Design for ease of assembly
Design for assembly, otherwise referred to as DFM, was first coined about 40 years ago by Boothroyd and Dewhurst. This team developed software and protocols for optimizing designs for manufacturing. You can buy the B&D software and other DFM software packages or apply common sense and some fundamental manufacturing knowledge to accomplish the same objectives. In addition to DFM software, 3D CAD software enables designers to visualize highly complex parts and assemblies, perform interference checks, verify mechanical motions and evaluate structural integrity well before any parts physically exist. 3D CAD software also allows you to visualize every assembly step of a product methodically. Optimizing designs for manufacturability has been simplified with all these excellent design tools if you know how to use them.
Optimizing assembly steps requires you to observe some critical parameters listed below:
- Always consider the part size and weight versus assembler safety and ease of handling
- Minimize motion steps during the assembly process, such as repositioning parts by rotation or translation
- Provide ease of access the all installed components. Provide enough space for fingers or hands in tight spaces.
- Standardize hardware whenever possible such as specifying the same screw sizes and lengths
- Eliminate assembly procedures requiring special skills or craftsmanship such as complicated alignment procedures, masked painting or decorating, and application of adhesives.
- Eliminate subjectivity or judgment during the assembly procedure. For example, assemblers should not be required to align adjustable mating covers visually. Covers should be designed to fit without any alignment.
- Carefully plan complex assemblies as a compilation of smaller subassemblies that can be individually tested before being integrated into the larger top assembly.
Define quality standards
Product quality is a critical consideration for manufacturing and design. Low-quality products are quickly branded with bad reputations, declining sales, and eventual removal from the market. Therefore, designers must clearly define quality expectations for all manufactured products and communicate these standards to the supply chain. Maintaining high-quality standards can be highly technical, requiring sophisticated equipment or procedures, or pretty straightforward. The critical considerations for preserving product quality include the following:
- Identify the essential parameters to inspect. For example, a valve’s primary function restricts flow based on pressures and viscosities. Therefore the valve seal might be the only feature to inspect. If the valve is within an expensive faucet, the surface finish, color and feel would also be vital features to examine.
- Define QC parameters critical to product performance, reliability, appearance, and safety. Identifying and establishing these boundaries is not always obvious and requires a solid engineering background. It’s imperative that the threshold for each test point realistically represents the warranty limits of the product.
- Ideally, inline production inspection procedures should be efficient and fast. Some products require 100% QC testing and verification, while others may only require random sample selection of one part from a population of 100, 1000, or 10,000 parts. Determination of lot size for random sampling is based on probability data and statistical algorithms.
Know your vendor’s capabilities and limitations
Your suppliers are your partners. The vendors you choose are critical contributors to the success of your product launch and market position. Your vendor’s technical capabilities, culture and staff should be compatible with your design and quality expectations. For example, if your design tolerances are too tight for the candidate manufacturer, you must either loosen the tolerances or switch to a vendor who can comply with your expectations. The candidate manufacturer must share the same business philosophy as your company. For example, they must demonstrate an ability to comply with your expectations for delivery dates, product quality, product verification, product appearance, and product function. These qualifications can be verified by interviewing customer references, inspecting QC equipment and protocols, inspecting manufacturing process controls, reviewing their product portfolio, and most importantly, talking to a broad cross-section of the staff.
All too often, a finished design is handed to the manufacturer at the end of the design cycle. At that stage of the development process, compromises or revisions are very difficult to implement. It’s always beneficial to collaborate with manufacturing vendors throughout the design and development process to avoid this dilemma. Your design can be optimized for manufacturing from the start based on invaluable feedback from your supplier. Your supplier can suggest alternatives for achieving the same objectives by pointing out expensive design features and replacing them with optimized options. These suggestions are much easier to implement during the early stages of design versus the latter phases of development after hundreds of decisions have been made.
Optimize your tolerance requirements for your suppliers
All manufactured products are produced to comply with specified dimensions that control size, weight, and performance. Single stand-alone parts often don’t require critical tolerances since they are not required to mate to another part. However, most pieces are designed to fit to another piece based on a specific relationship. These critical relationships are apparent for gears, locks, sliding mechanisms, hinged lids, mating enclosure covers, and millions of other applications. The fit between these parts is specified by your dimensions, your tolerance allowance, and the clearances you’ve provided. The tolerance range is a function of the following:
Tolerance control will vary between different materials. For example, if we examine the basic materials listed below, we can immediately appreciate the vast differences in basic properties within each material classification. Plastics and rubbers include various materials exhibiting an extensive range of properties. Rubbers can be as soft and conformal as wet clay or as rigid and stiff as some metals. Although metals also show a range of properties, their thermal properties and stiffness are typically much higher than plastics. Wood materials are generally porous and fibrous, with many densities and strengths. Wood is typically more rigid than plastic but less rigid than most metals. Ceramics and glasses are brittle heterogeneous materials fused into a solid, highly rigid form with high thermal resistance and strength. The last category of composites is often laminated combinations of very different materials to provide a synergistic new material.
