1、Overview of metal milling fundamentals
Definition of metal milling
Metal milling is a metal processing method that cuts the workpiece by means of a rotating multi-flute tool. In the milling process, the milling cutter rotates at high speed, which is the main movement, providing the main power of cutting, while the workpiece or the cutter makes a feed movement in a specific direction, so that the metal layer being cut is continuously removed by the cutter. Both synergistic action, cut away the excess material of the workpiece, and finally obtain the required shape, size and surface quality of the machining process. To mill a flat surface on a metal block, for example, the rotation of the milling cutter will be peeled off layer by layer of metal material, with the workpiece along the horizontal direction of the slow feed, a flat and smooth plane is gradually presented.
2、The evolution of metal milling
Since the introduction of milling machines in the late 19th century, metal milling began its journey in the industrial arena. In the early days, the milling machine mainly relies on manual operation, processing accuracy and efficiency is relatively limited. With the development of the times, the application of electrification technology so that the milling machine power performance has been improved. The rise of computer technology and CNC technology has brought revolutionary changes to metal milling, giving rise to CNC milling. Today, modern milling technology not only achieves high precision, high efficiency, high automation, but also has a flexible processing capability, can easily cope with a variety of complex processing needs, in the manufacturing industry plays an increasingly important role.
3、Metal Milling Equipment and Tools
1 – Types of milling equipment
- Vertical milling machine: Its spindle is perpendicular to the working table, and this structure makes it excellent in machining flat surfaces, grooves, step surfaces and so on. For example, when manufacturing small moulds, vertical milling machine can precisely mill the cavities and various detailed structures of moulds. It has the advantages of simple structure, easy operation and relatively moderate price, so it is widely used in small processing factories and mould manufacturing workshops.
- Horizontal milling machine: the milling cutter is mounted on a horizontal spindle, which is suitable for machining various shaft parts, gears and so on. For example, in the mechanical transmission system, the horizontal milling machine can precisely mill the gear tooth shape to ensure the transmission accuracy of the gear. It has a wide range of processing, for some large workpieces that need to be cut in the horizontal direction, horizontal milling machine can also be competent.
- Gantry milling machine: the representative of large-scale milling machine, with a large working table and high rigidity, mainly used for processing large workpieces, such as the bed of large machinery, beams and so on. Like the bed of heavy machine tools, the size is huge, which needs the powerful cutting capacity and stable machining performance of gantry milling machine to ensure the machining accuracy and quality.
2 – Milling tool types and materials
- Tool types: There are many types of milling tools, commonly end mills, end mills, disc milling cutters, keyway milling cutters and so on. End milling cutter is mainly used for processing plane, its cutting edge is distributed in the end and side of the tool; end milling cutter can be used for milling planes, grooves, step surfaces and contour shapes, etc.; disc milling cutter is commonly used for milling larger planes or grooves; keyway milling cutter is specially used for processing keyway. Different types of cutters are suitable for different machining tasks according to their structure and characteristics.
Tool material:
- High-speed steel: one of the most commonly used materials for manufacturing milling cutters, with good toughness and the ability to withstand large shocks and vibrations. In some of the cutting speed requirements are not high, but the need for tools with strong impact resistance processing occasions, such as rough machining cast iron parts, high-speed steel tool performance. However, its heat resistance is poor, cutting speed is relatively low.
- Cemented Carbide: Made from hard compounds of refractory metals and binders by powder metallurgical process. It has high hardness, wear resistance and heat resistance, and the permissible cutting speed is several times higher than that of high-speed steel. When machining non-ferrous metals such as aluminium alloys and copper alloys, as well as some common steels, carbide tools can improve machining efficiency and surface quality. However, it has relatively poor toughness.
- Ceramic tool: It has the characteristics of high hardness, high wear resistance, high heat resistance and high chemical stability. In high-speed cutting and machining of difficult-to-machine materials, such as hardened steel and high-temperature alloys, ceramic tools can give play to their advantages and achieve efficient, high-precision machining.
