Machined Investment Casting

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LeanWerks navigates the challenges of a complex jet engine part

Many shops avoid machining complex castings because of the variability of the beginning form and complicated work-holding requirements. Engineers like to design with investment castings because they offer intricacy of form with good dimensional precision relative to other casting processes. Despite their relatively good dimensional precision, investment castings still require machining processes to achieve precision fits and function in high performance assemblies.

Challenge

Material: Inconel 625. This aircraft grade high temperature alloy is extremely tough and eats cutting tools for breakfast, lunch, and dinner. Tools should be PVD coated carbide with positive rakes or ceramic for roughing.
Geometry: This casting has multiple features on all faces which creates a process flow and work-holding challenge.
Tolerances: Form, size, and position tolerances all present challenges on this part. The surface inside the housing has to be flat <.001″, while the end flange diameters have to be held to ± .001″ size tolerance and coaxial within Ø.001, and the ports on the OD have positional requirements of < Ø.005″ to primary, secondary, and tertiary datums.

Solution

“All solutions are simple” is a mantra and an ideal at LeanWerks. We start with a foundation of great people that includes journey level tradespeople and degreed engineers. Their expertise is strengthened by robust problem-solving processes to ensure successful results and manufacturing plans. In this case, the solution turned out to be a combination of work-holding and tool life management.

Work-holding: LeanWerks engineered a locating plate to precisely hold the casting after the first flange is turned. The locating plate stays with the part through all subsequent machining operations. The remaining machined features are accessed with accuracy and confidence since the locating plate easily loads on a fixture base at each subsequent machining operation. This is achieved by using hardened bushings in the locating plate and a combination of a hardened spherical pin and diamond pin on the fixture bases. The pin/bushing combination ensures a precise and repeatable location, and with the addition of hardened and ground mounting plates for the fixture base and rotary fourth axis for our milling operations, the center of rotation is also held true.

Tool Life Management: Speeds, feeds, and tool paths must be finely tuned when cutting Inconel as there is little margin for error. The key to consistent performance, once the foundation is established, is managing tool life. LeanWerks uses a stringent program of cutting tool management so tools are changed before they fail, reducing scrap and increasing throughput. Broken tools not only cause unneeded and costly delays to replace the broken tool, but they often create a scrapped part or can cause damage to the fixture or machine tool. That is especially true in materials like Inconel as a dull tool hardens the material and often creates forces so high that the part is over cut, damaged by the work-holding, or ripped from the work-holding and/or machine tool causing sever damage. When operating to a tight schedule or working to ramp up to increased demand, tool failures can be devastating and must be avoided. LeanWerks uses experienced master machinists like Frank Purcell to inspect the cutting tools and create standard tool change increments through the development of a new program. LeanWerks uses charts like the one shown below to document the standards needed to run a part consistently so it can be run with confidence by a machine operator.

Results

Machined castings that meet specification, that is the result we expected and achieved. When demand for these parts doubled due to unexpected events that shifted the commercial aerospace market, LeanWerks was able to add resources and increase throughput quickly because of our simple and well-documented solution.