CNC machining aluminum parts process knowledge

Aluminum is the most widely used and widely used metal material among non-ferrous metals, and its application range is still expanding. There are more than 700,000 kinds of aluminum parts produced using aluminum materials.

Today, I will introduce to you the process flow of CNC processing aluminum parts and how to avoid CNC processing deformation.

1.The advantages and characteristics of aluminum are as follows

1) Low density

The density of aluminum is about 2.7g/cm3, which is only 1/3 of iron or copper.

2) Strong plasticity

Aluminum has good ductility and can be made into various products by pressure processing methods such as extrusion and stretching.

3) Corrosion

Aluminum is a metal with a strong negative charge. Under natural conditions or after anodizing, a protective oxide film will form on the surface, and its corrosion resistance is much better than steel.

4) Easy to strengthen

The strength of pure aluminum is not high, but it can be increased by anodizing.

5) Convenient surface treatment

Surface treatment can further enhance or change the surface properties of aluminum. The aluminum anodizing process is quite mature and stable in operation, and is widely used in the process of CNC processing aluminum parts.

6) Good conductivity and easy to recycle

2.CNC aluminum processing technology

1) Cold punching

Using aluminum pellets, the material is formed in one go by an extruder and a die. It is suitable for cylindrical products or product shapes that are difficult to achieve by stretching process, such as oval, square, and rectangular products.

The tonnage of the machine used is related to the cross-sectional area of ​​the product. The gap between the upper die punch and the lower die tungsten steel is the wall thickness of the product. When the upper die punch and the lower die tungsten steel are pressed, the vertical gap to the bottom dead center is the top thickness of the product.

1. Advantages: short mold opening cycle and lower development cost than the stretching die.
2. Disadvantages: long production process, large fluctuations in product size during the process, and high labor costs.

2) Stretching exercise

Using aluminum skin. Suitable for non-cylindrical (aluminum products with arcs) using continuous mold machines and molds for multiple deformations to meet the shape requirements.

1. Advantages: more complex and multiple deformation products have stable size control during the production process and a smoother product surface.
2. Disadvantages: high mold cost, relatively long development cycle, and high requirements for machine selection and precision.

3.Surface treatment of CNC processed aluminum parts

1) Sandblasting (shot peening)

The process of using the impact of high-speed sand flow to clean and roughen the metal surface.

Using this method to treat the surface of CNC processed aluminum parts, the workpiece surface can obtain a certain degree of cleanliness and different roughness, so that the mechanical properties of the workpiece surface are improved, thereby improving the fatigue resistance of aluminum parts.

While increasing the adhesion between it and the coating, it also prolongs the durability of the coating film, and is also conducive to the leveling and decoration of the paint. We often see this process in various Apple products.

2) Polishing

Use mechanical, chemical or electrochemical effects to reduce the surface roughness of CNC processed aluminum parts and obtain a bright and smooth surface.

The polishing process is mainly divided into: mechanical polishing, chemical polishing, and electrolytic polishing. After mechanical polishing and electrolytic polishing, CNC processed aluminum parts can achieve an effect close to the mirror surface of stainless steel. This process gives people a sense of high-end simplicity, fashion and future.

3) Drawing

Metal wire drawing is a manufacturing process that uses sandpaper to repeatedly scrape lines out of aluminum sheets. Wire drawing can be divided into straight drawing, random drawing, spiral drawing, and thread drawing.

The metal wire drawing process can clearly show every tiny silk mark, making the metal matte have a delicate hair luster, and the product has a sense of fashion and technology.

4) High gloss cutting

Use an engraving machine, strengthen the diamond knife on the engraving machine spindle, rotate at high speed (generally 20,000 rpm), cut the parts, and produce local highlight areas on the surface of the product.

The brightness of the cutting highlight is affected by the milling and drilling speed. The faster the drilling speed, the brighter the cutting highlight, and vice versa, the darker it is, and it is easy to produce cutting lines. High gloss and bright cutting is particularly widely used on mobile phones, such as iPhone 5.

In recent years, some high-end TV metal frames have adopted high gloss CNC milling technology. Coupled with anodizing and wire drawing technology, the TV is full of fashion and technology.

5) Anodizing

Anodizing refers to the electrochemical oxidation of metals or alloys. Under the corresponding electrolyte and specific process conditions, aluminum and its alloys form an oxide film on aluminum products (anodes) due to the effect of external current.

