Design for manufacturing (DFM) in Composites
DFM (design for manufacturing) can be defined as a practice for designing products, keeping manufacturing in mind. DFM starts by taking a plain sheet of paper and identifying a product’s functional, performance, and other requirements. It utilizes rules of thumb, best practices, and heuristics to design the part. Best practices for a highquality product design are to minimize the number of parts, create multifunctionality in the part, minimize part variations, and create ease of handling. DFM involves meeting the end use requirements with the lowest-cost design, material, and process combinations.
In the past, several product problems arose because of poor design. The designers were not aware of the various manufacturing techniques available on the market, nor the capabilities of each manufacturing technique. As a result, products were heavy, had many parts and thus many assembly operations, and resulted in poor quality and increased cost. To effectively design the product, manufacturing knowledge needs to be incorporated into product design. The designer should know how the process and design interact. In general, the real challenge in designing composite products is to develop a good understanding not only of engineering design techniques, but also of processing and material information.
The purpose of DFM is to:
• Narrow design choices to optimum design (Figure)
• Perform concept generation, concept selection, and concept improvement
• Minimize product development cycle time and cost
• Achieve high product quality and reliability
• Simplify production methods
• Increase the competitiveness of the company
• Have a quick and smooth transition from the design phase to the production phase
• Minimize the number of parts and assembly time
• Eliminate, simplify, and standardize whenever possible
DFM Implementation Guidelines
The main objective of DFM is to minimize the manufacturing information content in the product without sacrificing functional and performance requirements. DFM can also be applied for a product that is already in production or on the market. The main objective here will be to make the product more cost competitive. The following DFM guidelines are applicable to products made of composites, metals, and plastics.
Minimize Part Counts
There is good potential for part integration by questioning the need for separate parts. At General Motors, Ford, Chrysler, GE, IBM, and other companies, DFM strategies have reduced the total number of part counts by 30 to 60% in many product lines. Composite materials offer good potential for part integration. Minimization of part counts can result in huge savings by eliminating the need for assembly, inventory control, storage, inspection, transportation, and servicing. According to Huthwaite,3 “the ideal product has a part count of one.” In general, more than one part is needed if there is a relative motion requirement, a different materials requirement, a different manufacturing requirement, or an adjustment requirement. An example of part integration is the steel identification badge clip that has four different parts but can be replaced by a single injection molded plastic part. Another example is the monocoque composite bicycle frame. Do not perform part intergration if design becomes overly complex, heavy, or difficult to manufacture.
A typical automobile, airplane, or luxury yacht consists of thousands of parts to meet various functional or performance needs. For example, a Heloval 43-meter luxury yacht from CMN Shipyards is comprised of about 9000 metallic parts for hull and superstructure and over 5000 different types of parts for outfitting.
To determine if a part is a potential candidate for elimination, the following questions should be asked:
1. Do the parts move relative to each other?
2. Is there any need to make parts using a different material?
3. Will the part require removal for servicing or repair?
4. Will there be a need for adjustment?
If the answers to the above questions are “no,” then the part is a potential candidate for replacement. The following guidelines can be used to minimize the number of parts.
• Question and justify the need for a separate part. Ask the four
questions above; and if the answer is “no,” then redesign the prod-
uct by eliminating the separate part.
• Create multifunctionality features in the part.
• Eliminate any product feature that does not add any value for the
customer.
• Use a modular design.
Eliminate Threaded Fasteners
Avoid the use of screws, nuts, bolts, and other fasteners in the product. It is estimated that driving a screw into the product costs almost 6 to 10 times the cost of a screw. The use of fasteners increases inventory costs and add complexity in assembly. Fasteners are used to compensate for dimensional variation, to join two components, or for part disassembly. The use of fasteners creates the potential for a part to become loose during service. IBM has used this philosophy to redesign its printer, eliminating many screws and replacing them with snap fit assembly. The resulting design had 60% less parts and 70% reduced assembly time.
Snap fits are used with plastics or short fiber composite parts and provide ease of assembly due to the lack of any installation tool requirement. General concerns regarding the use of snap-fits include strength, size, servicing, clamp load, etc.
