How to cool a vertical machining center during high-speed milling?

(1) Hardness of workpiece material

If the hardness of the workpiece material is 42HRC, compressed air cooling can usually achieve better results. The processing characteristics of high-speed milling of high-hardness materials are: high cutting temperature; under the action of cold work hardening, the chips will become harder than the parent material. When cutting such materials, if cutting fluid is used for cooling, the tool may be subjected to thermal shock caused by intermittent heating and cooling, and the rapid change in temperature can easily cause the carbide cutting edge to break. On the contrary, if compressed air is used for cooling, not only can the tool temperature be kept constant, but the chips can also be blown away from the cutting area, avoiding the damage to the tool caused by the re-cutting action of high-hardness chips.

(2) Type of workpiece material

If the hardness of the workpiece material is less than <42HRC, the cooling method should be determined according to the type of workpiece material. When milling sticky materials (such as aluminum, soft stainless steel, etc.). ) At high speed, cutting fluid is usually required for cooling. Cutting fluid can lubricate the tool, allowing the chips to easily slide out of the chip groove and separate from the back angle of the tool. When milling most tool steels (such as P20, H13, S7, NAK55, D2, etc.) at high speeds, compressed air cooling may be the right choice. If workpiece material is found to adhere to the tool during machining, it may indicate that cutting fluid is needed; however, it may also indicate that a different tool coating needs to be selected.

(3) Tool coatings

TiCN coating and titanium aluminum nitride (TiAlN) coating are the two most commonly used tool coatings for high-speed milling of tool steel. TiCN coating is more suitable when ball end mills are milling workpiece materials with a hardness of less than 42HRC at a cutting speed of less than 800sfm (or round end mills of the same material at a cutting speed of less than 600sfm). If the hardness of the material being machined or the cutting speed is higher than the above cutting parameter range, it is best to choose TiAlN coating.

TiCN coating has good adaptability to cooling of cutting fluid. Although a sharp change in cutting temperature may still cause the carbide cutting edge to break, machining within the above cutting parameter range generally does not produce high cutting temperatures sufficient to cause thermal shock hazards.

On the contrary, TiAlN coatings with good cutting performance at high temperatures for machining center machines are not suitable for cooling with cutting fluids. When the coating is cut at high temperatures, a hard and smooth aluminum oxide layer can be formed on the outer surface of the coating, which helps to improve the cutting performance of the tool. (In fact, the "Exalon" TiAlN coating developed by Milstar Corporation in the United States has more advanced high-temperature cutting performance. A solid lubricating layer is added to the outer surface of this TiAlN coating, which makes it easier for chips to slide along the cutting edge of the tool.) The requirements for tool coatings for milling graphite electrode workpieces in machining center machines are generally not strict, and TiAlN coatings or diamond coatings can be used. Although both coatings can be cooled by compressed air to achieve good cutting results, many machining workshops are still willing to use cutting fluids because cutting fluids help remove dust generated during machining.

(4) Surface finish requirements

When CNC machining centers use ball-end milling cutters for high-speed milling, cutting fluid cooling may be required to obtain a high surface finish on the workpiece. Since the cutting speed at the end of the ball end mill is zero, the cutting fluid can play a good lubricating role. When a typical ball end mill is used for micro-feed finishing, the workpiece material in the low-speed cutting area at the end of the milling cutter may be stuck in the "belly". The residual material in a red-hot state is dragged by the tool through the workpiece and may weld to the workpiece surface, thereby destroying the surface finish of the workpiece. (To solve this problem, some CNC machining center spherical profile machine-clamped blade milling cutters, such as the "super-finishing" blades of Milstar Corporation in the United States, can eliminate this "chisel edge" by improving the design of the blade. Cutting fluid can reduce the impact of chip welding by lubricating the tool and the workpiece, and obtain a higher surface finish. Based on this consideration, even in the processing occasions where TiAlN coated tools are used, the method of cutting fluid cooling should be adopted. Although the tool life may be shortened, sometimes part of the tool life needs to be sacrificed in order to meet the surface finish requirements.