How to determine the optimal cutting speed for CNC turning

Determining the optimal cutting speed for CNC turning requires a comprehensive consideration of multiple factors. The following are the specific methods:

The tool matches the material of the workpiece

Tool material: Different tool materials allow for different maximum cutting speeds. For instance, the high-temperature cutting speed of high-speed steel tools is less than 50m/min, that of carbide tools can reach over 100m/min, and that of ceramic tools can be as high as 1000m/min.

Workpiece material: The hardness of the workpiece material will affect the cutting speed of the tool. When the same tool is used to process hard materials, the cutting speed should be reduced, while when processing softer materials, the cutting speed can be increased. For example, when cutting workpieces with high strength and hardness, since the cutting tool is prone to wear, the cutting speed should be selected to be lower. Brittle materials such as cast iron, although they have relatively low strength, form fragmented chips during cutting. The heat is concentrated near the cutting edge and is not easily dissipated. Therefore, the cutting speed should be set lower. When cutting non-ferrous metals and non-metallic materials, a higher height can be selected.

Processing stage and precision requirements

Rough machining stage: First, consider choosing the largest possible depth of cut when the rigidity of the machine tool allows. Secondly, choose a larger feed rate; Finally, determine an appropriate cutting speed based on the allowable service life of the cutting tool. Increasing the depth of cut can reduce the number of tool passes, improve processing efficiency, and increasing the feed rate is beneficial for chip breaking.

In the finishing stage: Emphasis should be placed on how to ensure the processing quality, and on this basis, the processing efficiency should be improved as much as possible. Therefore, a smaller depth of cut and feed rate should be selected to increase the processing speed as much as possible. For workpieces with higher requirements for surface roughness quality, the cutting speed should be set higher.

The correlation influence of cutting parameters

Cutting depth and feed rate: When the cutting depth and feed rate are large, the cutting resistance is also large, and the cutting heat will increase. Therefore, the cutting speed should be reduced. For instance, when the depth of cut and feed rate increase, both the cutting heat and the cutting force are relatively large. Therefore, the cutting speed should be appropriately reduced.

Material removal rate consideration: The material removal rate is directly proportional to the product of the cutting speed, feed rate and cutting depth. Any change in these three cutting parameters will affect the cutting volume of the workpiece per minute. However, if the increase in cutting speed is used to reflect the rise in cutting rate, it implies that the improvement in productivity is achieved through the wear and tear of cutting tools. Therefore, both productivity and tool durability should be comprehensively considered.

The status of machine tools and cutting tools

Machine tool condition: Before starting processing operations on a lathe, the condition of the machine tool should always be taken into consideration. Old machines used in production operations for processing hard or abrasive materials often cause a large amount of clearance or wear on the mechanical components of the machine, which may lead to unsatisfactory results. Moreover, it may be necessary to lower the speed and feed parameters recommended by the mold manufacturer a little to prevent the machine's operating speed from exceeding its processing capacity.

Tool life: If a long tool usage time (life) is required, a lower cutting speed should be adopted. Conversely, a higher cutting speed can be adopted. For instance, cutting speed has a significant impact on tool durability. When the cutting speed increases by 50%, the tool durability decreases by approximately 80%.

Formula calculation and empirical adjustment

Formula calculation: After knowing the cutting speed Vc, the spindle speed N can be inferred in reverse through the formula Vc = π× workpiece diameter D× rotational speed N/1000. The spindle speed S (r/min) can also be calculated based on the cutting speed υ (mm/min) by the formula S = υ×1000/πD (where D is the diameter of the workpiece or tool in mm).

Experience adjustment: In actual production, the situation is rather complex. Generally, the cutting parameters can be adjusted according to the recommended value range in the process manual or the tool selection manual provided by the tool manufacturer. Meanwhile, the analogy method can also be adopted in combination with practical experience to determine it.