Minimize Variations
Part dimensional variation as well as property variation are the major sources of product defects and nonconformities. Try to use standard parts off the shelf and avoid the use of special parts. Eliminate part variations such as types of bushings or O-rings, seals, screws, or nuts used in one application. The same size would mean the same tool for assembly and disassembly. This guideline aims to reduce part categories and the number of variations in each category, thus providing better inventory control and part interchangeability.
Easy Serviceability and Maintainability
Design the product such that it is easy to access for assembly and disassembly. The part should be visible for inspection and have sufficient clearance between adjacent members for scheduled maintenance using wrench, spanner, etc.
Minimize Assembly Directions
For product assembly, minimize assembly direction. While designing the product, think about the assembly operations needed for various part attachments. It is preferable to use one direction; z-direction assembly operation allows gravity to aid in assembly. A one-direction assembly operation minimizes part movement as well as the need for a separate assembly station. It is better in terms of an ergonomics point of view as well.
Provide Easy Insertion and Alignment
When there are more than two parts in a product, the mating parts need to be brought close by performing insertion or alignment. Some guidelines for easy insertion and alignment are:
• Provide generous tapers, chamfers, and radii for easy insertion and assembly.
• Provide self-locating and self-aligning features where possible.
• Avoid hindrance and obstruction for accessing mating parts.
• Avoid excessive force for part alignment.
• Design parts to maintain location.
• Avoid restricted vision for part insertion or alignment.
Consider Ease for Handling
In an assembly plant, various parts are kept in separate boxes near the assembly station. Workers pick up those parts and assemble them using adhesive bonding or mechanical fastening or by slip fit or interference fit. Avoid using parts such as springs, clips, etc., which are easy to nest and become interlocked. It disrupts the assembly operation and creates irritation for the worker. For smooth assembly operation and ease of handling, parts should not be heavy and should not have many curves, thus reducing the potential for entanglement. To avoid physical fatigue of the worker, part and assembly locations should be easy to access. Parts should be symmetric to minimize handling and aid in orienting. Add features that help guide the part to its desired location. The following suggestions can improve part handling. These suggestions are more applicable for a high volume production environment.
• Minimize handling of parts that are sticky, slippery, fragile, or have sharp corners or edges.
• Keep parts within operator reach.
• Avoid situations in which the operator must bend, lift, or walk to get the part.
• Minimize operator movements to get the part. Avoid the need for two hands or additional help to get the part.
• Avoid using parts that are easy to nest or entangle.
• Use gravity as an aid for part handling.
Design for Multifunctionality
Once an overall idea of the product’s functions is gleaned, one can design individual components such that they provide maximum functionality. It is preferable to use molding operations that provide net shape or near netshape parts. For example, an injection molded composite housing part meets the structural requirement of the product and has built in features for alignment, self locating, mounting, and a bushing mechanism. This technique helps minimize the number of parts.
Design for Ease of Fabrication
In composite part fabrication, product design cannot be made effective without knowledge of the manufacturing operations. Each manufacturing process has its strengths and weaknesses. The product design should be tailored to reap the benefits of the selected manufacturing process. For example, if close tolerances are required on the inside diameter of a tube, then filament winding is preferred compared to a pultrusion process. The design should be simplified as much as possible because it helps in manufacturing and assembly and thus in cost savings. Workers and others who are dealing with the products can easily understand simplified design.
Prefer Modular Design
A module is a self contained component that is built separately and has a standard interface for connection with other product components. For example, a product that has 100 parts can be designed to have four or five modules.
Each module can be independently designed and improved without affecting the design of the other modules. Modular design is preferred because it helps in the final assembly, as well as in servicing where a defective module can be easily replaced by a new module. Modular design can be found in aerospace, automotive, computer, and other products. For example, steering systems, bumper beams, and chassis systems are separate modules designed, produced, and improved upon by independent organizations and assembled in the vehicle. In each of these modules, there are many other modules, which are again designed by various groups of the organization.
