Design for Assembly classification system

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What is Design for assembly classification?

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 judgement 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 & manipulateParts present handling difficulties
Parts can be grasped and manipulated by one hand without the aid of grasping tools Thickness > 2 mmThickness ≤ 2 mmThickness > 2 mmThickness ≤ 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
0123456789
(α + β) < 360 °01.131.431.881.692.181.842.172.652.452.98
360° ≤ (α + β) < 540 °11.51.82.252.062.552.252.573.0633.38
540° ≤ (α + β) < 720 °21.82.12.552.362.852.572.93.383.183.7
(α + β) = 720 °31.952.252.72.5132.733.063.553.344

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 toolsParts need tweezers for grasping and manipulationParts need standard tools other than tweezersParts need special tools for grasping and manipulation
Parts can be manipulated without optical magnificationParts require optical magnification for manipulation
Parts are easy to grasp and manipulateParts present handling difficulties (a)Parts are easy to grasp and manipulateParts present handling difficulties (a)
Thickness > 0.25 mmThickness ≤ 0.25 mmThickness > 0.25 mmThickness ≤ 0.25 mmThickness > 0.25 mmThickness ≤ 0.25 mmThickness > 0.25 mmThickness ≤ 0.25 mm
0123456789
α≤180°0≤β≤180°43.66.854.357.65.68.356.358.677
β=360°547.254.75868.756.75988
α=360°α≤β≤80°64.88.055.558.86.89.557.559.889
β=360°75.18.355.859.17.19.557.8510.1910

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 difficultiesParts 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
0123456789
84.14.55.15.66.7555.255.856.357

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 partsParts can be handled by one person without mechanical assistanceParts 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 manipulateParts present handling difficulties (a)Parts are easy to grasp and manipulateParts present handling difficulties (a)
α ≤ 180° α = 360° α ≤ 180° α = 360° α ≤ 180° α = 360° α ≤ 180° α = 360°
0123456789
92323344579

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.

 

 

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