Introduction to Design for (Cost Effective) Assembly and Manufacturing

DFA and DFM definition

DFA is the method of design of the product for ease of assembly. DFM is the method of design for ease of manufacturing of the collection of parts that will form the product after assembly.

Concerns

DFA is concerned only with reducing product assembly cost. DFM is concerned with reducing overall part production cost.

Similarities between DFA and DFM

• reduce material, overhead, and labor cost
• shorten the product development cycle time
• utilize standards to reduce cost

Analysis steps:

1. Concept design
2. Design for assembly -> Optimize design for part count and assembly
3. Design for manufacturing -> Optimize design for production readiness
4. Detailed design

Design for Assembly Principles

• Minimize part count
• Design parts with self-locating features
• Design parts with self-fastening features
• Minimize reorientation of parts during assembly
• Design parts for retrieval, handling, & insertion
• Emphasize ‘Top-Down’ assemblies
• Standardize parts (minimum use of fasteners)
• Encourage modular design
• Design for a base part to locate other components
• Design for component symmetry for insertion

Count Parts & Interfaces

• List number of parts (Np)
• List number of interfaces (Ni)

Part standardization:

• Within the assembly station
• Within the full assembly
• Within the assembly plant
• Within the corporation
• Within the industry

DFA Complexity Factor

Square root of summation of number of parts and number of part-to-part interfaces

Fasteners

A study by Ford Motor Co. revealed that threaded fasteners were the most common cause of warranty repairs. This finding is echoed in more recent survey of automotive mechanics, in which 80% reported finding loose or incorrect fasteners in cars they serviced.

Fastener cost from least to most expensive:

1. snap fit
2. plastic bending
3. riveting
4. screwing

Handling Time: based on assembly process and complexity of parts

• How many hands are required?
• Is any grasping assistance needed?
• What is the effect of part symmetry on assembly?
• Is the part easy to align/position?

Handling Difficulty

• Size
• Thickness
• Sharpness
• Weight
• Fragility
• Flexibility
• Slipperiness
• Stickiness
• Necessity for using 1) two hands, 2) optical magnification, or 3) mechanical assistance

Assembly metrics:

• Error proofing
• Handling Index
• Insertion Index
• 2nd Op. Index

Analyze all metrics:

1. Reduce part count & type - Part count efficiency and DFA Complexity Factor
2. Error Proofing - Error Index
3. Ease of handling - Handling Index
4. Ease of insertion - Insertion Index
5. Eliminate secondary ops - 2nd Op. Index

DFMA Principles

• Minimize Part Count
• Standardize Parts and Materials
• Create Modular Assemblies
• Design for Efficient Joining
• Minimize Reorientation of parts during Assembly and/or Machining
• Simplify and Reduce the number of Manufacturing Operations
• Specify ‘Acceptable’ surface Finishes for functionality