4、Metal milling in various industries
Aerospace
1 – Fit between component machining requirements and milling: The aerospace sector demands near-excessive precision and surface quality from its components. The turbine blades of aircraft engines, which operate in an extreme environment of high temperature, high pressure and high speed, have complex shapes and extremely high aerodynamic performance requirements. Metal milling through multi-axis linkage of CNC technology, can accurately process the complex surface of the turbine blade, to ensure that the shape accuracy of the blade reaches the micron level, and the surface roughness is controlled at a very low level. Meanwhile, for large structural parts such as the magazine of an aero-engine, wing beams and fuselage frames of an aircraft, metal milling is able to achieve highly efficient material removal under the premise of guaranteeing dimensional accuracy, meeting the design requirements of aerospace products in terms of lightweight and high-strength.
2 – Typical case study: Take the machining of wing beams of a certain type of aircraft as an example, the wing beams are made of high-strength aluminium alloy material, with a length of several meters, its cross-section shape is complex, and the thickness accuracy of each part is strictly required. Using a large five-axis CNC milling machine, through the optimisation of tool paths and cutting parameters, the milling of the complex shape of the wing beam can be completed in a single clamping to ensure the dimensional accuracy and surface quality of each part, which greatly improves the machining efficiency and reduces the errors caused by multiple clamping.
Automotive manufacturing industry
1 – Aluminium alloy and other materials machining characteristics and milling advantages: aluminium alloy and other lightweight materials are used in large quantities in automobile manufacturing to reduce the weight of the vehicle and improve fuel economy. Aluminium alloy materials have good cutting performance, but are prone to deformation, tool sticking and other problems in the machining process. Metal milling can effectively solve these problems by selecting appropriate tool materials, tool geometry parameters and cutting parameters. For example, the use of carbide cutting tools with higher cutting speeds and smaller feeds can reduce the phenomenon of sticking tools during the machining of aluminium alloys and ensure the finish of the machined surface. At the same time, for the cylinder block of the automobile engine, transmission gears and other key components, metal milling can accurately process a variety of complex hole systems, tooth shapes and other structures, to ensure the accuracy and quality of the parts, and improve the performance and reliability of the car.
2 – Example of automotive parts milling: In the processing of automotive engine cylinder block, there are multiple cylinder bores, water jackets, oil passages and other complex structures inside the cylinder block. The use of machining centres for milling can accurately ensure the cylindricity and roundness of the cylinder bores and the positional accuracy between the bores, ensuring good sealing and power output of the engine. At the same time, by optimising the milling process, such as adopting high-speed milling technology, the machining efficiency can be improved and the production cost can be reduced.
5、Electronic equipment manufacturing field
1 – Precision parts machining requirements and milling realisation: The field of electronic equipment manufacturing requires a high degree of precision and miniaturisation of components. For example, components such as the motherboard bracket inside a mobile phone or the fins of a computer radiator are tiny in size, but require sub-millimetre or even micron-level precision. Metal milling is able to realise the machining of these precision parts with the help of high-precision CNC equipment and tiny tools. In the processing of mobile phone motherboard bracket, through CNC milling can accurately process a variety of mounting holes, positioning grooves and other structures to ensure that the bracket and the motherboard and other parts of the precise fit. For the fins of computer radiator, milling can process the thin and dense fin structure, increase the heat dissipation area and improve the heat dissipation efficiency.
2 – Milling effect of electronic equipment shell: The shell of electronic equipment not only needs to have certain strength and protection performance, but also has high requirements for appearance and texture. Metal milling can use aluminium alloy, stainless steel and other materials, through precision milling processing of the shell of a variety of shapes and details, such as chamfering of the frame, the key groove and so on. At the same time, through the surface treatment process, such as anodic oxidation, sandblasting, etc., can further enhance the corrosion resistance of the shell and decorative, to meet the consumer’s aesthetic demand for the appearance of electronic equipment.
6 、The Multiple Purposes of Metal Milling
Precise shaping to meet design needs
Ability to machine complex shapes: In many areas of manufacturing, parts are being designed in increasingly complex and varied shapes. Metal milling, thanks to multi-axis technology, especially CNC milling machines with five or even more axes, allows tools to cut the workpiece from multiple angles. For example, in the manufacture of impellers in the aerospace industry, the blade shape of the impeller is twisted and complex, which is difficult to reach by traditional machining methods. Metal milling, on the other hand, allows the tool path to be programmed and controlled to precisely mill the unique shape of each blade to meet its strict aerodynamic design requirements. In the medical device field, customised implants such as hip replacements need to be precisely matched to the patient’s bone structure. Metal milling is able to process metal materials into complex shapes that are individually tailored to the patient’s bone data, ensuring that the implant fits the human skeleton well.
