“Selection Principles of Three Elements in CNC Machine Tool Cutting”.
In metal cutting processing, correctly selecting the three elements of CNC machine tool cutting – cutting speed, feed rate, and cutting depth is of crucial importance. This is one of the main contents of the metal cutting principle course. The following is a detailed elaboration of the selection principles of these three elements.
I. Cutting Speed
Cutting speed, that is, linear speed or circumferential speed (V, meters/minute), is one of the important parameters in CNC machine tool cutting. To select an appropriate cutting speed, multiple factors should be considered first.
Cutting speed, that is, linear speed or circumferential speed (V, meters/minute), is one of the important parameters in CNC machine tool cutting. To select an appropriate cutting speed, multiple factors should be considered first.
Tool materials
Carbide: Due to its high hardness and good heat resistance, a relatively high cutting speed can be achieved. Generally, it can be above 100 meters/minute. When purchasing inserts, technical parameters are usually provided to clarify the range of linear speeds that can be selected when processing different materials.
High-speed steel: Compared with carbide, the performance of high-speed steel is slightly inferior, and the cutting speed can only be relatively low. In most cases, the cutting speed of high-speed steel does not exceed 70 meters/minute, and is generally below 20 – 30 meters/minute.
Carbide: Due to its high hardness and good heat resistance, a relatively high cutting speed can be achieved. Generally, it can be above 100 meters/minute. When purchasing inserts, technical parameters are usually provided to clarify the range of linear speeds that can be selected when processing different materials.
High-speed steel: Compared with carbide, the performance of high-speed steel is slightly inferior, and the cutting speed can only be relatively low. In most cases, the cutting speed of high-speed steel does not exceed 70 meters/minute, and is generally below 20 – 30 meters/minute.
Workpiece materials
For workpiece materials with high hardness, the cutting speed should be low. For example, for quenched steel, stainless steel, etc., in order to ensure the tool life and processing quality, V should be set lower.
For cast iron materials, when using carbide tools, the cutting speed can be 70 – 80 meters/minute.
Low-carbon steel has better machinability, and the cutting speed can be above 100 meters/minute.
The cutting processing of non-ferrous metals is relatively easy, and a higher cutting speed can be selected, generally between 100 – 200 meters/minute.
For workpiece materials with high hardness, the cutting speed should be low. For example, for quenched steel, stainless steel, etc., in order to ensure the tool life and processing quality, V should be set lower.
For cast iron materials, when using carbide tools, the cutting speed can be 70 – 80 meters/minute.
Low-carbon steel has better machinability, and the cutting speed can be above 100 meters/minute.
The cutting processing of non-ferrous metals is relatively easy, and a higher cutting speed can be selected, generally between 100 – 200 meters/minute.
Processing conditions
During rough machining, the main purpose is to quickly remove materials, and the requirement for surface quality is relatively low. Therefore, the cutting speed is set lower. During finish machining, in order to obtain good surface quality, the cutting speed should be set higher.
When the rigidity system of the machine tool, workpiece, and tool is poor, the cutting speed should also be set lower to reduce vibration and deformation.
If the S used in the CNC program is the spindle speed per minute, then S should be calculated according to the workpiece diameter and cutting linear speed V: S (spindle speed per minute) = V (cutting linear speed) × 1000 / (3.1416 × workpiece diameter). If the CNC program uses a constant linear speed, then S can directly use the cutting linear speed V (meters/minute).
During rough machining, the main purpose is to quickly remove materials, and the requirement for surface quality is relatively low. Therefore, the cutting speed is set lower. During finish machining, in order to obtain good surface quality, the cutting speed should be set higher.
When the rigidity system of the machine tool, workpiece, and tool is poor, the cutting speed should also be set lower to reduce vibration and deformation.
If the S used in the CNC program is the spindle speed per minute, then S should be calculated according to the workpiece diameter and cutting linear speed V: S (spindle speed per minute) = V (cutting linear speed) × 1000 / (3.1416 × workpiece diameter). If the CNC program uses a constant linear speed, then S can directly use the cutting linear speed V (meters/minute).
