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Castings - Designing with Machining in Mind

For most cast parts, machining is an integral part of its manufacturing live cycle. In this post, we go over why it is important that machining considerations are to be incorporated into the process as early as possible, and before the pattern is fabricated.

Basic fixturing principles

A workpiece (i.e. the casting), has 12 degrees of freedom as shown below in figure 1. This includes both a linear and rotational movement, in both directions, acting on the X, Y, and Z axes. These degrees of freedom must be restricted through the use of workholding (e.g. chucks, clamps, collets, vises, etc.)

workpiece block showing degrees of freedom when in free state.
figure 1 : degrees of freedom

Additionally, the part must be positioned in a manner to properly capture the geometric requirements (we'll be posting a series on GD&T later). Meeting the geometric requirements established in the drawings will allow the part to function as intended. Below in figure 2, a rectilinear component is show using the 3-2-1 principle. Using the 3-2-1 principle, the workpiece is setup in the workholding, in sequential order from primary to tertiary datums. This sequence is important, as it not only satisfies design requirements, but is also the sequence used to inspect the part for geometric and dimensional verification. The fixturing sequence also provides a mechanism to produce consistent parts.

Locating of rectilinear workpiece on datums
figure 2 : 3-2-1 principle

While the 3-2-1 principle is suitable for rectilinear based components, other systems are used for other classes of components. Cylindrical components for example, could be located in a lathe chuck. In this system, the face of the workpiece is placed against the step on the chuck jaw, and the cylindrical surface of the workpiece is clamped with the chuck jaws.

Other systems exist, so it is important that the workholding approach is confirmed before the pattern is built.

Features to use for locating points

As-cast surfaces that can be manufactured to ensure consistent dimensional and geometric stability, should be used to locate the workpiece. These features include walls and bosses, as well as lugs that have been specially created to aid in the machining of the workpiece.

Using features that are influenced by flash, gating, riser’s etc., should be avoided. These regions will be prone to inconsistent dimensional and geometric stability, due to the nature of cut-off and grinding departments using hand tools. Features created by cores, including cored holes, should also be avoided as they could suffer from both the aforementioned problems, as well as core shift.

Workholding and clamping

There are 3 links in the machining chain that effect overall rigidity in the machining process. The machine, the tooling, and the workholding. While the machine, and tooling are outside the scope of this discussion, the workholding may play an important role in the design of the casting.

Due to the high forces exerted on the workpiece during machining, caused by both the cutting tool and the clamping, catastrophic deformation of the workpiece is possible. Carefully planning out locating points, as well adding additional material, may be necessary to prevent deformation which would cause a rejected part.

Rigidity in the machining chain is important not only in reducing deformation of the workpiece, but also in increasing the life of the cutting tool. In a sufficiently rigid system, tool chatter is eliminated, and the tool is able to run at increased, and often a more optimal rate.

Final thoughts

In summary, it is important to include a machinist in the design lifecycle of the casting, prior to making the pattern. Ensuring that the correct features and attributes are included on the casting will lead to a consistently machinable part, that meets the customers requirements. This is accomplished by the correct use of datums, and location points, as well as the stability of the part, based on the specific workholding application being used to hold the part in the machine.

A properly designed casting that incorporates post processing such as machining, can save both time and money, as the risk of scrap is reduced, and the ease of setup and repeatability is increased and streamlined.


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