Machining allowance of non-ferrous metals and their alloys
Due to their unique physical and chemical properties, non-ferrous metals and their alloys are widely used in aerospace, automotive, and electronic communications applications. Machining allowance, the thickness of material removed from the rough cut to the finished part during machining, is crucial for ensuring machining quality, improving production efficiency, and reducing costs for non-ferrous metal and alloy parts. Determining machining allowance for non-ferrous metals and their alloys requires comprehensive consideration of factors such as material properties, machining methods, and part precision requirements. Compared to ferrous metals, this approach presents unique and complex requirements.
The material properties of non-ferrous metals and their alloys have a significant impact on machining allowances. Aluminum and aluminum alloys are relatively soft and have good plasticity. They are prone to plastic deformation and tool sticking during machining. If the machining allowance is insufficient, surface defects (such as scratches and dents) in the previous process may not be completely removed. If the allowance is too large, it will increase material consumption and machining time, reducing production efficiency. Copper and copper alloys have good thermal conductivity and high toughness. Cutting heat is easily diffused during machining, but chips are not easy to break. Excessive machining allowances will increase cutting forces and cause deformation of parts. Magnesium alloys have low density and high strength, but are relatively brittle. Excessive machining allowances will cause cracks due to excessive cutting forces, affecting part quality. Therefore, for different non-ferrous metals and their alloys, the appropriate machining allowance range needs to be determined based on their hardness, plasticity, toughness and other characteristics. For example, the rough machining allowance for aluminum and aluminum alloys is generally 2-5mm, and the fine machining allowance is 0.5-1mm; the rough machining allowance for copper and copper alloys is 1.5-4mm, and the fine machining allowance is 0.3-0.8mm.
The processing method is an important basis for determining the machining allowance of non-ferrous metals and their alloys. Different processing methods have different machining accuracy and surface roughness, and the required machining allowance is also different. Cast blanks have poor surface roughness (generally Ra25-12.5μm) and large shape errors. Therefore, a larger roughing allowance is required to remove defects such as oxide scale, pores, and sand holes on the casting surface. For example, the roughing allowance for aluminum castings is usually 3-6mm, and that for copper castings is 2-5mm. Forged blanks have better surface quality (Ra12.5-6.3μm) and smaller shape errors. The roughing allowance can be appropriately reduced. The roughing allowance for aluminum forgings is 2-4mm, and that for copper forgings is 1.5-3mm. For cold-rolled or cold-drawn nonferrous metal profiles, due to their high dimensional accuracy (tolerance grades up to IT8-IT10) and low surface roughness (Ra3.2-1.6μm), machining allowances can be further reduced. Generally, roughing allowances are 1-2mm, and finishing allowances are 0.1-0.5mm. Furthermore, grinding offers higher precision than turning and milling, so grinding allowances are typically smaller than those for turning and milling. For example, the grinding allowance for aluminum and aluminum alloys is 0.1-0.3mm, and for copper and copper alloys it is 0.05-0.2mm.
The required precision and surface quality of a part play a decisive role in determining machining allowances. High-precision parts (tolerance grades IT5-IT7) and parts with low surface roughness requirements (Ra0.8-0.025μm) require larger machining allowances to gradually eliminate errors in previous processes and ensure final accuracy. For example, aluminum parts used in precision instruments can have a total machining allowance of 5-8mm, with roughing removing the majority of the allowance, and semi-finishing and finishing gradually increasing accuracy. For parts with average precision (IT8-IT11) and less stringent surface roughness requirements (Ra6.3-1.6μm), the machining allowance can be reduced appropriately, typically to 2-5mm. In addition, the structural shape of the parts will also affect the processing allowance. For thin-walled, slender and other easily deformed non-ferrous metal parts, in order to reduce deformation during the processing process, smaller processing allowances and multiple processing methods are required. For example, in the processing of thin-walled aluminum cylinders, the processing allowance is controlled at 0.5-1mm each time, and the design size is gradually reached through multiple passes to avoid deformation of the parts due to excessive single allowance.
Process factors during machining also play a crucial role in determining machining allowances. The accuracy and rigidity of the machine tool can affect machining errors. When machining with low-accuracy and low-rigidity machines, the machining allowance should be appropriately increased to compensate for these errors. Tool wear can lead to a decrease in surface quality and dimensional errors, so the impact of tool wear should be considered when determining machining allowances. Generally, an additional allowance of 0.1-0.3mm should be added. Fixture positioning errors and clamping distortion can also affect part machining accuracy. For parts machined with complex fixtures, the machining allowance should be appropriately increased to ensure that the part still meets design requirements after eliminating the effects of positioning errors and clamping distortion. Furthermore, non-ferrous metals and their alloys have a large coefficient of thermal expansion. Temperature fluctuations during machining can cause dimensional changes in the part. Therefore, the impact of temperature variations should be considered when determining machining allowances. For example, when machining aluminum and aluminum alloy parts in a high-temperature environment, an additional allowance of 0.05-0.1mm should be added to compensate for dimensional changes caused by thermal expansion.
The main methods for determining machining allowances for non-ferrous metals and their alloys include empirical estimation, table lookup, and calculation. The empirical estimation method relies on operator experience and actual machining conditions to determine the allowance. It is suitable for small-batch production and parts with low precision requirements. While simple and easy to use, it offers lower accuracy. The table lookup method, based on relevant process manuals and standards, provides recommended machining allowances for different materials, machining methods, and precision requirements. It is currently the most widely used method in production. For example, the Metalworking Process Manual provides detailed specifications for machining allowances in different machining processes for non-ferrous metals such as aluminum, copper, and magnesium, as well as their alloys. The calculation method uses a formula to determine machining allowances based on factors such as prior process errors, surface defect depth, and part deformation. This method offers higher accuracy and is suitable for machining high-precision and large, complex parts. The calculation formula is: Z = Zmin + Δ, where Z is the machining allowance, Zmin is the minimum necessary allowance (the minimum allowance required to account for surface defects and prior process errors), and Δ is an additional allowance to account for various process factors. In actual production, the three methods are usually used in combination. First, the approximate allowance range is determined by the table lookup method, and then adjustments are made based on experience and calculation methods to obtain a reasonable processing allowance.
