The cost of assembly is determined by the number of parts in the assembly and the ease with which the parts can be handled, inserted, and fastened. Design can have a strong influence in both areas. Reduction in the number of parts can be achieved by elimination of parts (e.g., replacing screws and washers with snap or press fits, and by combining several parts into a single component). Ease of handling and insertion is achieved by designing so that the parts cannot become tangled or nested in each other, and by designing with symmetry in mind. Parts that do not require end-to-end orientation prior to insertion, as a screw does, should be used if possible. Parts with complete rotational symmetry around the axis of insertion, like a washer, are best.
When using automatic handling it is better to make a part highly asymmetric if it cannot be made symmetrical.
For ease of insertion, a part should be made with chamfers or recesses for ease of alignment, and clearances should be generous to reduce the resistance to assembly.
Self-locating features are important, as is providing unobstructed vision and room for hand access. Figure 13.15 illustrates some of these points.
The guidelines for design for assembly can be grouped into three classes: general, handling, and insertion.
1. Minimize the total number of parts: A part that is not required by the design is a part that does not need to be assembled. Go through the list of parts in the assembly and identify those parts that are essential for the proper functioning of the product. All others are candidates for elimination. The criteria for an essential part , also called a theoretical part, are:
●The part must exhibit motion relative to another part that is declared essential.
●There is a fundamental reason that the part be made from a material different from all other parts.
● It would not be possible to assemble or disassemble the other parts unless this part is separate, that is it is an essential connection between parts.
● Maintenance of the product may require disassembly and replacement of a part.
● Parts used only for fastening or connecting other parts are prime candidates for elimination.
Designs can be evaluated for efficiency of assembly with Eq. (13.10), where the time taken to assemble a “theoretical” part is taken as 3 seconds. 24 Designassembly efficiency
= 3× “theoretical” minimum number of parts total assembly time for all parts
A theoretical part is one that cannot be eliminated from the design because it is needed for functionality. Typical first designs have assembly efficiencies of 5 to 10 percent, while after DFA analysis it is typically around 20 to 30 percent.
2. Minimize the assembly surfaces: Simplify the design so that fewer surfaces need to be prepared in assembly, and all work on one surface is completed before moving to the next one.
3. Use sub-assemblies: Subassemblies can provide economies in assembly since there are fewer interfaces in final assembly. Subassemblies can also be built and tested elsewhere and brought to the final assembly area. When subassemblies are purchased they should be delivered fully assembled and tested. Products made from subassemblies are easier to repair by replacing the defective subassembly.
4.Mistake-proof the design and assembly: An important goal in design for assembly is to ensure that the assembly process is unambiguous so that the operators cannot make mistakes in assembling the components. Components should be designed so that they can only be assembled one way. The way to orient the part in grasping it should be obvious. It should not be capable of being assembled in the reverse direction. Orientation notches, asymmetrical holes, and stops in assembly fixtures are common ways to mistake-proof the assembly process. For more on mistake-proofing, see Sec. 13.8.
Guidelines for Handling
5. Avoid separate fasteners or minimize fastener costs: Fasteners may amount to only 5 percent of the material cost of a product, but the labor they require for proper handling in assembly can reach 75 percent of the assembly costs. The use of screws in assembly is expensive. Snap fi ts should be used whenever possible.
When the design permits, use fewer large fasteners rather than several small ones.
Costs associated with fasteners can be minimized by standardizing on a few types and sizes of fasteners, fastener tools, and fastener torque settings. When a product is assembled with a single type of screw fastener it is possible to use auto-feed power screwdrivers.
6. Minimize handling in assembly: Parts should be designed to make the required position for insertion or joining easy to achieve. Since the number of positions required in assembly equates to increased equipment expense and greater risk of defects, quality parts should be made as symmetrical as their function will allow.
Orientation can be assisted by design features that help to guide and locate parts in the proper position. Parts that are to be handled by robots should have a flat, smooth top surface for vacuum grippers, or an inner hole for spearing, or a cylindrical outer surface for gripper pickup.
Guidelines for Insertion
7. Minimize direction: All products should be designed so that they can be assembled from one direction. Rotation of an assembly requires extra time and motion and may require additional transfer stations and fixtures. The best situation in assembly is when parts are added in a top-down manner to create a z-axis stack.
8. Provide unobstructed access for parts and tools: Not only must the part be designed to fi t in its prescribed location, but there must be an adequate assembly path for the part to be moved to this location. This also includes room for the operator’s arm and tools, which in addition to screwdrivers, could include wrenches or welding torches. If a worker has to go through contortions to perform an assembly operation, productivity and possibly product quality will suffer after a few hours of work.
9. Maximize compliance in assembly: Excessive assembly force may be required when parts are not identical or perfectly made. Allowance for this should be made in the product design. Designed-in compliance features include the use of generous tapers, chamfers, and radii. If possible, one of the components of the product can be designed as the part to which other parts are added (part base) and as the assembly fixture. This may require design features that are not needed for the product function.[/b]
study note of FDM : DESIGN FOR ASSEMBLY (DFA)