Design for Assembly (DFA) manual handling classification systematically quantifies how features affect the manual handling of parts during assembly.
What is the Design for assembly classification?
So what is Design for Assembly? It can be defined as a method of designing items with the simplicity of assembly in mind.
Design for assembly should be considered at all 4 stages of the product design process, especially during the embodiment stage of the design.
It’s difficult to analyze and compare design solutions systematically without a methodology to quantify an engineering product design. To overcome this issue Boothroyd and Dewhurst came up with a system to quantify a design using simple manual assembly techniques.
Boothroyd and Dewhurst’s classification system, systematically quantify how features affect the manual handling of the part during the assembly. They divided the classification and assembly method into two main categories. Manual handling and manual insertion and graded them according to how difficult it is to handle and insert by allocating time for each activity.
Full classification and definition can be found in their book (“Product design for manufacture and assembly” (Boothroyd Dewhurst Inc 1999)
Benefits of a DFA classification system
- It quantifies the process
- Quick results
- Simple and easy to use
- It eliminates subjective judgment during the design review
- Ideas, reasoning, and decision during this review can be documented
DFA Manual handling times
One hand without tools
| One hand | Parts are easy to grasp & manipulate | Parts present handling difficulties | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Parts can be grasped and manipulated by one hand without the aid of grasping tools | Thickness > 2 mm | Thickness ≤ 2 mm | Thickness > 2 mm | Thickness ≤ 2 mm | |||||||
| Size > 15 mm | 6 mm ≤ Size > 15 mm | Size < 6 mm | Size > 6 mm | Size ≤ 6 mm | Size > 15 mm | 6 mm ≤ Size > 15 mm | Size < 6 mm | Size > 6 mm | Size ≤ 6 mm | ||
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| (α + β) < 360 ° | 0 | 1.13 | 1.43 | 1.88 | 1.69 | 2.18 | 1.84 | 2.17 | 2.65 | 2.45 | 2.98 |
| 360° ≤ (α + β) < 540 ° | 1 | 1.5 | 1.8 | 2.25 | 2.06 | 2.55 | 2.25 | 2.57 | 3.06 | 3 | 3.38 |
| 540° ≤ (α + β) < 720 ° | 2 | 1.8 | 2.1 | 2.55 | 2.36 | 2.85 | 2.57 | 2.9 | 3.38 | 3.18 | 3.7 |
| (α + β) = 720 ° | 3 | 1.95 | 2.25 | 2.7 | 2.51 | 3 | 2.73 | 3.06 | 3.55 | 3.34 | 4 |
Table 1. DFA Manual handling times – one hand without tools (Boothroyd, Dewhurst, & Knight, 2011)
One hand with tools
| One hand with grasping aids | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Parts can be grasped and manipulated by one handle but only with the use of grasping tools | Parts need tweezers for grasping and manipulation | Parts need standard tools other than tweezers | Parts need special tools for grasping and manipulation | |||||||||
| Parts can be manipulated without optical magnification | Parts require optical magnification for manipulation | |||||||||||
| Parts are easy to grasp and manipulate | Parts present handling difficulties (a) | Parts are easy to grasp and manipulate | Parts present handling difficulties (a) | |||||||||
| Thickness > 0.25 mm | Thickness ≤ 0.25 mm | Thickness > 0.25 mm | Thickness ≤ 0.25 mm | Thickness > 0.25 mm | Thickness ≤ 0.25 mm | Thickness > 0.25 mm | Thickness ≤ 0.25 mm | |||||
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | |||
| α≤180° | 0≤β≤180° | 4 | 3.6 | 6.85 | 4.35 | 7.6 | 5.6 | 8.35 | 6.35 | 8.6 | 7 | 7 |
| β=360° | 5 | 4 | 7.25 | 4.75 | 8 | 6 | 8.75 | 6.75 | 9 | 8 | 8 | |
| α=360° | α≤β≤80° | 6 | 4.8 | 8.05 | 5.55 | 8.8 | 6.8 | 9.55 | 7.55 | 9.8 | 8 | 9 |
| β=360° | 7 | 5.1 | 8.35 | 5.85 | 9.1 | 7.1 | 9.55 | 7.85 | 10.1 | 9 | 10 | |
Table 2. DFA Manual handling times – one hand with tools (Boothroyd, Dewhurst, & Knight, 2011)
With both hands
| Two hands for manipulation | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Parts severely nest or tangle or are flexible but can be grasped and lifted by one hand ( with the use of grasping tools if necessary)(b) | Parts present no additional handling difficulties | Parts present additional handling difficulties (Sticky, delicate, slippery etc) (a) | ||||||||
| α ≤ 180° | α = 360° | α ≤ 180° | α = 360° | |||||||
| Size > 15 mm | 6 mm ≤ Size > 15 mm | Size < 6 mm | Size > 6 mm | Size ≤ 6 mm | Size > 15 mm | 6 mm ≤ Size > 15 mm | Size < 6 mm | Size > 6 mm | Size ≤ 6 mm | |
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | |
| 8 | 4.1 | 4.5 | 5.1 | 5.6 | 6.75 | 5 | 5.25 | 5.85 | 6.35 | 7 |
Table 3. DFA Manual handling times – one hand with tools (Boothroyd, Dewhurst, & Knight, 2011)
Both hands and assistance for large parts
| Two hands or assistance required for large size | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Two hands or persons or mechanical assistance required for grasping and transporting parts | Parts can be handled by one person without mechanical assistance | Parts severely nest or tangle or are flexible (b) | Two persons or mechanical assistance required for parts manipulation | ||||||||
| Parts do not severely nest or tangle and are not flexible | |||||||||||
| Part weight < 10 lb ( ≈ 4.5 kg) | Parts are heavy ( > 10lb (≈ 4.5 kg)) | ||||||||||
| Parts are easy to grasp and manipulate | Parts present handling difficulties (a) | Parts are easy to grasp and manipulate | Parts present handling difficulties (a) | ||||||||
| α ≤ 180° | α = 360° | α ≤ 180° | α = 360° | α ≤ 180° | α = 360° | α ≤ 180° | α = 360° | ||||
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 9 | 2 | 3 | 2 | 3 | 3 | 4 | 4 | 5 | 7 | 9 | |
Table 4. DFA Manual handling times – one hand with tools (Boothroyd, Dewhurst, & Knight, 2011)
References
Boothroyd, G., Dewhurst, P., & Knight, W. A. (2011). Product design for Manufacture and Assembly. London: CRC Press.