Analyze the parameter setting techniques for CNC turning of quenched and tempered steel
When CNC turning quenched and tempered steel, parameter Settings need to comprehensively consider material properties, tool selection, cutting parameters and cooling strategies. The following are specific tips:
Cutting speed
The cutting speed has a significant impact on the durability of the cutting tool. Increasing the cutting speed can shorten the processing time and improve efficiency. However, if the linear speed is too high, the cutting temperature will rise and the tool durability will be greatly reduced. For example, if the linear speed is 20% higher than the specified linear speed of the sample, the tool life will be reduced to half of the original. If it is increased to 50%, the tool life will only be one fifth of the original. When processing at low cutting speeds (20-40 m/min), the workpiece is prone to vibration and the tool durability is also low.
Quenched and tempered steel has a relatively high hardness, so the cutting speed should be appropriately reduced. Taking the hardness HRC25-30 of 40Cr alloy steel after quenching and tempering treatment as an example, the rough machining speed can be set at 600-900 r/min, and the finish machining speed can be set at 1200-1500 r/min. For instance, if the material of a certain upper cover is 45# steel with a quenched and tempered hardness of HRC28-32, when processing the thread of M105X2, if it is processed as a non-quenched 45# steel part, the tool linear speed should be between 180 and 200m/min. However, when quenched to HRC28-32, it is more appropriate to reduce the linear speed to around 120m/min for processing.
Feed rate
The feed rate is closely related to the surface roughness of the processed surface. Usually, the feed rate is determined according to the surface roughness requirements. The feed rate should be greater than the width of the chamfer; otherwise, the chips cannot be broken. Generally, it is about twice the width of the chamfer. When the feed rate is large, the thickness of the chip layer increases, the cutting force increases, the cutting temperature rises, and the wear of the rear face increases. However, its impact on the tool durability is smaller than that of the cutting speed. When the feed rate is small, the wear at the back is large, and the durability of the cutting tool decreases rapidly. The feed rate is between 0.1 and 0.4, which has a relatively small impact on the rear cutter face, depending on the specific situation.
During rough machining, under the premise of ensuring the quality of part processing, a higher feed rate should be selected to increase productivity. The main factors limiting the feed rate include the strength and rigidity of the tool holder, insert, machine tool, workpiece, etc. Generally, the selection range should be between 100 and 200mm/min. When performing cutting, deep hole processing or high-speed steel tool processing, the feed rate should be appropriately reduced. Generally, it is advisable to select 20-50mm /min. When performing semi-finishing or finishing, the feed rate is usually selected based on the surface roughness requirements of the part to be processed. If the surface roughness requirement is low, the feed rate will be small, but it should not be too small either. If the feed rate is too small, the cutting thickness will be too thin, which will instead increase the surface roughness and aggravate the wear of the tool. Under normal circumstances, the feed rate during finish turning should be selected within the range of 0.10-0.20mm/r.
The depth of the cut
The depth of cut is determined based on the allowance of the workpiece, its shape, the power of the machine tool, its rigidity and the rigidity of the tool. Changes in the depth of cut have a significant impact on the tool life. For instance, when performing peeling and turning on a hot-rolled D80 round steel piece, assuming the maximum and minimum external dimensions of the round steel piece are 82 and 78 respectively due to its ellipticity, the depth of the first cut must be less than 78. As the tool tip is continuously processed all the time, it can effectively ensure that the tool tip does not chipping, thereby increasing the service life of the tool. For workpieces of different materials or those of the same material but with different heat treatment hardness, the cutting depth during processing will vary. The decision should be made based on the actual situation.
Taking 40Cr alloy steel as an example, the back cut for rough machining can be set at 2-4mm, and the back cut for finish machining can be set at 0.08-0.12mm.
Tool selection
Quenched and tempered steel has a relatively high hardness. It is recommended to use CBN tools. For instance, when processing 40Cr alloy steel, it is recommended to use CBN tools in the finishing stage and increase the concentration of the cutting fluid to 8% - 10%.
Cooling strategy
The coolant should be supplied in sufficient quantity, with a flow rate of no less than 8L/min. Special attention should be paid to aligning the cooling pipelines during the inner hole processing. For materials with high hardness such as quenched and tempered steel, a composite cooling scheme can be adopted, such as micro-lubrication (MQL) technology. Atomized cutting oil of 5-50 ml /h can be precisely sprayed into the cutting area, reducing thermal deformation and lowering the liquid consumption by 90% at the same time. The high-pressure internal cooling system, by directly flushing the blade tip with 5-10 mpa high-pressure coolant, effectively suppresses built-up edge and accelerates chip removal. Low-temperature cooling, using liquid nitrogen or CO₂ cold air (-50℃ to -30 ℃), can reduce the temperature in the cutting zone by more than 200℃, which is particularly suitable for the processing of high-hardness stainless steel.
