In this Blog lists some general considerations for product design strategy and presents assembly strategy, fastening strategy, assembly motions, and test strategy. These strategies are important aspects of the concept/architecture phase
All engineers should learn the instructions from equivalent assemblies from relevant products and formulate action plans and deliverables to leverage the best and avoid the worst. This also involves raising and resolving assembly issues based on instructions.
Simplify assembly with fewer parts, off-the-shelf parts that come assembled, and parts that are combined into monolithic circuit boards, castings, stampings, extrusions, and molded parts. Design for assembly without need for any skill or judgment and minimize manual tasks, for instance, by using connectors instead of wiring lugs and hand soldering.
Design teams should strive to design products to eliminate the need to apply any liquids for fastening, bonding, or sealing. Eliminate the need for calibration or any kind of tweaking.
Design easy assembly features with self-jigging parts or parts that are aligned with pins, slots, or other features. Design symmetrical parts that don’t have to be oriented. At each workstation, minimize part variety and standardize on one fastener per workstation.
Combining parts is a technique that can be used to reduce the part count and simplify assembly, provided the combined parts don’t get so big and complex that they require expensive tooling or large mega-machine tools.
The combined parts could provide the following benefits: eliminate the need to manufacture the interface features, hold their tolerances, and save the time and cost of assembly.
It may be possible to fabricate combined parts on a single machine tool in a single setup. Examples include many parts combined into a monolithic plastic or machined part; many integrated circuits combined into VLSI or ASICs; and multiple circuit boards combined into one, thus eliminating card cages and inter-board wiring operations.
The criteria for combining parts involves asking the following three questions:
1. When the product is in operation, do adjacent parts move with respect to each other?
2. Must adjacent parts be made of different materials?
3. Must adjacent parts be able to separate for assembly or service?
If all three answers are no, consider combining the parts into one. It is important to remember that every interface between parts requires geometrical features to be designed and manufactured plus all interface tolerances need to be held.
ASSEMBLY DESIGN GUIDELINES
Guidelines throughout this book will use the following numbering system for instructional clarity. Each company is encouraged to develop the numbering system optimal for its operations. The use of several categories of guidelines allows the addition of new guidelines to the appropriate category rather than at the end of a single list.
If new guidelines are added next to related ones, they will be considered together when the designer is dealing with that subject. In this way, newer guidelines will be less likely to be overlooked than if they were just added to the end of one long list of general guidelines.
- Assembly Strategy = A
- Fastening = F
- Motion of assembly =M
- Test = T
- Standardization = S
- Part Shape = P
- Handling by automation = H
- Quality and reliability = Q
- Repair and Maintenance = R
If guidelines are to be used as checklists, they should be worded to optimize usefulness in checklists. Then the team and management would note on the checklist whether the guideline has been obeyed or how much the product deviates from a certain goal, say, zero or 100%.
1. Understand manufacturing problems/issues related products.
In order to learn lessons from the past and not repeat past mistakes, it is important to understand all problems and issues with current and past products with respect to manufacturability, introduction into production, quality, repairability, serviceability, regulatory test performance, and so forth. This is especially true if the previous engineering is being leveraged into new designs. In a checklist, this could be checked “completed” with a lessons learned report as a deliverable.
2. Design for efficient fabrication, processing, and assembly; identify difficult tasks, and avoid them by design.
Concurrently engineer the assembly sequence while designing the product. Designing for easy parts fabrication, material processing, and product assembly is a primary design consideration. Even if labor cost is reported to be a small percentage of the selling price, problems in fabrication, processing, and assembly can generate enormous overhead costs, cause production delays, and demand the time of precious resources.
3. Eliminate over constraints to minimize tolerance demands.
An over constraint happens whenever there are more constraints than the minimum necessary; for instance, joining two rigid frames with four bolts, guiding a rigid platform on four rigidly mounted bearings, or trying to precisely align two parts with multiple round pins inserted into round holes. (The solutions for these are shown below.)