II. Feed Rate
Feed rate, also known as tool feed rate (F), mainly depends on the surface roughness requirement of the workpiece processing.
Feed rate, also known as tool feed rate (F), mainly depends on the surface roughness requirement of the workpiece processing.
Finish machining
During finish machining, due to the high requirement for surface quality, the feed rate should be small, generally 0.06 – 0.12 mm/revolution of the spindle. This can ensure a smooth machined surface and reduce surface roughness.
During finish machining, due to the high requirement for surface quality, the feed rate should be small, generally 0.06 – 0.12 mm/revolution of the spindle. This can ensure a smooth machined surface and reduce surface roughness.
Rough machining
During rough machining, the main task is to quickly remove a large amount of material, and the feed rate can be set larger. The size of the feed rate mainly depends on the tool strength and generally can be above 0.3.
When the main relief angle of the tool is large, the tool strength will deteriorate, and at this time, the feed rate cannot be too large.
In addition, the power of the machine tool and the rigidity of the workpiece and tool should also be considered. If the machine tool power is insufficient or the rigidity of the workpiece and tool is poor, the feed rate should also be appropriately reduced.
The CNC program uses two units of feed rate: mm/minute and mm/revolution of the spindle. If the unit of mm/minute is used, it can be converted by the formula: feed per minute = feed per revolution × spindle speed per minute.
During rough machining, the main task is to quickly remove a large amount of material, and the feed rate can be set larger. The size of the feed rate mainly depends on the tool strength and generally can be above 0.3.
When the main relief angle of the tool is large, the tool strength will deteriorate, and at this time, the feed rate cannot be too large.
In addition, the power of the machine tool and the rigidity of the workpiece and tool should also be considered. If the machine tool power is insufficient or the rigidity of the workpiece and tool is poor, the feed rate should also be appropriately reduced.
The CNC program uses two units of feed rate: mm/minute and mm/revolution of the spindle. If the unit of mm/minute is used, it can be converted by the formula: feed per minute = feed per revolution × spindle speed per minute.
III. Cutting Depth
Cutting depth, that is, cutting depth, has different choices during finish machining and rough machining.
Cutting depth, that is, cutting depth, has different choices during finish machining and rough machining.
Finish machining
During finish machining, generally, it can be below 0.5 (radius value). A smaller cutting depth can ensure the quality of the machined surface and reduce surface roughness and residual stress.
During finish machining, generally, it can be below 0.5 (radius value). A smaller cutting depth can ensure the quality of the machined surface and reduce surface roughness and residual stress.
Rough machining
During rough machining, the cutting depth should be determined according to the workpiece, tool, and machine tool conditions. For a small lathe (with a maximum processing diameter of less than 400mm) turning No. 45 steel in the normalizing state, the cutting depth in the radial direction generally does not exceed 5mm.
It should be noted that if the spindle speed change of the lathe uses ordinary frequency conversion speed regulation, then when the spindle speed per minute is very low (lower than 100 – 200 revolutions/minute), the motor output power will be significantly reduced. At this time, only a very small cutting depth and feed rate can be obtained.
During rough machining, the cutting depth should be determined according to the workpiece, tool, and machine tool conditions. For a small lathe (with a maximum processing diameter of less than 400mm) turning No. 45 steel in the normalizing state, the cutting depth in the radial direction generally does not exceed 5mm.
It should be noted that if the spindle speed change of the lathe uses ordinary frequency conversion speed regulation, then when the spindle speed per minute is very low (lower than 100 – 200 revolutions/minute), the motor output power will be significantly reduced. At this time, only a very small cutting depth and feed rate can be obtained.
In conclusion, correctly selecting the three elements of CNC machine tool cutting requires comprehensive consideration of multiple factors such as tool materials, workpiece materials, and processing conditions. In actual processing, reasonable adjustments should be made according to specific situations to achieve the purposes of improving processing efficiency, ensuring processing quality, and prolonging tool life. At the same time, operators should also continuously accumulate experience and be familiar with the characteristics of different materials and processing technologies so as to better select cutting parameters and improve the processing performance of CNC machine tools.