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Conversion of a fabricated Rocker Arm to a casting.

A manufacturer of agricultural equipment that is conducting a complete design review to improve their products’ competitiveness and performance asked Castech Solutions to assist.

Many components had originally been designed as fabricated parts as it was the most economical and simplest option at the time. With the company success and increased production volumes, it made sense to reassess if some components could be supplied at a lower cost by modifying the manufacturing process. One of the possibilities to be explored was the conversion to a casting.

One of the first parts to be considered was a fabricated Rocker Arm shown in Image 1..

  

Image 1. Rocker Arm manufactured by fabrication of steel sheet and tube.

Based on the current drawing (Image 2) Castech Solutions provided a budget quotation which indicated that a significant cost reduction was possible. With this assurance the company together with Castech were able to commit the time and money needed to develop the casting design.



Image 2. Drawing supplied for the budgeting quote stage.

Castech recommended investment casting as the most appropriate manufacturing process because it delivered: 
  • Good dimensional accuracy which reduced to a minimum the machining operations.
  • Good surface finish and definition.
  • The target price. 

Because the new Rocker design would be manufactured by casting, the engineers could approach the new design with a degree of freedom not available as a fabrication. Working closely with the customer a CAD solid model was defined and optimised using Finite Element Analysis and casting simulation software.

The collaborative design process ensured that the final design resulted in optimum performance and manufacturability of the finished part. Some of the benefits were:

  • Tailoring the section thickness and overall design to put metal where it was needed according to loading conditions in service; see Image 3. 
  • Cast on Logo to promote the customer’s “Brand”
  • Part identification for quality traceability.
  • Overall improvement in the appearance of the part.
Image 3. 3D model of the new design of Rocker Arm and results of the FEA showing the distribution of stresses on the new design.

Two choices of material were considered: carbon steel and ductile iron. Due to its weld ability and fracture toughness a quenched and tempered medium carbon steel was chosen.

Once the design was finalised and approved, Castech developed the manufacturing specification. Because of Castech’s involvement in the design process, there was a clear understanding of the product requirements that was translated into the specified NDT, dimensional and material quality assurance documentation.

For example, the position of the machined bores. The important requirement regarding their position is that the wall section thickness between the machined bore and the casting is constant, to avoid weak areas.

Appropriate machining setup datum’s were noted on the drawing. A dimensional inspection procedure was defined to ensure that the wall thickness variations did not exceed 10% and the range for the average section thickness was specified. 
The requirement and inspection procedure was clearly explained in the Product Data Sheets. 
The Test Certificates for dimensional inspection were designed to conveniently record the results; see Image 4.


Image 4. Definition of dimensional measurements for the position of the bores and Test Certificate detail showing how they are recorded.

Before the manufacture of the pattern and samples commenced Castech engineers conducted a technical review with the manufacturing partner's engineers. Issues as the positioning of the holes were explained to the machining engineers and inspectors to ensure the manufacturer understood all of the product requirements and critical areas.

Samples were manufactured (Image 5) and inspected by our team of inspectors in China. The inspection results were positive and the sample was shipped to our customer to be validated. Before launching mass production it was important to check that the new design integrated well in the equipment assembly. During this validation, it was noticed that a small design change was needed. A new model was generated and a new sample was cast and inspected before mass production started.


Image 5. Side to side comparison of the fabricated and cast rocker arm. Lettering in the cast part has been hidden. 
This project has demonstrated that:
  • Converting a fabricated part to a casting can result in a significant cost reduction.
  • The freedom of shape provided by the casting process allowed for an optimization of the geometry.
  • Logo, lettering and traceability can be added at no extra cost.
  • A collaborative design process can result in superior products offering unique advantages because of the capability of the casting process at a lower unit cost.