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Practical DFMA: Practical Guidelines for Great Engineering Design 2
Design components for simple fabrication
Parts should be designed to use the lowest cost material that will satisfy the requirements, as well as minimize waste and production processing time. Attempt to design parts using stock sizes and near-net shapes. To assist this, obtain information about supplied material dimensions and standard profiles from materials stockists. Keep this handy (paper or PC) and refer to it during design development or value engineering.
Designing parts for easy fabrication is based on the following principle - the most suitable manufacturing process is used to make the part and likewise the part is designed for the most suitable manufacturing process. Again, suppliers can advise on the most appropriate manufacturing processes. This includes the use of jigs and setting-up times to fabricate parts.
It follows that secondary and finishing processes such as grinding, reaming, honing, polishing, painting, plating, buffing etc. may need to be avoided wherever possible (particularly for parts concealed within the finished product). From a practical point of view (and one that meets the corrosion resistant requirements of components), secondary processes can be avoided by specifying surface finishes and tolerances carefully, and then selecting primary manufacturing processes which meet these requirements.
Also consider selecting materials which do not require finishing. Any extra cost of such materials, should be less than the combined cost of an alternative material and the finishing process (not to mention the lead-time). Additionally some plating processes now have environmental considerations and concerns. Parts should be designed to use simple fabrication processes.
Avoid or minimize the use of fasteners
Mechanical fasteners add significant cost to manufacturing and assembly. The cost of installing fasteners significantly exceeds the fasteners themselves. If fasteners must be used, minimize the number, as well as the variation in size and type. Can guidelines be set for these? Can any snap-fit components be used instead? If fasteners must be used, self-tapping screws and captive washers can assist cost minimization. Fasteners are particular troublesome during automation, where they are difficult to feed, require costly fittings, fixtures and feeders, as well as monitoring for correct torque. Avoiding or minimizing the use of fasteners significantly reduces cost, quality issues and assembly lead-times.
Minimize assembly directions
All parts should be assembled from one direction, as far as possible. This minimizes assembly time and simplifies the operation. The best solution is to use gravity and assemble parts in a top-down fashion (along the z-axis). Consider designing sub-assemblies to be assembled like this, before joining then to the main assembly.
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Maximize compliance during assembly
Manufactured parts are not perfectly made or identical, although they may look the same. This sometimes leads to assembly problems such as tolerance stack-up, misalignment during the mating of components and difficulty inserting parts during assembly. To solve these problems, compliance features are designed into parts. These include generous radii, location points for manufacturing fixtures, guiding features such as leads, generous chamfers and tapers for easy insertion. Poor compliance can lead to quality problems, part rejection and increased assembly lead-time. Clearly, all of these increase costs.
Positioning of parts during manufacture is costly, often requiring jigs and set-up time. Any movement during assembly increases the cost further. Because of this, parts should be designed to make positioning as easy to achieve as possible. Also production and assembly processes should maintain the desired position. Attempt to minimise the number of positions a part should be placed in during fabrication. For assembly, consider using symmetrical designs and identify the correct position by marking the part. Also, assist orientation by including features in the design which help guide and locate parts.
Eliminate or simplify adjustments
Mechanical adjustments add to costs in a variety of areas such as fabrication, assembly, testing and reliability. They also over-complicate parts. Can adjustments be eliminated or reduced? It is important to understand the original need for adjustments. Following this, can they be eliminated or reduced?
Minimise flexible components
Flexible components such as wiring are difficult to handle during assembly, particularly during automated assembly. Also, wear can quickly occur on soft materials like plastics, where they rub or touch during reciprocating assembly movement. Consider fixing these in place or using rigid connectors to eliminate this potential problem.
As you can see Henry W. Stoll provides some excellent guidelines to assist value engineering and cost reduction. Here are more hints and tips to assist you review products and reduce costs:
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