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Engineering LibreTexts

1: Design for Manufacturing (DFM)

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Fictional Picture of Robotic Manufacturing Lab
Figure 1.1: Fictional Picture of Robotic Manufacturing Lab

Introduction to Design for Manufacturing

Design for Manufacturing (DFM) is a methodical approach to product design that optimizes the manufacturing process by integrating design and production considerations early in the design cycle. Similar, is Design for Assembly (DFA), and Design for Manufacturing and Assembly (DFMA). The primary goal of DFM/DFA/DFMA is to minimize manufacturing costs, increase product quality, and shorten time-to-market. All while ensuring that the design meets functional requirements. By considering manufacturing constraints during the design phase, mechanical designers can avoid costly redesigns, minimize waste, and enhance the efficiency of production systems.

DFM bridges the gap between the creative process of design and the practical realities of manufacturing. It requires collaboration between mechanical designers, manufacturing engineers, and other stakeholders to align product specifications with available processes, materials, and equipment. This chapter explores the principles, methodologies, and applications of DFM, providing tools and strategies to create manufacturable designs.

Key Principles of DFM

DFM is guided by several core principles that simplify manufacturing and assembly while maintaining product performance. These principles are:

  1. Minimize Part Count: Reducing the number of components in a product reduces the assembly time, material costs, and potential failure points. For example, combining multiple features into a single part can streamline production.
  2. Standardize Components: Using purchased (off-the-shelf) or standardized parts reduces costs and simplifies inventory management. The fewer parts in the design, the fewer parts to keep track of.
  3. Simplify Manufacturing Processes: Designs should leverage processes that are efficient and repeatable, such as minimizing complex machining operations or favoring injection molding over multi-step fabrication for mass produced parts.
  4. Design for Assembly (DFA): A subset of DFM, DFA focuses on simplifying the assembly process by making sure parts are easy to handle, orient, and join. This includes designing parts with self-aligning features or making parts symmetrical as opposed to a right and left-handed part.
  5. Material Selection: Choose materials that are compatible with the manufacturing process and readily available. For example, Hot Rolled Steel is readily available and is a great selection for welding.
  6. Tolerancing and Geometry: Specify tolerances that are achievable with standard manufacturing techniques, avoiding overly tight tolerances that increase costs without significant functional benefit.
  7. Minimize Secondary Operations: Reduce the need for post-processing steps (e.g., painting, grinding, or drilling) by designing features that can be produced in a single operation.

DFM/DFA/DFMA Methodologies

Several methodologies support the implementation of DFM. Two widely adopted approaches are outlined below:

Boothroyd-Dewhurst DFM Method

Developed in the 1980’s by Geoffrey Boothroyd and Peter Dewhurst, this method emphasizes quantitative analysis of design efficiency, particularly in assembly. It involves:

  • Part Count Reduction: Evaluating whether each part is necessary by asking:
  1. Can this part move in relation to another part?
  2. Is the part made of a different material?
  3. Does the part need to be a separate part for assembly or maintenance?

If the answer to all 3 is "no," the part can potentially be eliminated or combined with other parts into one single part.

  • Assembly Time Estimation: Using standardized tables to estimate handling and insertion times based on part geometry and assembly complexity:

There is no universally standardized table for estimating handling and insertion times based solely on part geometry and assembly complexity. Such tables are often proprietary, context-specific, or derived from established methodologies like the Boothroyd Dewhurst Design for Assembly (DFA) method, which provides structured data but requires specific inputs tailored to the product in question. The following are guidelines for assembly time estimation practices.

Overview of Assembly Time Estimation

Estimating handling and insertion times in manufacturing and assembly processes involves analyzing the physical characteristics of parts (e.g., size, weight, symmetry, and geometry) and the complexity of the assembly tasks (e.g., orientation requirements, alignment difficulty, and fastening methods). These estimates are critical for optimizing design efficiency, reducing manufacturing costs, and improving productivity. While this book cannot reproduce a copyrighted table verbatim (e.g., from Boothroyd et al.'s "Product Design for Manufacture and Assembly"), these are a generalized framework based on DFA principles, which are widely accepted in engineering practice. This framework will include illustrative categories and approximate time values.