The tolerance control for each material classification will vary according to the material properties.
|Plastic Processing||Tolerance range||Key|
|Injection molding||1||4||Very loose|
|Sheet metal fabrication||1|
|Water Jet cutting||2|
|Ceramics and Glass|
|Water jet cutting||2|
- Plastics and rubbers – these soft materials inherently exhibit wider tolerance variations that should be accounted for in your designs. Softer rubbers are more challenging to control dimensionally than harder ones.
- Metals- tolerance variations for metals depend more on the manufacturing process versus the metal. Metals like steel or aluminum are typically cast, forged, extruded, sheet formed, or machined. Each manufacturing process has its unique tolerance limitations.
- Wood – Wood materials are offered directly from the tree or synthetically created materials like plywood or chipboard. These variations are typically fabricated by cutting, routing, or planing. Tolerance control for wood-based materials is highly dependent on moisture content, fiber structure, and density. Therefore wood materials should be dried to controlled moisture levels before being processed.
- Ceramics and glass – Ceramics and glass materials can be fabricated to very tight tolerances because of their high rigidity and hardness. Dimensional control of cast ceramic or glass parts is very poor. However, exact tolerances can be maintained with post-machining operations such as CNC milling, grinding or turning.
- Composites – Composite dimensions and tolerances are typically achieved by post-processes such as CNC water jet cutting, routing or saw cutting. These tolerances can be effectively controlled depending on the equipment.
The manufacturing process has the most significant effect on tolerance control and limits. Below is a shortlist of the most common manufacturing processes utilized in the industry today and their tolerance range. Designers should understand the general limitations of each process when detailing a product design for manufacturing—expecting a sand caster to mold a 4-foot long aluminum casting within a 1/16″.
Part geometry is another critical consideration to optimize manufacturing quality, performance, and productivity when controlling tolerances. Complex parts with many features are more challenging to fabricate and control and require more capital investment. However, complex parts have been typically designed to consolidate many individual parts and hopefully decrease assembly time. There are times when multifunctional pieces can become too awkward to handle, ship, or assemble. Designers should iteratively assess the value of developing a single multifunctional part versus many individual parts by evaluating the costs/benefits trade-offs of the previous parameters. Sometimes part complexity can drive plastic tooling prices so high the investment can’t be justified. Designers should identify those features that are critical to overall part performance. You don’t want to overly specify every dimension as essential, significantly increasing part cost and reject rate.
Part geometry should also be structurally reliable, safe, and secure. Designers must continuously refine the design to ensure it can be easily manufactured without compromising structural integrity. Optimizing structural integrity sometimes requires rigorous FEA structural analyses, testing, and prototyping. For example, the features of a plastic molded chassis must functionally perform, minimize assembly labor, and be easily molded without overly complicated tooling. These optimized trade-offs require a great deal of knowledge in many fields. It’s always wise to design parts with the most generous tolerances whenever possible.
Size of the part
Tolerance limitations are highly dependent on part size. The manufacturing processes, materials and equipment will vary based on part size. Large parts cannot be manufactured to the same tolerance limits as smaller parts. For example, computer chips are typically manufactured within microns and sub-micron tolerances using precision photo-etching methods versus large welded steel frames, typically fabricated within hundredths of an inch. Although there is no specific correlation between part size and tolerances, the previous statement regarding part size is accurate. Large parts can be very precisely manufactured if the materials are rigid and the machinery used is highly precise. Examples of larger parts that can be precisely manufactured include lathe-turned shafts, CNC machined metal castings and precision ground surface plates.
Another part size-based consideration for tolerancing parts is flexibility. Part covers or panels can often be very flexible when not mounted to a frame. The latter is especially true for plastic covers which are inherently flexible. It’s challenging to measure flexible parts since they tend to sag, twist, or bend when not appropriately supported. Therefore tolerances for large flexible covers should be based on specific cover support during measurements. Support methods should relieve the part of any unintentional distortion.
Define special testing and QC requirements
A significant consideration when designing a product for manufacturing is quality. Product quality standards should be quantitatively defined by the designer so manufactured parts can consistently be produced based on the same standards. These standards can be required for every manufactured part or randomly selected samples that have been chosen for verification. The percent of inspected components depends on the product’s safety, risk, and expectations. Engineers must comprehensively examine all aspects of the product, including its intended use, misuse, malfunction, warranties, reliability, and service. Identified features of the product must then be isolated by the engineer, who would then write appropriate test procedures.
Random sample testing requires extensive probabilistic risk analysis to determine the frequency of samples to be inspected per production lot. Examples of products that should be 100% inspected before shipment include chain saws, computer chips, razor blades, gas engines, microscopes, all medical devices. Examples of products that might warrant random sampling inspection might include hand calculators, low-cost speakers, dinnerware, and low-cost pens.
Engineers must design production inline inspection procedures and equipment to be accurate, fast, and representative of the evaluated desired parameters. Overly complicated or slow testing methods will restrict productivity and eventually be abandoned.
I hope this two-part article has provided you with greater insight into the number of factors that must be accounted for when designing for manufacturing. If you have any questions or comments about the contents of this article, please feel free to call me at 516-482-2181 or submit your comments to firstname.lastname@example.org.