consistent tolerance?
3. Will the overall pattern shrinkage factor hold true on the critical surface of the cast- ing (shown in Figure 2)?
T e customer requested an overall
tolerance of plus or minus 0.25 in. on all dimensions, so the initial plan was to produce a one-off fi rst article large elbow from single-use polystyrene pattern tooling, inspect it and then make the necessary adjustments on the balance of the pattern equipment. Bay Cast decided the castings would be produced in CNC-cut polystyrene tooling from customer-supplied CAD models so any necessary adjustments could be made quickly. Also, if major changes were required, a pattern could be immediately recut. A 0.19-in. pat- tern shrinkage allowance was selected, but with 0.25 in. of stock added to the perimeter of the pattern so some extra material would be available for fi t adjustments during fi nishing.
Producing a First Article T e three-axis geometry of the
elbow pieces made traditional layout of the rough castings nearly impossible to achieve, especially with foundry rigging still attached. To get a better idea of the geometry of the casting at shakeout and move forward on fi rst article approval, the metalcasting supplier decided to outsource a complete 3-D scan of the part. T e casting was sent to Diversi- fi ed Tooling Group, Sterling Heights, Mich., to utilize the company’s equip- ment and software for high density white-light scanning of surface geom- etry. T e results were then converted into an .STL fi le and analyzed against the original build model using mesh processing software. In the case of the elbow piece, the surface geometries of the scan data and the build model were compared in a “best fi t” scenario. T e results were submitted to the
customer for review and preliminary approval. T ese parts were originally intended to be manufactured from bent and rolled fabrications. Bay Cast dis- cussed with the customer dimensional requirements and preferred reporting method to facilitate approval on all of the castings. Based on the data, only the cope edge was beyond the 0.25-in.
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Fig. 2. The casting’s long, fl at edge presented a unique challenge for engineers.
tolerance originally requested, which could be remedied easily by remov- ing the extra material from the surface prior to shipping. From the customer’s standpoint, the surface scans provided an excellent visual tool with regard to
the shape of the casting. T e customer also requested analysis of the section views (in inches) based on a supplied 2-D drawing. One of the drawbacks of analy- sis software is in slicing the models
www.littlestownfoundry.com
Littlestown PA 17340 • 800-471-0844
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Doing It Right since 1916
Sept/Oct 2015 | METAL CASTING DESIGN & PURCHASING | 31
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