The function of design interacts with non-design engineering considerations and functions of a product. Four key elements include: design for manufacturing, design for assembly, design for inspection, and design for quality, also known as quality by design.
Design for Manufacturing
In the design process, every component manufactured must have the manufacturing processes considered. Can the component be manufactured? Is there any type of special tooling, fixtures, or other workholding equipment required? Can it be done at a reasonable cost?
A competent design engineer has a working knowledge of these considerations, and from experience, knows the general tolerance levels the manufacturing processes can hold. The designer also has general knowledge and experience in the different manufacturing processes, both roughing and finishing, such as turning, milling, grinding, shaping, hobbing, etc.
To evaluate these, it usually requires meetings with manufacturing engineers and representatives from purchasing and the accounting departments to get the costs of the processing established.
It is easy to design a product with components that cannot be manufactured, or at a reasonable cost, with reasonable accommodations.
In design for manufacturing, many items are evaluated.
- The type of raw material selected: Steel, iron, aluminum, etc.
- The type of heat treatment options for steel parts: Carburized and hardened, nitriding, quenched and tempered.
- Tolerances: Tighter tolerances are more difficult to hold and are more expensive to produce. Use the loosest tolerances that will make the parts function properly in the assembly.
- Size: Make the parts as small as possible to also function correctly.
- Fits and finishes: Smother finishes are more expensive to produce than rougher finishes.
- Production yields and losses: The scrap rate for the particular part being considered is a direct and measurable cost.
Design for Assembly
One of the main concepts for design for assembly can be found in Japanese manufacturing processes. It’s called poka-yoke, which translates to mistake proofing in English, a means of simplifying the assembly process and eliminating or reducing human error. One application of this concept is that a particular part will only fit in the assembly one way, thus eliminating the possibility of the part being assembled incorrectly. The rules of poka-yoke are: 1) Try not to spend. 2) Simpler is better; 3) Don’t make them optional; 4) Don’t confuse gauges with mistake proofing; and 5) No decision-making. Poka-yoke is one of the 13 pillars in the Toyota Production System.
Other considerations are to combine two or more parts into one part or using one part in multiple places, with either vertical or horizontal assembly, which means assembling from one plane/axis only and eliminating parts from the assembly if possible. This also can lead to cost savings.
Furthermore, it is important that the gearbox can be assembled without great difficulty or cost. As the design engineer is designing the gearbox, this should always be top of mind. As with design for manufacturing, this involves meeting with other departments, such as representatives of the assembly department tasked with assembling the product.
There are usually several different ways to assemble any given gearbox, and the design engineer, along with others, should consider as many ways as possible while choosing the best and usually the simplest way. Conceptually, this can be a challenge for the design engineer as this requires the ability to think in three dimensions while looking at a two-dimensional layout or model on a computer screen.
The main idea is to reduce the part count in the assembly or to make the parts easier to assemble. Reducing assembly costs is the ultimate goal.
Design for Inspection
Design for inspection involves specifying parts that can be easily inspected with the most inexpensive, simple equipment such as gauges or micrometers. It is helpful if standard inspection equipment be used rather than special purpose-built bespoke inspection devices. This also usually involves working with process and manufacturing engineers.
Design for Quality
Finally, there is design for quality, also known as quality by design (QbD), a concept first initiated by quality expert Joseph M. Juran. In his book, “Juran on Quality by Design: The New Steps for Planning Quality into Goods and Services” published in 1992, Juran believed that quality could and should be planned for. Customer satisfaction and reliability are the goals. This involves specifying the minimum quality level necessary to ensure satisfactory performance of the product, acceptable failure rates, and customer satisfaction.
All of the aforementioned procedures and processes should be in the thought processes of the design engineer. They are most easily accomplished in the design stage of a project, where it is least costly to optimize the design and make necessary changes from any one of these non-design considerations. At this stage where the design is on drawings or as a digital image, the designer’s time is the only expense — versus a manufactured part, tool, piece of inspection equipment, or other piece that costs money to produce.
These are some of the non-design related considerations that the competent design engineer should contemplate while designing a gearbox or any other assembly.