CNC machining, a subtractive manufacturing process, involves precisely removing material until the final product is achieved. Understanding the distinction between feed rate and cutting speed is crucial for machinists. When designing CNC-machined parts, optimizing these parameters becomes essential, with cutting speed influencing tool longevity and power consumption, while feed rate determines machining duration and surface roughness. This article will explore the comparison of feed rate versus cutting speed and detail the derivation process for each.
What is Cutting Speed?
Cutting speed is typically characterized as the relative velocity between the workpiece surface and the cutting tool, often denoting how swiftly the workpiece moves past the tool’s cutting edge. This metric is measured in meters per minute (m/min) or feet per minute (ft/min). Its importance in CNC machining lies in its role in determining key parameters like cutting temperature, power consumption, and tool lifespan. This distinct influence on various parameters establishes a significant contrast between feed rate and cutting speed.
What is Feed Rate?
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. For turning and boring processes, it is measured in inches per revolution (ipr) or millimeters per revolution (mpr). In milling processes, machinists use inches per minute (ipm) or millimeters per minute (mpm). Calculating the feed rate involves considering the number of flutes (or teeth) on the cutting tool and determining the feed rate for each tooth.
The Difference Between Feed Rate and Cutting Speed:
Cutting Temperature and Tool Life
Cutting temperature plays a vital role in distinguishing feed rate from cutting speed. Elevated cutting temperatures can adversely impact parameters like tool life and surface finish. The disparity lies in the extent of their influence on cutting temperature and tool life. Feed rate exhibits a relatively lower effect on cutting temperature and tool life compared to cutting speed. Thus, the difference between feed rate and cutting speed is characterized by their respective impacts on cutting temperature and tool life.
Surface Roughness and Scallop Marks
Scallop marks, also referred to as feed marks, are inherent in CNC machined prototypes and parts, serving as the primary contributor to surface roughness. The feed rate plays a direct role in the manifestation of scallop marks on parts. Consequently, an elevated feed rate correlates with increased scallop marks and surface roughness. In contrast, cutting speed exerts no influence on scallop marks, thus having no impact on surface finishes.