2 – Achieve high precision dimensional control: For all types of parts, accurate dimensions are key to their performance and assembly. Metal milling in the machining process, the CNC system can accurately control the relative motion between the tool and the workpiece, the error can be controlled within a very small range. Take the piston of automobile engine as an example, the dimensional accuracy of the piston directly affects the power output and fuel economy of the engine. Through metal milling, the diameter, height and other dimensions of the piston can be accurately processed, the tolerance can be controlled at ±0.01mm or even smaller, to ensure that the piston can run smoothly in the cylinder, and the cylinder wall to maintain a good seal. In the manufacture of electronic equipment, the dimensional accuracy of small parts is extremely high, metal milling can also meet, such as mobile phones, the tiny internal metal connectors, the dimensional accuracy of the micron level, to ensure the high performance and stability of electronic equipment.
7、Material removal to achieve target form
1 – Efficient removal of excess material: In metal machining, a large amount of excess material often needs to be removed from the raw material to the final part. Metal milling offers high metal removal rates, especially in the roughing phase. For example, when machining the blanks of large machine parts, the use of high-powered milling machines and highly efficient milling tools can quickly remove a large amount of material and bring the workpiece initially close to the target shape. In the case of machining the bed of a large machine tool, for example, the roughing milling process can remove most of the residual material in a short period of time, laying the foundation for the subsequent finish machining and greatly reducing the machining cycle.
2 – Control of material removal and precision: In addition to efficient material removal, metal milling can also accurately control the amount of material removed to achieve the transition from roughing to finishing. In roughing, most of the residue is removed quickly with larger depth of cut and feed to improve processing efficiency; while in the finishing stage, the amount of material removed in each cut is precisely controlled by reducing cutting parameters, such as lowering the depth of cut and decreasing the feed, as well as selecting high-precision tools and optimising the tool path, so as to achieve very high dimensional accuracy and surface quality requirements. For example, in the manufacture of moulds, the surface of the moulds is finely processed by rough milling for rapid prototyping and then fine milling, so that the surface roughness of the moulds reaches Ra0.8μm or even lower, which meets the stringent requirements of the moulds for surface quality.
8、Surface finishing to improve surface quality
1 – Reduce surface roughness: surface roughness is one of the important indicators of the surface quality of parts. Metal milling through the selection of appropriate tools, cutting parameters and the use of advanced processing technology, can effectively reduce the surface roughness of parts. For example, in the milling of aluminium alloy parts, the use of sharp carbide cutting tools, and the use of high-speed milling process, while reasonably controlling the use of cutting fluid, can make the parts surface roughness greatly reduced. In the manufacture of optical instruments, for the metal frames of some mirrors, extremely low surface roughness is required to ensure the mounting accuracy and optical performance of optical elements. Through precision milling and subsequent grinding process, the surface roughness of the frame can reach below Ra0.1μm to meet the high precision requirements of optical instruments.
2 – Improvement of surface integrity: Good surface integrity not only includes low surface roughness, but also involves the organisational structure of the surface layer, residual stresses and other aspects. During metal milling, reasonable cutting parameters can reduce work-hardening and residual stresses in the surface layer and avoid defects such as cracks on the surface. For example, when processing high-temperature alloy parts of aircraft engines, by optimising the milling parameters, such as reducing the cutting speed, increasing the feed, and at the same time with the appropriate cutting fluid for cooling and lubrication, the surface integrity can be effectively improved, and the service life of the parts in high temperature and high pressure environments can be increased.
9、Feature processing to build functional structures
1 – Creating various types of slot and hole structures: Slots and holes are common functional structures in many machine parts. Metal milling can accurately machine slots and holes of various shapes and sizes. For example, keyway milling cutters can be used to machine keyways with high precision and good surface quality for connecting shafts and transmission parts to ensure effective power transmission. In the manufacture of circuit boards, a large number of tiny over-holes need to be processed on the metal substrate, and the position and diameter of the over-holes can be precisely controlled by CNC milling to meet the electrical connection requirements of the circuit board. For some special shapes of grooves, such as dovetail grooves, T-shaped grooves, etc., metal milling can also be easily realised, widely used in mechanical assembly and positioning and other fields.