DFM (design for manufacturing) can be defined as a practice for designing products, keeping manufacturing in mind. DFM starts by taking a plain sheet of paper and identifying a product’s functional, performance, and other requirements. It utilizes rules of thumb, best practices, and heuristics to design the part. Best practices for a highquality product design are to minimize the number of parts, create multifunctionality in the part, minimize part variations, and create ease of handling. DFM involves meeting the end use requirements with the lowest-cost design, material, and process combinations.
In the past, several product problems arose because of poor design. The designers were not aware of the various manufacturing techniques available on the market, nor the capabilities of each manufacturing technique. As a result, products were heavy, had many parts and thus many assembly operations, and resulted in poor quality and increased cost. To effectively design the product, manufacturing knowledge needs to be incorporated into product design. The designer should know how the process and design interact. In general, the real challenge in designing composite products is to develop a good understanding not only of engineering design techniques, but also of processing and material information.
The purpose of DFM is to:
• Narrow design choices to optimum design (Figure)
• Perform concept generation, concept selection, and concept improvement
• Minimize product development cycle time and cost
• Achieve high product quality and reliability
• Simplify production methods
• Increase the competitiveness of the company
• Have a quick and smooth transition from the design phase to the production phase
• Minimize the number of parts and assembly time
• Eliminate, simplify, and standardize whenever possible
DFM Implementation Guidelines
The main objective of DFM is to minimize the manufacturing information content in the product without sacrificing functional and performance requirements. DFM can also be applied for a product that is already in production or on the market. The main objective here will be to make the product more cost competitive. The following DFM guidelines are applicable to products made of composites, metals, and plastics.
Minimize Part Counts
There is good potential for part integration by questioning the need for separate parts. At General Motors, Ford, Chrysler, GE, IBM, and other companies, DFM strategies have reduced the total number of part counts by 30 to 60% in many product lines. Composite materials offer good potential for part integration. Minimization of part counts can result in huge savings by eliminating the need for assembly, inventory control, storage, inspection, transportation, and servicing. According to Huthwaite,3 “the ideal product has a part count of one.” In general, more than one part is needed if there is a relative motion requirement, a different materials requirement, a different manufacturing requirement, or an adjustment requirement. An example of part integration is the steel identification badge clip that has four different parts but can be replaced by a single injection molded plastic part. Another example is the monocoque composite bicycle frame. Do not perform part intergration if design becomes overly complex, heavy, or difficult to manufacture.
A typical automobile, airplane, or luxury yacht consists of thousands of parts to meet various functional or performance needs. For example, a Heloval 43-meter luxury yacht from CMN Shipyards is comprised of about 9000 metallic parts for hull and superstructure and over 5000 different types of parts for outfitting.
To determine if a part is a potential candidate for elimination, the following questions should be asked:
1. Do the parts move relative to each other?
2. Is there any need to make parts using a different material?
3. Will the part require removal for servicing or repair?
4. Will there be a need for adjustment?
If the answers to the above questions are “no,” then the part is a potential candidate for replacement. The following guidelines can be used to minimize the number of parts.
• Question and justify the need for a separate part. Ask the four
questions above; and if the answer is “no,” then redesign the prod-
uct by eliminating the separate part.
• Create multifunctionality features in the part.
• Eliminate any product feature that does not add any value for the
customer.
• Use a modular design.
Eliminate Threaded Fasteners
Avoid the use of screws, nuts, bolts, and other fasteners in the product. It is estimated that driving a screw into the product costs almost 6 to 10 times the cost of a screw. The use of fasteners increases inventory costs and add complexity in assembly. Fasteners are used to compensate for dimensional variation, to join two components, or for part disassembly. The use of fasteners creates the potential for a part to become loose during service. IBM has used this philosophy to redesign its printer, eliminating many screws and replacing them with snap fit assembly. The resulting design had 60% less parts and 70% reduced assembly time.
Snap fits are used with plastics or short fiber composite parts and provide ease of assembly due to the lack of any installation tool requirement. General concerns regarding the use of snap-fits include strength, size, servicing, clamp load, etc.
