Design for Manufacturability

Design for Manufacturability (DFM), also called Design for Assembly, is a design effort which will lower the cost of manufacturing by combining design elements with the right processes to minimize material and labor involved in the product.

The DFM effort is focused on efficient manufacturing, ease of correct assembly, and minimum labor.

DFM is highly dependent on the expected volume of the product. Looking at extreme examples, compare volumes of 1 per month to 1,000 per month. The higher volume makes it more likely to use material fabrication processes like Injection Molding, Casting, and Stamping. It is also more likely that the assembly and test processes will use automated equipment and special production setup (e.g., a work cell rather than batch build). This is the balance between product cost and design cost. With low volumes, the product cost can be higher to save on design cost, as if design cost were a setup cost for manufacturing – which is not an unreasonable way of thinking about it. As volumes increase, it makes sense to spend more on design, tooling, and setup to lower the manufactured cost. 

The lifetime of the product also plays into this calculation. If the product is going to be manufactured for a brief time, even at a reasonable volume, you would tend to use less tooling and setup and lean more on assembly labor. The cost of the design and fabrication of the tooling must be spread out over the total amount of the product to understand the most efficient path. It sounds simple, but it is unlikely anyone has clear knowledge of the numbers involved. 

If you know at design time that you are only building one unit (e.g., a custom boat), the amount of tooling and automated equipment is easy to estimate – little to none pending quality considerations. If you are building 10,000 per year for many years, again the tooling is easy to estimate – spend on tooling and equipment to lower production cost. It gets more difficult when the trade-offs are not as obvious, with volumes of a few dozen to a few hundred per year. 

There is another consideration for DFM, and that is to use it to improve quality. There are some operations that are inherently riskier than others and you should consider utilizing a DFM process to de-risk manufacturing. Cable assemblies are a good example. Crimping connectors on the end of a wire has some surprising subtleties. From the quality of stripping the insulation to the degree of compression of the terminal, there are judgements to be made and a standard for judging that quality. We find that it is better to utilize a cable manufacturer who has automated and calibrated equipment to remove some of the judgement and apply process control. You can build your own cable assemblies but be careful. Product Resources has more than 150 different calibrated hand crimp tools for the various terminals that we need to crimp (at $400 each on average), plus automated wire stripping equipment – and we still outsource every cable we can to companies with more process automation. Printed circuit board assembly transitioned to automated processes long ago and even for all but the simplest prototypes the manufacturing is sent to companies that have invested the capital in automated assembly and test equipment. 

Here are some general areas where attention to DFM can improve quality and reduce cost: 

  • Component Fabrication Processes – Be familiar with the capabilities of the manufacturing processes used on the components. Design within the process’ capabilities and avoid tight tolerances.  
  • Minimize Part Count – Can components be combined to eliminate separate parts? Instead of bolting two parts together, could they be made as a single part? 
  • Utilize Common Parts – the more you can use the same part, the more you will benefit from the economies of scale. For example, if you have four distinct parts that need to be mounted by brackets, could you design one bracket that could mount all four parts? It may mean more holes in the bracket, or a larger bracket than necessary for one part, but in the end, you may save money and assembly time by having a higher volume of fewer parts to inventory (and order, count, kit, and maintain drawings, etc.). 
  • Prevent Incorrect Assembly – it can be satisfying to design a symmetrical part. It may be better to deliberately break that symmetry so that it is not possible to install the part incorrectly. Even if the mistake will be caught later because an associated part cannot be installed, you have tripled the assembly time. 
  • Optimize Tolerances – it is tempting and even easy to allow CAD systems to display 3-decimal place accuracy for all dimensions (even in mm). This is a mistake for a couple of reasons. It will drive the cost up if tight tolerances must be held for dimensions where it is not needed. Also, it will tend to hide the accuracy requirement for dimensions where it is truly needed. The better method is to go with the loosest tolerances possible for the part, and only tighten up the ones where it is necessary. 
  • Minimize Number of Different Fasteners / Tools – It will reduce assembly time by limiting the “searching” for the right fastener or tool. Instead of having 15 different bins of parts and tools to install them, you will have only a few. 
  • Safety and Ergonomics for Personnel – proper tooling and design of workstations can reduce repetitive strain injuries for workers. One example – screwdrivers can be replaced with electric screwdrivers with torque settings where possible. Saves injury and makes sure fasteners are always torqued properly and repeatably…and you need fewer of these screwdrivers with fewer fasteners. Be aware of workstations where the employee must lift boxes or parts from low shelves or the floor, especially if they twist their body while doing it. Even a light load will eventually cause back problems for them. 
  • Disassembly – this may apply more for field service and depot repair but consider what parts may need to be replaced and organize them so that it is easier to remove them and replace them without complete teardown of the product. The design of the Field Replaceable Unit (FRU) as a subassembly is important for service and can be a subassembly in manufacturing as well. That way, you will build more of those assemblies with some destined for manufacturing and some destined for inventory as a spare part.