Over constraints are costly and can cause quality problems and compromise functionality because the design will work only if all parts are fabricated to tight, maybe unrealistic, tolerances. Fortunately, over constraints are easy to avoid by specifying the exact number of constraints that will do the job: not enough constraints will result in an extra degree of freedom (something is loose); too many constraints will result in troublesome over constraints.
4. Provide unobstructed access for parts and tools.
Each part not only must be designed to fit in its destination location, but also must have an assembly path for entry into the product. This motion must not risk damage to the part or product and, of course, must not endanger workers.
Equally important is access for tools and the tool operator, whether that is a worker or robot arm, which usually requires more access room thana worker’s hand. Access may be needed for screwdrivers, wrenches, welding torches, electronic probes, and so forth. Remember that workers may be assembling these products all day, and having to go through awkward
contortions to assemble each product can lead to worker fatigue, slow throughput, poor product quality, and even worker injury. Access is also needed for field repair, where the tools may be simpler and possibly bulkier.
5. Make parts independently replaceable.
Products with independently replaceable parts are easier to repair because the parts can be replaced without having to remove other parts first. The order of assembly is more flexible because parts can be added in any order. This could be a valuable asset in times of shortages, in which case the rest of the product could be built and the hard-to-get part added when it arrives.
6. Order assembly so the most reliable goes in first, the most likely to fail goes in last.
If parts must be added sequentially, make sure the most likely to fail are the easiest to remove. This is important for both factory assembly and field repair.
7. Make sure options can be added easily.
Another advantage of independently replaceable parts is the ease of adding options later, either in the factory or in the field. Future options should be anticipated, and the product should be designed to accept these options. Considerations include allowing space for added parts, mounting holes, part access, tool access, software reconfiguration, extra utility capacity, and, of course, the safety of those performing the upgrade.
8. Ensure the product’s life can be extended with future upgrades.
Early consideration of the product-upgrading strategy could be crucial to extending the life of a product. Advances in technology should be anticipated so the product can be upgraded without a complete redesign. Modular design concepts can be used to allow modules that are prone to obsolescence to be replaced with upgraded ones. Extending product life through upgrading allows products to generate even more profit after the development and introduction costs have been paid off. Figure 3.3 shows the value of upgrades.
9. Structure the product into modules and subassemblies, as appropriate.
The use of subassemblies can streamline manufacturing because subassemblies can be built and tested separately. Subassemblies could be built in specialized departments, which is especially advantageous if those processes are different from those of the product; for instance, clean room assembly (assembly in a dust-free room).
If the entire product consists of a collection of pretested subassemblies, product testing may be eliminated or reduced to only a final go/no- go test before product shipment. In designs where potential quality problems are concentrated in one subassembly, test and diagnostic attention could be focused there. The remainder of the product may then rely on process controls.
10. Use liquid adhesives and sealants as a last resort.
For fastening, thoroughly pursue alternatives, such as screws or nuts coated with retention compound, fasteners with deformed threads, and optimal use of lockwashers. Design to eliminate the need for liquid sealants; for instance,with optimal enclosures and built-in seals. Long drying times can compromise flow manufacturing.
Design to eliminate the need to use sealants for arc prevention; for instance, with optimal spacing or snap-in insulating barriers or partitions. Seal with premeasured off-the- seal for manufactured solutions, such as rigid gaskets, compliant gaskets, custom-molded elastomeric gaskets, or O-rings, all of which should be self-jigging in the product.
If liquid adhesives and sealants are justified, make a thorough selection; be sure to optimize part alignment and repair strategy; standardize on the same application procedure (to avoid procedural errors); standardize on one adhesive per workstation (to avoid picking the wrong one); and avoid gaps, cracking, or structural weakness when glue shrinks. If justified, automate with robotic adhesive applicators or pick-and-place machines.