Generalized Framework for Handling and Insertion Times

Below is a structured table that outlines key factors affecting handling and insertion times, along with indicative time ranges. These values are derived from general DFA methodologies and assume manual assembly conditions. Times are expressed in seconds and represent averages for a skilled operator under standard conditions. Note that actual times may vary based on operator experience, tooling, and specific design details.

Table 1.1: Estimated Handling Times Based on Part Geometry

Handling refers to the process of grasping, transporting, and orienting a part prior to insertion.

Part Geometry/Handling Factor

Description

Estimated Time (seconds)

Small, Simple Parts

Lightweight (<0.5 kg), symmetric, easy to grasp

1.5 - 2.0

Medium, Symmetric Parts

Moderate size (0.5-2 kg), no special gripping needs

2.0 - 3.0

Large or Heavy Parts

Heavy (>2 kg), requires two hands or tools

3.5 - 5.0

Asymmetric Parts

Requires specific orientation, moderate complexity

2.5 - 4.0

Fragile or Flexible Parts

Needs careful handling to avoid damage

3.0 - 5.0

Parts with Handling Aids

Features like grips or tabs simplify grasping

1.5 - 2.5

Note
  • Times increase if parts are tangled, sticky, or require precise orientation.
  • Handling aids (e.g., chamfers, tabs) reduce time by improving “grasp ability”.

Table 1.2: Estimated Insertion Times Based on Assembly Complexity

Insertion involves placing a part into an assembly, aligning it, and securing it (e.g., via fasteners or snap-fits).

Assembly Complexity/Insertion Factor

Description

Estimated Time (seconds)

Simple Insertion, No Resistance

Straight-line motion, self-aligning features

1.0 - 2.0

Moderate Alignment Required

Single-direction insertion, minor adjustments

2.0 - 3.5

Difficult Alignment

Multiple directions or tight tolerances

3.5 - 5.0

Obstructed Access

Limited visibility or restricted motion

4.0 - 6.0

Multiple Fasteners

Requires screws, clips, or additional securing

5.0 - 8.0 (per fastener)

Snap-Fit or Self-Locking Features

No additional fasteners, designed for quick fit

1.5 - 2.5

Note
  • Times increase if parts are tangled, sticky, or require precise orientation.
  • Handling aids (e.g., chamfers, tabs) reduce time by improving “grasp ability”.

Methodology for Application

To utilize this framework effectively:

  1. Classify Part Geometry: Assess each part’s size, weight, symmetry, and handling features. Select the appropriate category from Table 1.1.
  2. Evaluate Assembly Complexity: Determine the insertion task’s difficulty based on alignment, access, and fastening requirements. Choose from Table 1.2.
  3. Calculate Total Time: For each assembly operation, sum the handling time and insertion time. Multiply by the number of identical operations if applicable.
  4. Adjust for Context: Refine estimates based on specific conditions (e.g., automation, operator skill, or part material).
Example Calculation

Consider assembling a small, symmetric plastic cover (0.3 kg) with a snap-fit mechanism:

  • Handling: Small, simple part → 1.5 seconds.
  • Insertion: Snap-fit, self-locking → 1.5 seconds.
  • Total Time per Operation: 1.5 + 1.5 = 3.0 seconds.

For 10 identical covers: 3.0 × 10 = 30 seconds.

Limitations and Recommendations

This generalized table provides a starting point. It is not a specific DFA analysis. For more accurate estimates:

  • Consult DFA Literature: Refer to sources like Boothroyd, Dewhurst, and Knight’s "Product Design for Manufacture and Assembly" for detailed charts and codes specific to your parts.
  • Use Software Tools: Tools like Boothroyd Dewhurst DFA software can generate precise time estimates based on CAD models and assembly sequences.
  • Conduct Empirical Testing: Validate estimates with physical prototypes or time studies in your manufacturing environment.

DFM Guidelines for Specific Manufacturing Processes

To maximize manufacturability, designers must tailor their approach to the constraints and capabilities of the chosen manufacturing process. In Chapter 2 we will explore detailed DFM guidelines for six common processes: injection molding, casting, sheet metal fabrication, welding, EDM Machining and CNC machining. These guidelines help mechanical designers create designs that are efficient, cost-effective, and feasible to produce.

