Dimensional Verification of Steel and Iron Castings.

by David Jolin, Quality Assurance Manager, Spokane Industries

One key benefit of steel and iron castings is the ability to be formed into complex, organic shapes that are not easily duplicated in fabricated, or even forged parts. Because of this, significant cost and labor savings can be achieved with castings, but these complex shapes can be quite difficult to inspect with traditional dimensional inspection techniques. The typical dimensional testing toolbox requires for numerous specialty items to complete the task adequately.

In addition to the complex shapes that are common for castings, an iron or steel cast surface will be textured by the molding material that the molten metal was poured into, usually bonded sand. This surface texture can affect the accuracy and repeatability of the inspection if care is not taken during the measurement process.

One important issue that complicates the dimensional inspection of iron and steel castings is the draft angle that is required on patterns that are used in sand molds. Draft angles are a manufacturing requirement of the sand mold process that allows the pattern to be drawn back out of the sand after the impression is made. These draft angles are rarely shown on casting blueprints and solid models but are usually noted on the prints as: Draft not to exceed 1.5 Degrees, or something similar.

The dimensional inspection of castings has traditionally relied on the standard hand-tools that reside in most inspectors toolboxes: height gauges, calipers, radius gauges, snap gauges, tape measures, etc. These hand tools continue to play an important role in the inspection process, but, because of the unique issues with castings as noted above, they cant always be relied upon for the complete dimensional inspection that may be required. In addition to standard hand tools, Spokane Industries uses both a traditional, table-based Mitutoyo CMM (Coordinate Measuring Machine) that has a 24x24x18 working volume and a Faro-Arm with an 8-foot sphere working volume.

The traditional CMM is primarily used for the measurement of smaller, investment castings produced in our lost-wax facility as well as for the castings that are further processed by machining. The Faro Arm is a portable, articulating-arm CMM that allows for much more complete and detailed measurements than would be possible with either hand tools or the traditional CMM. Spokane Industries uses a Faro Arm that has an accuracy of plus or minus .003 of an inch. Although this accuracy is not as precise as a standard, table-based CMM it is more than acceptable for the tolerances usually applied to iron and steel castings.

The Faro Arms measurement software contains all the standard measurement tools that are common in most measurement software packages: plane, line, distance, circle, etc., but the Faro Arm gives the dimensional inspector the ability to digitally trace the contours of the casting and compare the CAD data directly against the trace. This tracing capability of the Faro Arm is accomplished by moving the tip of the arm against the feature of the casting that needs measuring.

The software will record the path of the tip as small, stitched-line segments or digital points. These line segments or points can then be measured in the software. Because of this free-form recording of the actual shape of the casting within the software, the dimensional inspector is able to record the true shape of the casting that can be measured, viewed on-screen, reviewed via email, and rechecked even if the casting is no longer present. These features allow Spokane Industries more accurate and quicker dimensional inspections of castings that can be communicated with our customers via traditional dimensional reports, CAD/actual casting scan overlay, or a combination of both.

David Jolin, Quality Assurance Manager at Spokane Industries states, "another advantage of this scanning capability is to reverse-engineer existing castings that may not have a blueprint or cad-data. This is especially helpful if a customer has only a casting to provide to Spokane Industries to copy. Spokane Industries can scan the casting with the Faro Arm, generate a blueprint and CAD model and submit these back to the customer for review and approval. Once approved, Spokane Industries would then create the pattern for the molding operation, and then pour a first part sample. This sample can then be confirmed back against the scan of the original part or to the created and approved blueprint as well."

The realm of dimensional inspection has moved far beyond the days of hand-written dimensional reports listing the results to the nearest fraction of an inch. The advancements that the digital age and computer-aided inspection systems have spawned allow us to produce parts and inspect them with more detail and precision than ever before. This increased detail results in a broader understanding of the casting process that encourages engineers to design even more complex castings. As customer expectations mature, so does the ability of our QA department to meet and exceed them.