNC machine programmers, you can consistently expect top-notch machining quality whenever you collaborate with us. Reach out to BOYI today for all your CNC machining needs.

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