DFM Process Workflow

The DFM process typically follows these steps:

  1. Conceptual Design: Develop initial ideas while considering basic manufacturing feasibility.
  2. Process Selection: Identify suitable manufacturing methods (e.g., casting, machining, additive manufacturing) based on volume, material, and complexity.
  3. Detailed Design: Refine the design with specific manufacturing constraints in mind, such as tool access or mold parting lines.
  4. Prototyping and Validation: Build prototypes to test manufacturability and identify issues.
  5. Feedback and Iteration: Incorporate feedback from manufacturing teams to optimize the design.
  6. Production Release: Finalize the design for full-scale manufacturing.

Collaboration between design, manufacturing, and assembly teams is critical at each stage to ensure alignment.

Case Study: DFM Optimization of a Gearbox Housing for an Industrial Machine in Green Bay, WI
AI generated image of a cutaway gearbox showing internal gears
Figure 1.2: AI generated image of a cutaway gearbox showing internal gears
Background

A hypothetical mid-sized manufacturing firm, Green Bay Industrial Solutions (GBIS), located in Green Bay, Wisconsin, specializes in producing custom components for industrial machinery used in sectors such as paper processing, packaging, and material handling—industries prominent in Northeast Wisconsin. GBIS received a contract from a regional OEM (Original Equipment Manufacturer) to design and manufacture a gearbox housing for a heavy-duty conveyor system. The gearbox housing, a critical component, needed to encase a high-torque gear assembly, withstand operational stresses, and be produced cost-effectively in batches of 500 units annually.

The initial design, provided by the OEM’s engineering team, was functional but not optimized for manufacturability. GBIS applied DFM principles to refine the design, leveraging local machining capabilities and expertise to reduce production costs and improve assembly efficiency.

Objective

The primary goals of the DFM process were:

  1. Minimize manufacturing costs by simplifying fabrication and reducing material waste.
  2. Enhance assembly efficiency by optimizing part geometry for handling and insertion.
  3. Ensure structural integrity under operational loads (e.g., 500 Nm torque, 50°C operating temperature).
  4. Meet the OEM’s delivery timeline of 12 weeks from design approval to first batch shipment.
Initial Design Challenges

The OEM’s original gearbox housing design presented several manufacturability issues:

  • Complex Geometry: The housing featured multiple undercuts and thin-walled sections (2 mm thick in some areas), requiring intricate machining and increasing tool wear.
  • Material Selection: Specified as cast iron (ASTM A48 Class 30), the material was durable but costly and time-consuming to cast and machine.
  • Assembly Complexity: The design included 12 separate mounting holes with tight tolerances (±0.01 mm), necessitating precise alignment during assembly and increasing insertion time.
  • Weight: At 15 kg, the part was heavy, complicating handling and raising shipping costs.
DFM Analysis and Redesign Process

GBIS employed a cross-functional team of design engineers, machinists, and quality specialists to conduct a DFM analysis. The process utilized local resources, including CNC machining capabilities at BM Corporation and collaboration with TM Manufacturing. Steps included:

  1. Geometry Simplification:
    • Undercuts were eliminated by redesigning the housing with straight walls and draft angles (2°), enabling easier mold release if cast or simpler milling if machined.
    • Thin-walled sections were thickened to 4 mm where feasible, balancing weight and manufacturability while maintaining strength (verified via finite element analysis).
  2. Material Optimization:
    • After evaluating alternatives, the team proposed switching to ductile iron (ASTM A536 Grade 65-45-12), which offered comparable strength, better machinability, and reduced casting defects compared to gray cast iron.
    • A cost-benefit analysis showed a 15% reduction in material and processing costs.
  3. Feature Reduction:
    • The number of mounting holes was reduced from 12 to 8 by integrating mounting points into a flange design, decreasing drilling time and simplifying alignment during assembly.
    • Chamfers and fillets (R3 mm) were added to edges, reducing stress concentrations and easing tool access.
  4. Tooling and Process Alignment:
    • The redesign aligned with GBIS’s in-house CNC milling and turning capabilities, avoiding the need for specialized tooling or outsourcing.
    • A single setup was devised for machining the housing’s critical features, reducing setup time by 30%.
  5. Assembly Considerations:
    • Self-aligning features, such as tapered locating pins, were incorporated to reduce insertion time from 6 seconds to 2 seconds per operation.
    • The weight was reduced to 12 kg by optimizing wall thickness and removing non-critical material, easing manual handling.
Manufacturing Process
  • Fabrication: The housing was cast using a local foundry in Green Bay, followed by CNC machining at GBIS’s facility. The simplified geometry allowed for a reusable sand mold, lowering per-unit casting costs.
  • Machining: A 4-axis CNC mill was used to finish the housing’s mating surfaces and mounting flange, achieving tolerances of ±0.05 mm—less stringent than the original ±0.01 mm but sufficient for the application.
  • Quality Control: Coordinate Measuring Machine (CMM) inspection ensured dimensional accuracy, with a focus on critical mating surfaces.
Results

