Design for Assembly (DfA) and Design for Manufacturing (DfM) are the industrialization steps implemented in the part or system design and assembly/manufacturing process.
Design for Assembly: DfA is the design methodology of the part or system for the ease of assembly.
Design for Manufacturability: DfM is the design methodology for the ease of manufacturing of the parts collection that will form after assembly the part or system.
DfA and DfM both strive an optimum in their respective assembly and manufacturing design. This can lead to a conflict between ease of Assembly versus ease of manufacturing. Examples are for DFA have only one part which could mean a very difficult to manufacturere part. DfA and DfM can't be seen seperate so a new defenition is on Assembly and Manufacturing.
Design for manufacturing and Assembly: DfMA
The objective is to design a part or system which has minimized the mounting and connection risks (DfA) and followed the Design for Manufacturability (DfM) rules. The importance of designing for assembly/manufacturing is underlined by the fact that about 70% of assembly/manufacturing costs of a part or system (cost of materials, mounting, connecting and processing) are determined by design decisions, with production decisions (such as process planning or machine tool selection) responsible for only 20%. The designer plays a key role in the development of the part or system assembly/manufacturing process. If designs are not designed for Assembly or Manufacturability, improvements in the assembly process will have very limited results.
The design methodology is first to design for assembly followed by design for manufacturing which will lead to an optimum when feedback form manufacturing are used to influence the assembly design so an optimum is obtaind in DfA and DfM.
The heart of any design for assembly and manufacturing system is a group of design principles or guidelines that are structured to help the designer reduce the mounting, connection and processing risks. The following is a listing of the DfA and the DfM rules.
Design for Assembly (DfA)
Design for Assembly (DfA) is the design methodology for the ease of assembly of the part or system. DfA focusses only on the assembly aspects and must lead to an optimum solution with feedback from the Design for Manufacturing (DfM).
Industralization DfA Rules:
Develop a Modular Design
The use of modules in part or system design simplifies manufacturing activities such as inspection, testing, assembly, purchasing, redesign, maintenance, service, and so on. One reason is that modules add versatility to product update in the redesign process, help run tests on the sub module before the final module is put together, and allow the use of standard components to minimize product variations.
Use Standard Parts
Standard parts have a lower risk in part defects, parts physical out of spec or parts functional out of spec than custom-made parts due to the high availability of these parts. With standard parts the reliability, cost price and availability is well ascertained.
Minimize Total Number of Parts
Each part will contribute with the part risks, mounting risks and connection risks to the assembly Zero Hour Defect Rate (ZHDR). With less defect opportunities, with equal risks DPMO's, the Zero Hour Defect Rate (ZHDR) will be lower. This must be discussed during the design phase to ensure minimum part count. Minimizing the number of parts minimize also the development, purchasing, inventory, handling, mounting, processing, equipment engineering, inspection, testing etc. time. In general, it reduces the level of intensity of all activities related to the part during its
entire life. A part that does not need to have relative motion with respect to other parts, does not have to
be made of a different material, or that would make the assembly or service of other parts extremely
difficult or impossible, is an excellent target for elimination. Some approaches to part-count reduction
are based on the use of one-piece structures and selection of manufacturing processes such as injection
molding, extrusion, precision castings, and powder metallurgy, among others.
Minimize number of Different Parts
By minimizing the number of different parts the chance on mounting risks will become lower. This is special true for fasteners in the mechanical assembly or electronic parts as resistors, capacitors, ... in the electronic assembly. Mounting the wrong part risk will be lower. Examples are fastener with wrong length or material.
Design Multi-functional Parts
Multi-functional parts reduce the total number of parts in a design, thus, obtaining the benefits when minimizing the number of parts. Some examples are a part to act as both an electric conductor and as a structural member, or as a heat dissipating element and as a structural member. Also, there can be elements that besides their principal function have guiding, aligning, or self-fixturing features to facilitate assembly, and/or reflective surfaces to facilitate inspection, etc.
