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
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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