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Material Specification and Machining of Precision Castings

As we said before, before purchasing any Precision Casting, there are many things you need to consider. This article will discuss the design, material specifications,and machining of castings to give you more advice before purchasing any.

Generally speaking, to achieve the most efficient production and the highest quality product, the part should be designed to take advantage of the flexibility of the casting process. The supplier foundry must have either the designer's drawings or pattern equipment and know the length of the run.

Castings are generally furnished with unmachined surfaces, unless specified. To take advantage of the casting process, the supplier foundry should also know which surfaces are to be machined and where datum points are located. The acceptable dimensional tolerances must be indicated when a drawing is provided. Tolerances are normally decided by agreement between the supplier foundry and customer. Close cooperation between the customer's design engineers and the supplier foundry is essential to optimize the casting design.

On the other hand, industry standard specifications provide the casting customer with the tools necessary to establish criteria for almost any casting application. These specifications do not preclude special requirements that the customer's technical staff members may require. Variations from standard specifications can result in misunderstandings, higher costs and disqualification of potential supplier foundries. If exception is taken to a provision in the main body of a specification requirement (as opposed to taking exception to a supplemental requirement of a specification), the resulting casting cannot be held to compliance with that specification.

Generally, mechanical properties may be verified by the use of test bars cast either separately or attached to the castings. The mechanical properties obtained represent the quality of the steel, but do not necessarily represent the properties of the castings themselves, which are affected by solidification conditions and rate of cooling during heat treatment, which in turn are influenced by casting thickness, size and shape. In particular, the harden ability of some grades may restrict the maximum size at which the required mechanical properties are obtainable. Short of destructive testing of an actual casting sample, the use of a test bar is the best measure of the steel quality.

As far as we know, tolerance refers to the dimensional accuracy achievable for a given production method. For the green sand casting process, mold expansion, solidification shrinkage, and thermal contraction all influence the tolerance of the finished part. Consequently, there are limits for tolerances in an as-cast part. Subsequent machining is commonly employed when a tighter tolerance is required.

In the final analysis the supplier foundry is responsible for giving the designer a cast product that is capable of being transformed by machining to meet the specific requirements intended for the function of the part. To accomplish this goal a close relationship must be maintained between the customer's engineering and purchasing staff and the casting producer.