An other approach is to reuse parts as much as possible. Different parts or systems can share parts that have been designed for multi-use. These parts can have the same or different functions when used in different systems. In order to do this, it is necessary to identify the parts that are suitable for multi-use. For example, all the parts (purchased or made) can be sorted into two groups: the first containing all the parts that are used commonly in all products. Then, part families are created by defining categories of similar parts in each group. The goal is to minimize the number of categories, the variations within the categories, and the number of design features within each variation. The result is a set of standard part families from which multi-use parts are created. After organizing all the parts into part families, the manufacturing processes are standardized for each part family. The production of a specific part belonging to a given part family would follow the manufacturing routing that has been setup for its family, skipping the operations that are not required for it. Furthermore, in design changes to existing products and especially in new product designs, the standard multi-use components should be used.
Avoiding adjustments is still the best way to design a part or system. When adjustments are required minimize the risks by making the adjustment results unambiguous.
Design for non-symmetrical mounting
When orientation is important use non Symmetrical parts with an odd number of connection points or make use of part locators which ensures the correct rotation. When parts are non critical on rotation design for symmetry.
Minimize Assembly Directions
Minimize assembly directions. All parts should be assembled from one direction. If possible, the best way to add parts is from above, in a vertical direction, parallel to the gravitational direction (downward). In this way, the effects of gravity help the assembly process, contrary to having to compensate for its effect when other directions are chosen.
Use Gravity when Mounting Parts
When mounting parts into an assembly use gravity by the connections (glue, screw,..)
Use no Hand tools
When using tools to connect parts in an assembly the connection risk can be minimized when tools are use with reproducible results such as an torque controlled screw driver instead of a hand tool. The mounting risk can be further minimized when this torque controlled screwdriver is use in a lean set up were the mounting actions are controlled with a limit on the number of handlings per workstep.
Handling consists of positioning, orienting, and fixing a part. To facilitate orientation, symmetrical parts should be used when ever possible. If it is not possible, then the asymmetry must be exaggerated to avoid failures. Use external guiding features to help the orientation of a part. The subsequent operations should be designed so that the orientation of the part is maintained. Also, magazines, tube feeders, part strips, and so on, should be used to keep this orientation between operations. Avoid using flexible parts - use slave circuit boards instead. If cables have to be used, then include a dummy connector to plug the cable (robotic assembly) so that it can be located easily. When designing the product, try to minimize the flow of material waste, parts, and so on, in the manufacturing operation; also, take packaging into account, select appropriate and safe packaging for the product.
Design for Manufacturing (DfM)
Design for Manufacturing (DfM) is the design methodology for the ease of manufacturing of the (Mono) parts that will form after assembly the part or system. DfM focusses only on the manufacturing of the mono parts which must lead to an optimum solution with feedback from the Design for Assembly (DfA).
Industralzation DfM Rules:
Minimize Manufacturing Steps
Minimize the number of manufacturing or processing steps during part manufacturing. This is in conflict with the Design for Assembly (DfA) requirement which wants to minimize the number of parts. This optimazation is part of the feedback loop between the DfA versus the DfM reviews. This must be weighted between the Functionality, Zero Hour Defect Rate, Total Cost, Cycle Time and reliability.
Design for Ease of Manufacturing
Design for ease of manufacturing. Select the optimum combination between the material and assembly process to minimize the overall manufacturing cost. In general, final operations such as painting, polishing, finish machining, etc. should be avoided. Excessive tolerance, surface-finish requirement, and so on are commonly found problems that result in higher than necessary production cost.
Tien-Chien chang, Richard A. Wysk, and Hsu-Pin Wang. Computer-Aided Manufacturing, Second Edition, Pages 596 to 598. Prentice Hall 1998
Litirature on Quality
George, Michael l., ''What is Lean Six Sigma'', 2003, McGraw Hill, ISBN-10: 007142668X
George, Michael l., ''The Lean Six Sigma Pocket Toolbook'', 2004, McGraw Hill, ISBN-10: 0071441190
Morgan, John, ''Lean Six Sigma For Dummies'', 2012, Wiley Publishing, Inc., ISBN-10: 1119953707
Gygi, Craig, ''Six Sigma for Dummies'', 2005, Wiley Publishing, Inc., ISBN: 0-7645-6798-5
Webber, Larry, ''Quality Control for Dummies'', 2012, Wiley Publishing, Inc., ISBN-10: 0470069090
Kemp, Sid, ''Quality Management Dymistifeid'', 2006, McGraw Hill Education, ISBN: 0-07-144908-6