There are many factors that a design engineer must take into account when designing a sand casting. All of these factors depend on what the casting is going to be used for and what other processes it must go through after the casting is poured. For instance, if the casting is going to be used on a motorcycle and is going to be visible, chances are it is either going to be painted/powder coated or machined to a smooth finish. Or if the casting is going to be used as a housing or part of a structure where it must maintain rigidity, heat treating may be required. Chances are that no matter what the casting is going to be used for, it will require some type of machining.

For castings that are going to be machined, it is important for the design engineer to include extra material (when necessary) where the casting can be attached to a machine fixture during machining. A machine fixture is a fixture that is specifically designed to hold the casting in place during machining. If the casting shifts within the machine fixture AT ALL during machining, it can ruin the entire casting which must be scrapped further increasing costs. In the picture above, you can see the machine fixture (red) that we manufactured specifically for that casting. Also, you can see the excess material on the casting that is used to hold the casting in the machine fixture, after machining these tabs will be cut off and machined smooth. However, not every casting requires excess material, or tabs as I call them, for machining. These tabs are usually added to small castings or castings with odd designs that would make it difficult to create a machine fixture for. How do you know what you need to include in you casting design?

The best thing a design or casting engineer can do is consult the engineer(s) who will be designing the machine fixture for the casting. Doing so will determine what needs to be included in the design to accommodate machining in the most cost effective way. Failure to consult the engineer can be a huge mistake as it may incur additional machining costs. Most of the time the additional machining costs stem from added time needed to produce a machine fixture that will hold the casting steady during machining without doing any damage to the casting itself. If you don't have the luxury to consult the engineer that will produce the machine fixture, the next best option is to seek Engineering Assitance.

There are many factors that one must consider when designing a casting, but failure to take secondary operations into account has the potential to dramatically increase the overall cost of the project. Always consult the engineer that is going to produce the machine fixture for the casting or if that's not an option seek engineering assistance.

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