Anodizing can not only solve the defects of CNC processed aluminum parts such as surface hardness and wear resistance, but also prolong the service life of CNC processed aluminum parts and enhance the aesthetics. It has become an indispensable link in the surface treatment of CNC processed aluminum parts and is currently the most widely used and very successful process.

6) Two-color anode

Two-color anodizing refers to anodizing on a product to give different colors to specific areas. The two-color anodizing process is rarely used in the television industry because of its complex process and high cost; but the contrast between the two colors can better reflect the high-end and unique appearance of the product.

4.Process measures to reduce the deformation of aluminum alloy CNC processed parts

There are many reasons for the deformation of aluminum parts, which are related to the material, part shape, production conditions, etc. The main reasons are as follows: deformation caused by internal stress of the blank, deformation caused by cutting force, cutting heat, and deformation caused by clamping force.

1) Reduce the internal pressure of hair culture

Using natural or artificial aging, vibration treatment and other methods, the internal stress of the blank can be partially eliminated. Pretreatment is also an effective process method. For fat and big blanks, due to the large machining allowance, the deformation after CNC machining is also large. If the excess part of the blank is cut off by CNC machining in advance, the machining allowance of each part can be reduced, which can not only reduce the deformation of subsequent CNC machining, but also release some internal stress after a period of pre-CNC machining.

2) Improve the cutting ability of the tool

The material and geometric parameters of the tool have an important influence on the cutting force and cutting heat. The correct selection of the tool is crucial to reduce the deformation of CNC machining aluminum parts.

① Reasonable selection of tool geometric parameters.

1. Rake angle: Under the condition of maintaining the strength of the blade, the rake angle is appropriately selected to be larger. On the one hand, it can grind a sharp edge, and on the other hand, it can reduce cutting deformation, make chip removal smooth, and thus reduce cutting force and cutting temperature. Avoid using tools with negative rake angles.
2. Back angle: The size of the back angle has a direct impact on the wear of the back tool face and the surface quality of CNC machining. Cutting thickness is an important condition for selecting the back angle. During rough milling, due to the large feed rate, heavy cutting load, and high heat generation, the tool requires good heat dissipation conditions, so a smaller back angle should be selected. During fine milling, the cutting edge is required to be sharp, the friction between the back tool face and the CNC machining surface is reduced, and the elastic deformation is reduced, so a larger back angle should be selected.
3. Helix angle: In order to make milling smooth and reduce milling force, the helix angle should be as large as possible.
4. Main deflection angle: Appropriately reducing the main deflection angle can improve heat dissipation conditions and reduce the average temperature of the machining area.

② Improve tool structure.

1. Reduce the number of milling cutter teeth and increase chip space. Due to the large plasticity of aluminum materials, the cutting deformation is large during processing, and a larger chip space is required, so the chip groove bottom radius should be larger and the number of milling cutter teeth should be less.
2. Fine grinding of the teeth. The roughness value of the tooth edge should be less than Ra=0.4um. Before using a new knife, use a fine oil stone to lightly grind the front and back sides of the teeth several times to eliminate the burrs and fine serrations left when grinding. In this way, not only can the cutting heat be reduced, but the cutting deformation is also relatively small.
3. Strictly control the wear standard of the tool. After the tool is worn, the surface roughness value of the workpiece increases, the cutting temperature increases, and the deformation of the workpiece increases.

Therefore, in addition to selecting tool materials with good wear resistance, the tool wear standard should not be greater than 0.2mm, otherwise it is easy to produce built-up edge. During cutting, the workpiece temperature should generally not exceed 100℃ to prevent deformation.

3) Improve the clamping method of CNC processed aluminum parts

For thin-walled aluminum CNC processed parts with poor rigidity, the following clamping methods can be used to reduce deformation:

1. For thin-walled bushing parts, if a three-jaw self-centering chuck or a spring chuck is used for radial clamping, once it is released after CNC processing, the workpiece will inevitably deform. At this time, a method of axial end face clamping with good rigidity should be used. Use aluminum to process the inner hole of the part for positioning, insert the self-made threaded mandrel into the inner hole of the part, press the end face with a cover plate and tighten it with a nut. When processing the outer circle, clamping deformation can be avoided, so as to obtain satisfactory CNC processing accuracy.
2. When CNC processing thin-walled and thin-plate workpieces, it is best to use a vacuum suction cup to obtain a uniformly distributed clamping force, and then use a smaller amount of cutting for processing, which can well prevent the deformation of CNC processed aluminum parts.
3. In addition, the filler method can also be used. In order to increase the process rigidity of thin-walled workpieces, the medium can be filled inside the workpiece to reduce the deformation of the workpiece during clamping and cutting. For example, urea melt containing 3% to 6% potassium nitrate is poured into the workpiece. After CNC processing, immerse the CNC processed aluminum parts in water or alcohol, dissolve the filler and pour it out.