The DFM-optimized gearbox housing yielded significant improvements:

  • Cost Reduction: Manufacturing cost per unit decreased by 20% (from $150 to $120), driven by material savings, reduced machining time, and fewer process steps.
  • Time Savings: Production lead time for the first batch was reduced from 14 weeks to 10 weeks, meeting the OEM’s deadline.
  • Assembly Efficiency: Handling and insertion time dropped from 15 seconds to 8 seconds per unit, a 47% improvement, due to lighter weight and self-aligning features.
  • Performance: The redesigned housing passed all stress tests (500 Nm torque, 1,000-hour durability cycle), confirming no compromise in functionality.
Conclusion

This case study exemplifies how DFM principles can transform an industrial part’s design for efficient manufacturing. By leveraging local machining expertise and focusing on simplicity, material efficiency, and assembly optimization, GBIS delivered a cost-effective, high-quality gearbox housing that strengthened its partnership with the OEM.

DFM Tools and Software

Modern engineering relies on software to implement DFM effectively:

  • CAD Software: Tools like SolidWorks, Inventor or Autodesk Fusion 360 include DFM modules to analyze part geometry for manufacturability (e.g., draft analysis, mold flow simulation).
  • Simulation Tools: Finite Element Analysis (FEA) and process simulation (e.g., Moldflow) predict how designs behave during manufacturing.

Benefits and Challenges of DFM

The benefits of DFM include:

  • Cost Reduction: Lower material, labor, and tooling costs.
  • Improved Quality: Fewer defects due to simplified designs and processes.
  • Faster Time-to-Market: Reduced iterations and streamlined production.

Challenges include:

  • Upfront Effort: DFM requires more time in the design phase, which can conflict with tight project schedules.
  • Knowledge Gap: Designers may lack detailed manufacturing expertise, necessitating close collaboration with production teams.
  • Trade-Offs: Optimizing for manufacturing may compromise aesthetics or performance, requiring careful balance.

Practical Guidelines for DFM

To apply DFM effectively, consider the following:

  1. Engage Manufacturing Early: Consult production engineers during concept development.
  2. Leverage Prototyping: Use 3D printing or soft tooling to test designs affordably.
  3. Document Decisions: Maintain a DFM log to track trade-offs and rationale.
  4. Iterate Based on Feedback: Treat manufacturing as a partner, not an afterthought.

Conclusion

Design for Manufacturing is a critical discipline that ensures products are not only functional but also economically and efficiently produced. By embedding manufacturing considerations into the design process, mechanical designers can create robust, cost-effective solutions that meet market demands. As manufacturing technologies evolve—such as the rise of additive manufacturing, automation, and AI—DFM principles will continue to adapt, offering new opportunities for you to innovate.

Exercises
  1. Analyze a simple product (e.g., monitor arm in classroom) and propose three DFM improvements.
  2. Using the Boothroyd-Dewhurst method, evaluate a multi-part assembly (e.g., caster wheel and bracket assembly) and suggest ways to reduce part count.

Further Reading

Boothroyd, G., Dewhurst, P., & Knight, W. (2010). Product Design for Manufacture and Assembly.


This page titled 1: Design for Manufacturing (DFM) is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Bryan Guns (Northeast Wisconsin Technical College).

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