Minimize Variations
Part dimensional variation as well as property variation are the major sources of product defects and nonconformities. Try to use standard parts off the shelf and avoid the use of special parts. Eliminate part variations such as types of bushings or O-rings, seals, screws, or nuts used in one application. The same size would mean the same tool for assembly and disassembly. This guideline aims to reduce part categories and the number of variations in each category, thus providing better inventory control and part interchangeability.
Easy Serviceability and Maintainability
Design the product such that it is easy to access for assembly and disassembly. The part should be visible for inspection and have sufficient clearance between adjacent members for scheduled maintenance using wrench, spanner, etc.
Minimize Assembly Directions
For product assembly, minimize assembly direction. While designing the product, think about the assembly operations needed for various part attachments. It is preferable to use one direction; z-direction assembly operation allows gravity to aid in assembly. A one-direction assembly operation minimizes part movement as well as the need for a separate assembly station. It is better in terms of an ergonomics point of view as well.
Provide Easy Insertion and Alignment
When there are more than two parts in a product, the mating parts need to be brought close by performing insertion or alignment. Some guidelines for easy insertion and alignment are:
• Provide generous tapers, chamfers, and radii for easy insertion and assembly.
• Provide self-locating and self-aligning features where possible.
• Avoid hindrance and obstruction for accessing mating parts.
• Avoid excessive force for part alignment.
• Design parts to maintain location.
• Avoid restricted vision for part insertion or alignment.
Consider Ease for Handling
In an assembly plant, various parts are kept in separate boxes near the assembly station. Workers pick up those parts and assemble them using adhesive bonding or mechanical fastening or by slip fit or interference fit. Avoid using parts such as springs, clips, etc., which are easy to nest and become interlocked. It disrupts the assembly operation and creates irritation for the worker. For smooth assembly operation and ease of handling, parts should not be heavy and should not have many curves, thus reducing the potential for entanglement. To avoid physical fatigue of the worker, part and assembly locations should be easy to access. Parts should be symmetric to minimize handling and aid in orienting. Add features that help guide the part to its desired location. The following suggestions can improve part handling. These suggestions are more applicable for a high volume production environment.
• Minimize handling of parts that are sticky, slippery, fragile, or have sharp corners or edges.
• Keep parts within operator reach.
• Avoid situations in which the operator must bend, lift, or walk to get the part.
• Minimize operator movements to get the part. Avoid the need for two hands or additional help to get the part.
• Avoid using parts that are easy to nest or entangle.
• Use gravity as an aid for part handling.
Design for Multifunctionality
Once an overall idea of the product’s functions is gleaned, one can design individual components such that they provide maximum functionality. It is preferable to use molding operations that provide net shape or near netshape parts. For example, an injection molded composite housing part meets the structural requirement of the product and has built in features for alignment, self locating, mounting, and a bushing mechanism. This technique helps minimize the number of parts.
Design for Ease of Fabrication
In composite part fabrication, product design cannot be made effective without knowledge of the manufacturing operations. Each manufacturing process has its strengths and weaknesses. The product design should be tailored to reap the benefits of the selected manufacturing process. For example, if close tolerances are required on the inside diameter of a tube, then filament winding is preferred compared to a pultrusion process. The design should be simplified as much as possible because it helps in manufacturing and assembly and thus in cost savings. Workers and others who are dealing with the products can easily understand simplified design.
Prefer Modular Design
A module is a self contained component that is built separately and has a standard interface for connection with other product components. For example, a product that has 100 parts can be designed to have four or five modules.
Each module can be independently designed and improved without affecting the design of the other modules. Modular design is preferred because it helps in the final assembly, as well as in servicing where a defective module can be easily replaced by a new module. Modular design can be found in aerospace, automotive, computer, and other products. For example, steering systems, bumper beams, and chassis systems are separate modules designed, produced, and improved upon by independent organizations and assembled in the vehicle. In each of these modules, there are many other modules, which are again designed by various groups of the organization.
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