Why Castings Should Have Certain Machining Allowance?

The characteristics of the investment casting parts are high precision and low surface roughness, so most of the time casts surfaces are machined for better appearance. Machining allowance is the extra material left on the surface of the casting during the casting process. This extra material is intended to be removed during subsequent machining to ensure that the finished product reaches the required dimensions and surface finish. When casting, metal may shrink or deform, and machining allowances are designed to compensate for these changes and ensure that the machined part meets precise specifications and tolerances.

For how much the machining allowance should be depends on the casting processing method, the size of the processed surface, the casting structure, and other factors.

During the casting process, if there are many pores, slag blocks, sand holes and rough edges on the surface, more machining allowance needs to be reserved to correct these defects. For large castings, because they are more susceptible to deformation and the casting shape is less precise, larger machining allowances are also required to ensure that the final product meets the required specifications and surface finish.

When casting carbon steel, a carbon-loss layer will form on the surface, so the machining allowance should be equal to the thickness of this carbon-loss layer to ensure complete removal.

The size of the machining allowance is also related to the processing method. Typically, where a riser system (the part of a casting used to guide molten metal) is provided, a machining allowance of 2 to 4 mm is required to compensate for irregularities, while other surfaces typically only require a machining allowance of 0.5 to 1.5 mm. to achieve the desired surface effect.

carbon steel casting

Leave a Reply