Parameter strategies for CNC turning of deep holes in stainless steel

The parameter strategy for CNC turning of deep holes in stainless steel can be optimized from aspects such as cutting parameters, tool geometric parameters, programming instructions, and cooling strategies

Cutting parameters

Rotational speed: During rough machining, a medium rotational speed is adopted, such as 80 to 120m/min. For finish machining, it can be increased to 150 to 180m/min. Excessively high rotational speed can easily cause high temperatures, leading to tool oxidation, while too low a speed will intensify work hardening.

Feed rate: It should be combined with the surface quality requirements. For rough machining, it is recommended to be 0.15 to 0.3mm/r, and for finish machining, it should be controlled at 0.05 to 0.1mm/r.

Cutting depth: It should be avoided to be too small to prevent the accumulation of work hardening layers. For rough machining, 2 to 4mm is recommended, and for finish machining, 0.2 to 0.5mm.

Tool geometric parameters

Rake Angle: Under the premise of ensuring that the tool has sufficient strength, a larger rake Angle should be selected. This not only reduces the plastic deformation of the metal being cut, but also lowers the cutting force and cutting temperature, while reducing the depth of the hardened layer. The rake Angle for turning various stainless steels is approximately 12° to 30°. For martensitic stainless steel (such as 2Cr13), the rake Angle can be taken as a larger value. For austenitic and austenitic + ferritic stainless steels, the rake Angle should be taken as a smaller value. For stainless steel that has not undergone quenching and tempering treatment or has a relatively low hardness after quenching and tempering, a larger rake Angle can be taken. For workpieces with smaller diameters or thin walls, a larger rake Angle is recommended.

Relief Angle: Increasing the relief Angle can reduce the friction between the rear face of the tool and the machined surface, but it will lower the strength and heat dissipation capacity of the cutting edge. The reasonable value of the relief Angle depends on the cutting thickness. When the cutting thickness is small, a larger relief Angle should be selected. Stainless steel turning or boring tools usually have a relief Angle of 10° to 20° (for finish machining) or 6° to 10° (for rough machining).

The main deflection Angle: Reducing the main deflection Angle can increase the working length of the cutting edge, which is beneficial for heat dissipation. However, during the cutting process, it increases the radial force and is prone to vibration. It is usually taken as 45° to 75°. If the rigidity of the machine tool is insufficient, it can be appropriately increased.

Secondary deflection Angle: Usually taken as 8° to 15°.

The radius of the tool tip arc: To enhance the tool tip, a tool tip arc of 0.5 to 1.0mm is generally ground.

Edge Angle: To enhance the strength of the tool tip, the edge Angle is generally set at -8° to -3°. When cutting intermittently, a larger value of -15° to -5° is taken.

Programming instruction

The FANUC system provides two instructions, G73 and G83, for deep hole processing. G73 is the high-speed deep hole reverse chip removal drilling command, and G83 is the deep hole reverse chip removal drilling command. The instruction formats of the two are the same, but there are differences in the feed actions in the Z direction. When the G73 command is executed, the tool is retracted by a d value (set with parameters) after each feed. The G83 command, on the other hand, retreats to point R after each feed, meaning it completely exits the hole and then re-enters it. For deep hole processing, especially for those with a large length-to-diameter ratio, to ensure smooth breaking and chip removal, the G83 instruction should be given priority.

Cooling strategy

The cooling effect of traditional emulsions is limited. It is recommended to adopt a composite cooling solution: micro-lubrication (MQL) technology, which precisely sprays 5 to 50 ml /h of atomized cutting oil into the cutting zone, reducing thermal deformation and lowering fluid consumption by 90%. The high-pressure internal cooling system directly scour the blade tip with high-pressure coolant at 5 to 10MPa, effectively suppressing built-up edge and accelerating 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 processing high-hardness stainless steel.