Air-cooled cutting can generally be carried out by liquid nitrogen cooling and compressed air cooling, and the cutting area can be supplemented with oil mist lubrication to improve the surface finish. Air-cooled cutting can achieve better processing results, but the processing cost is higher. Dry cutting eliminates the need for cooling and lubrication, reducing processing costs and reducing environmental pollution. In order to achieve dry cutting, the tool coating must have two important functions: 1 to act as a thermal barrier between the tool and the workpiece to reduce the thermal stress acting on the tool base; 2 to act as a solid lubricant The role is to reduce the cutting friction and the adhesion of the chips to the tool. TiAlN coating is a high performance coating that better meets the above requirements.
Many new carbide tool grades (especially coated grades) use dry cutting for high cutting efficiency for higher cutting efficiency. In fact, for intermittent cutting, the higher the temperature in the cutting zone, the less suitable the use of cutting fluid. The addition of cutting fluid during milling allows the tool to withstand severe temperature changes (the milling insert is cooled as it is cut from the workpiece and the temperature rises again as it cuts into the workpiece). Although a similar heating-cooling cycle exists during dry cutting, the temperature change is much more moderate than when the cutting fluid is added. A drastic change in temperature can cause stress in the blade, causing cracks.
The coating thickness (typically 2 to 18 μm) has a significant impact on tool performance. For intermittent cutting with large impact force, rapid cooling and heating of the tool, the thin coating is better than the thick coating in temperature change, and the stress is small and crack is not easy. Therefore, the life of the thin coating blade during dry cutting can be 40% longer. In general, the PVD process can obtain a thinner coating than the CVD process, and it is more firmly bonded to the substrate. Therefore, PVD coating is often used for circular tools and milling inserts. In addition, due to the low deposition temperature of PVD coatings, it is widely used in sharp-edged cutting tools and large positive rake milling cutters and turning tools.
TiAlN coating is currently the best performing PVD coating for high speed dry cutting. Its high temperature continuous cutting performance is four times higher than that of titanium nitride (TiN) coating, which is mainly due to the coating surface during high temperature cutting. The amorphous alumina film formed on the chip/tool ​​interface after oxidation of the aluminum.
High-speed milling of AlSi H13/JIS SKD61 die steel (52HRC) with VC-MD model TiAlN coated milling cutter, TiN coated milling cutter and uncoated milling cutter (feed rate: 0.10mm/tooth; axial depth of cut 10mm) , radial cutting depth 0.5mm; down-milling, air-cooled), after the processing length of 50m, the wear of the flank surface around the tool shows: when the cutting speed V=100m/min, the processing time is 60min, TiAlN coated milling cutter The wear amount is only 1/2~1/3 of other tools. When V=600m/min, the TiAlN coated milling cutter can process the same length in only 10min, and the wear amount is increased by 2 times; TiN coated milling cutter and Uncoated milling cutters have a large amount of wear at V=200 m/min, and severe wear occurs when V continues to increase. Analysis of the trend of the wear curve shows that the wear curve of the TiAlN coated cutter has a smaller slope and a flatter trend; the slopes of the wear curves of the other two tools are larger. It shows that the wear amount of TiAlN coating changes little with the increase of cutting speed, which is very suitable for high speed cutting.
4. Cutting performance of TiAlN coated milling cutter for high speed milling of titanium alloy
Milling titanium alloy material (Ti-6Al-4V) with VC-2MS type TiAlN coated milling cutter (φ10mm) (milling depth: radial depth of cut 0.2mm, axial depth of cut 10mm, side milling; cooling pressure: 4.4M The relationship between milling speed and tool flank wear shows that titanium alloy is a difficult material. When TiAlN coated milling cutter is milled at low speed, the tool wear is small and the wear curve is flat. As the cutting speed continues to increase, the amount of tool wear increases slowly. However, when the cutting speed exceeds 10000 r/min, the amount of tool wear increases rapidly. The high temperature generated by intermittent cutting of the titanium alloy material causes a large temperature difference between the cutting edge of the insert and other parts, resulting in cracks in the cutting edge, and the crack propagation will cause the cutting edge to break and the blade to break.
Milling Ti-6Al4V with a six-blade TiAlN coated milling cutter (φ10mm), when changing the milling speed and milling depth (feed rate: 0.10mm/tooth; axial depth of cut 10mm, side milling), milling speed and radial The relationship between the depth of cut shows that the higher the cutting speed when milling titanium alloy materials, the smaller the radial depth of cut for normal milling. For example, at the critical speed (10000r/min), the radial depth of cut is 0.1mm to ensure normal milling. If the radial depth of cut is 0.2mm, the tool wear amount is large. When the rotational speed is further increased, the wear will increase rapidly. . It is safe to select the speed of 5000r/min, and the normal machining can be realized within the range of radial depth of cut of 0.4~0.6mm. The amount of chip removal is 15cc/min. The amount of chip removal usually decreases with the decrease of the radial depth of cut. The larger the chip discharge, the greater the processing power required. According to the cutting test results, when milling titanium alloy, it is reasonable to select the speed of 5000r/min and the radial depth of cut 0.4~0.6mm. At this time, the milling cutter has small wear and long life, large milling power and high processing efficiency.
5. Two efficient milling methods
(1) Arc milling slot
When milling the keyway, the diameter of the milling cutter should be smaller than the width of the keyway. Otherwise, the milling resistance is large, which affects the life of the milling cutter. If the φ10 milling cutter is used to machine the keyway with a groove width of 10mm, each cutting depth is 1mm, and it needs to be milled 10 times to complete the machining. In order to improve the processing efficiency, a circular arc milling method can be used. Rotary milling with a φ6 milling cutter along a certain arc can efficiently process a depth of 10 mm. The formula for calculating the arc milling groove is:
θ=arccos[1-2Rd(t-Rd)/φ(t-φ)]
Where: θ is the maximum milling angle, Rd is the radial depth of cut, t is the width of the groove being machined, and φ is the diameter of the milling cutter.
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