The Standard of Machinery Processing Allowance
Machining allowance, also known as machining tolerance, is the difference between the nominal value and the actual machined dimension of a part. When a part is machined, the machining allowance is predetermined, selected based on design considerations, production capabilities, and applicable standards.
The standard of machinery processing allowance is determined by the EN-ISO 8785 norm (The industrial tolerances dimensions and deviations from ISO). Major components, such as rotors and reducers, must pass the test mentioned in the standard before they can be put on the market. The standard has been established to properly define the inspection process and dimensional tolerances. Specifically, it establishes general dimensional tolerances, precision and process values that are used in production, including the clearance and gap tolerances. It also covers procedures for determining run-out and assembly limits.
The range of the machinery processing allowance is based on the complexity and the size of the part that is produced:
For simple components, such as fasteners and pins, the range of the processing allowance is 0.001 to 0.003 [mm].
For medium-sized components, such as bearings, the range of the process allowance is 0.002 to 0.005 [mm].
For large components, such as drive shafts and crankshafts, the range of the process allowance is 0.005 to 0.010 [mm].
The machining allowance can also be determined by the type of metal used, with values ranging from 0.002 to 0.008 [mm] for aluminum and 0.004 to 0.015 [mm] for steel.
Processing allowances are categorized into two types according to the types of materials:
For soft and ductile materials, such as aluminum and brass, a small machining allowance must be taken, which means that the part should have a maximum deviation allowance of 0.5 [mm].
For hard materials, such as steel and stainless steel, a large allowance should be taken, ranging from 0.5 to 1.0 [mm], depending on the complexity of the part.
Machining allowances are also determined by the type of process used:
For parts that are cast, the machining allowances range from 0.1 to 0.4 [mm].
For parts that are milled, the range of the process allowance is 0.2 to 0.8 [mm].
In addition, the machining allowance can also be affected by the type of tool used. For example, CNC milling machine may require a larger allowance as opposed to a manual milling machine.
The importance of processing allowance should not be underestimated. Not only does it help to ensure that parts fit correctly together, but it also helps to reduce the risk of deformation caused by high pressures and temperatures. Therefore, accurate allowances need to be determined and the systems need to be carefully managed in order to minimize any issues that might arise during the machining process.
