Providing Value With 3-D Scanning
One case study shows how 3-D scanning can trim lead times by quickly developing tooling for complex castings while ensuring dimensional accuracy.
Glenn McQuarter, Bay Cast Inc., Bay City, Michigan
(Click here to see the story as it appears in the November issue of Modern Casting.)
A casting buyer needed a large, curved component as part of a new project, but the design’s size (1,200 lbs.) and curved shape presented specific challenges that severely limited the available methods of manufacturing. Not only that, the turn-around time for the project was relatively short.
The customer approached Bay Cast Inc., Bay City, Mich., which specializes in heavy-sectioned castings and large-format machining services.
This specific project highlighted how metalcasters can provide added value with 3-D scanning as a method of ensuring the accuracy of first article castings and minimizing tooling work. The customer needed 16 large castings in a 1030 steel alloy, including 10 large elbows (1,200-lb. castings with a maximum envelope size of 132 x 58 x 15 in. and a maximum wall thickness of 1.44 in.) and six small elbows (500-lb. castings with a maximum envelope size of 93 x 42 x 8 in. and maximum wall thickness of 1.12 in.).
3-D scanning involves the use of a three-dimensional data acquisition device to acquire X, Y and Z coordinates (or points) from the surface of a physical object. The conglomeration of these points, known as a point cloud, then can be used to create a 3-D mesh and, eventually, a solid model in various CAD formats. These models then can be used in inspection, analysis, rapid manufacturing and reverse engineering efforts.
The customer initially pursued casting the parts, which were part of a new project, because of the difficult design. The only other viable method for production required rolling and shaping approximately 18 individuals pieces and then welding them together. Casting this component as a single part had the potential to lower costs and reduce lead times, as long as they could be cast to the desired shape.
Capabilities & Concerns
Timing on this particular project was critical because these castings were part of larger fabrication assemblies. Figure 1 shows the elbow piece in its final application. Bay Cast’s engineering department, with a minimum amount of experience with these castings’ geometry at the time, had major concerns about casting this particular project:
- Will parts with this much surface area and so little weight encounter a restraint to contraction on the large open sides?
- Can the length of the longest overall dimension be held to a consistent tolerance?
- Will the overall pattern shrinkage factor hold true on the critical surface of the casting (shown in Figure 2)?
The 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 first 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. pattern 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 fit adjustments during finishing.
Producing a First Article
The 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 first article approval, the metalcasting supplier decided to outsource a complete 3-D scan of the part. The casting was sent to Diversified Tooling Group, Sterling Heights, Mich., to utilize the company’s equipment and software for high density white-light scanning of surface geometry. The results were then converted into an .STL file 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 fit” scenario.
The results were submitted to the customer for review and preliminary approval. These parts were originally intended to be manufactured from bent and rolled fabrications. Bay Cast discussed 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. tolerance originally requested, which could be remedied easily by removing 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. The customer also requested analysis of the section views (in inches) based on a supplied 2-D drawing.
One of the drawbacks of analysis software is in slicing the models to compare cross-section geometries. Although no additional scanning work was necessary, this additional request required conversion of the scan data into a CAD file to overlay the build model and analyze the required sections.
Inspection performed on the first article small elbow piece showed similar results with additional material (0.4 to 0.6 in.) along the cope edge of the casting. Both large and small first articles were brought into tolerance by removal of the excess material during finishing. Additionally, tie bars were introduced to maintain dimensional stability through heat treatment (shown in Figure 4).
After making the required adjustments to the patterns, Bay Cast poured the remaining castings which were again inspected utilizing the same criteria as the first articles. All of the results were within customer requirements, and dimensional variations from casting to casting were minimal considering all of the pieces were poured from a separate polystyrene pattern. A summary of the variations are shown below in Tables 1 and 2.
3-D point cloud scanning has been used for a variety of applications in various industries. For the metalcasting industry in particular, the technology can be a tremendous tool in verifying a pattern’s dimension and inspecting a first-article casting for shrinkage and warpage.
From a machining standpoint, especially in large format jobbing operations, 3-D scanning technology can be used to qualify castings for rough or finish machining with an additional purpose of optimizing a machining strategy. This allows for “level machining,” which means that CNC milling equipment can be programmed based on a part’s scanned geometry. As a result, less spindle time is spent cutting “air” during the rough machining process, which potentially increases throughput and reduces costs.
In the case of the two different elbow castings, the data from 3-D scanning provided general insight into the metalcasting process with respect to dimensional repeatability of unusual geometries utilizing polystyrene pattern equipment. By casting these components, and ensuring they met required tolerances via 3-D scanning, the customer eliminated complex layout and assembly processes at its facility. The scan data provided proof that the component met its required shape.
This article is based on the paper, “Practical Application of 3-D Scanning in a Jobbing Foundry,” presented at the Steel Founders’ Society of America’s 2014 Technical & Operating Conference.