2 – Machining special features such as threads and gears: Threads and gears are important and indispensable components in mechanical transmissions. Metal milling can process threads and gears through specialised tools and machining processes. In the processing of threads, the use of thread milling cutter, through the CNC programming to control the helical movement trajectory of the tool, can be processed with high precision internal threads and external threads, compared with the traditional thread processing methods, has a higher machining accuracy and productivity, and can be processed in a variety of special specifications of the thread. For gear processing, the use of gear milling cutter on the milling machine in accordance with a specific tooth curve for cutting, can produce different modulus, number of teeth and tooth shape of the gear, to meet the needs of a variety of mechanical transmission systems, such as automotive transmissions in the gears, gears in the industrial gear reducer and so on.
10、Factors affecting the realisation of the purpose of metal milling
Tool selection and optimisation of cutting parameters
1 – Influence of the tool on the purpose of machining: The material of the tool, its geometry and the state of wear of the tool have a crucial influence on the fulfilment of the purpose of metal milling. Different tool materials are suitable for different workpiece materials and machining requirements. For example, high-speed steel tools have good toughness and are suitable for low-speed cutting and occasions where high impact resistance of the tool is required; carbide tools have high hardness and good wear resistance and are suitable for high-speed cutting and processing of materials with high hardness. The geometry of the tool, such as the front angle of the tool, the back angle, the edge inclination angle, etc., will affect the size of the cutting force, the shape of the chip and discharge, which in turn affects the machining accuracy and surface quality. Excessive tool wear can lead to machining dimensional deviation and increased surface roughness, so it is necessary to replace the tool in time or take measures to compensate for tool wear.
2 – Cutting parameter optimisation strategy: cutting parameters include cutting speed, feed and depth of cut. Reasonable optimisation of cutting parameters can improve machining efficiency and achieve multiple purposes of metal milling under the premise of ensuring machining quality. In roughing, in order to improve the material removal rate, a larger depth of cut and feed can be selected, and the cutting speed can be appropriately reduced to ensure tool durability. While in finishing, in order to obtain high dimensional accuracy and low surface roughness, it is necessary to reduce the depth of cut and feed, and increase the cutting speed. For example, when milling alloy steel parts, through the test and simulation analysis, it is determined that in the roughing stage, the depth of cut is 5mm, the feed is 0.3mm/r, and the cutting speed is 150m/min; in the finishing stage, the depth of cut is 0.1mm, the feed is 0.05mm/r, and the cutting speed is 300m/min, and it is able to achieve a better processing effect.
11、Equipment precision and stability guarantee
1 – The impact of machine tool accuracy on machining accuracy: the accuracy of the milling machine is the basis for ensuring the accuracy of metal milling machining. The geometric accuracy of the machine tool, such as the rotary accuracy of the spindle, the flatness of the table, the positioning accuracy of the coordinate axes, etc., directly affects the machining accuracy of the workpiece. A high-precision machine tool can ensure the accuracy of the tool movement trajectory, thus achieving precise shaping and size control. For example, in the processing of precision moulds, the positioning accuracy of the machine tool is required to reach ±0.005mm or even higher, in order to meet the strict requirements of moulds for accuracy. If the accuracy of the machine tool is not enough, it will lead to large deviation in the size of the processed parts, the shape error is obvious, and it can not meet the design requirements.
2 – Equipment stability and continuous machining capability: In metal milling, the stability of the equipment is essential to ensure quality and continuous machining capability. A stable machine structure reduces vibration and deformation, and avoids vibration-induced increases in surface roughness and dimensional deviations. The thermal stability of the machine is also critical, especially during long periods of continuous machining. Machine tools generate heat during operation, leading to thermal deformation of parts and affecting machining accuracy. Therefore, advanced milling machines are usually equipped with good cooling systems and thermal error compensation devices to ensure the stability of the equipment in long-time operation and the smooth realisation of machining purposes.
conclude
Importance of reasonable process planning: scientific and reasonable process planning is a key link in achieving the purpose of metal milling. Process planning includes the selection of machining methods, the arrangement of the machining sequence, the design of tool paths and so on. For example, when processing a complex mechanical part, it is necessary to determine whether roughing or finishing is to be carried out first, which parts are suitable for milling, and which parts need other machining methods to cooperate, according to the shape, size, material and processing requirements of the part.