4) Reasonable arrangement of the process

During high-speed cutting, due to the large CNC processing allowance and intermittent cutting, the milling process often produces vibration, affecting the CNC processing accuracy and surface roughness.

Therefore, the CNC high-speed cutting process can generally be divided into: roughing-semi-finishing-angle cleaning-finishing and other processes. For parts with higher precision requirements, sometimes secondary semi-finishing is required, and then finishing.

After roughing, the parts can be cooled naturally to eliminate the internal stress caused by roughing and reduce deformation. The remaining allowance after roughing should be greater than the deformation, generally 1 to 2 mm.

During finishing, the finished surface of the part should maintain a uniform CNC machining allowance, generally 0.2 to 0.5 mm, so that the tool is in a stable state during the CNC machining process, which can greatly reduce cutting deformation, obtain good CNC machining surface quality, and ensure product accuracy.

5.Operational skills to reduce deformation of CNC machining aluminum parts

In addition to the above reasons, in actual operation, the operation method is also very important.

1) For parts with large CNC machining allowances, in order to have better heat dissipation conditions during CNC machining and avoid heat concentration, symmetrical machining should be used during CNC machining.

For example, if a 90mm thick sheet needs to be processed to 60mm, if one side is milled immediately after the other side is milled, and the final size is reached in one go, the flatness can reach 5mm; if symmetrical processing with repeated feeding is adopted, each side is processed twice, and the final size is reached, the flatness can be guaranteed to be 0.3mm.

2) If there are multiple cavities on the plate-like parts, it is not advisable to use the sequential processing method of one cavity by one cavity during CNC processing, which is easy to cause uneven force and deformation of the aluminum CNC machined parts.

Use multi-layer processing, process all cavities at the same time on each layer, and then process the next layer, so that the parts are evenly stressed and deformation is reduced.

3) Reduce cutting force and cutting heat by changing the cutting amount. Among the three elements of cutting amount, the back cutting amount has a great influence on the cutting force.

If the machining allowance is too large, the cutting force of one pass will be too large, which will not only deform the aluminum alloy CNC machined parts, but also affect the rigidity of the machine tool spindle and reduce the durability of the tool. If the back cutting amount is reduced, the production efficiency will be greatly reduced.

However, the use of high-speed milling in CNC processing can overcome this problem. While reducing the amount of back cutting, as long as the feed rate is increased accordingly and the machine tool speed is increased, the cutting force can be reduced to ensure the efficiency of CNC processing.

4) The cutting sequence should also be paid attention to. Rough processing emphasizes improving the efficiency of CNC processing and pursuing the removal rate per unit time. Generally, reverse milling can be used, that is, the excess material on the surface of the blank is removed at the fastest speed and in the shortest time, basically forming the geometric contour required for finishing.

Finishing emphasizes high precision and high quality, and it is advisable to use down milling, because the cutting thickness of the cutter teeth gradually decreases from the maximum to zero during down milling, which greatly reduces the degree of CNC processing hardening and the degree of deformation of aluminum CNC processed parts.

5) Thin-walled workpieces are deformed due to clamping during CNC processing, and even cannot be finished. In order to minimize the deformation of the workpiece, the clamping piece can be loosened before finishing to the final size, so that the workpiece can return to its original state freely, and then gently clamped. As long as the workpiece can be fully clamped (based on the feel), the ideal CNC processing effect can be obtained.

In short, the point of action of the clamping force is best on the support surface, and the clamping force should act in the direction of good rigidity of the workpiece. Under the premise of ensuring that the aluminum CNC machined parts do not loosen, the smaller the clamping force, the better.

6) When processing parts with cavities, try not to let the milling cutter cut directly into the part like a drill bit when processing the cavity, which will cause insufficient space for the milling cutter to accommodate chips and poor chip removal, resulting in overheating, expansion, and cutter breakage and other adverse phenomena.

First drill out the hole with a drill bit of the same size or larger than the milling cutter, and then mill with the milling cutter. Alternatively, you can also use CAM software to create a spiral grooving program.