Boothroyd-Dewhurst DFA Method

The Boothroyd-Dewhurst (B-D) DFA method is used to minimise assembly times and costs by reducing the number of individual part components and optimising their design for handling and insertion [36]. As part of its optimisation approach, the B-D DFA method identifies the fundamental operations within manufacturing assembly and quantifies their difficulty using human completion times derived through empirical testing [29]. The results have been consolidated into two tables known as the B-D manual handling table and the B-D manual insertion table. A more in-depth discussion of these tables and their influential factors is given in [29], but an extract from the manual handling and insertion tables is shown in Figures 2.10 and 2.12 for reference. A summary of the factors given in [29] is given below.

Handling Factors

1. Requirement: The handling required to grasp and manipulate the object, which is dependent on the object’s size, weight, and handling difficulty.

2. Part Handling Difficulty: Ease at which the part can be handled. Parts can present handling difficulties if they nest or tangle, stick together, are fragile, slippery, or require caution during handling.

3. Part Symmetry: Total rotational symmetry of a part (α+β). Alpha and beta equal the rotational symmetry of the part about an axis perpendicular and parallel to the axis of insertion (see Figure 2.11).

4. Part Size: Length of the longest side of the minimum bounding prism that encloses the part.

5. Part Thickness: Length of the shortest side of the minimum bounding prism that encloses the part.

6. Part Weight: Weight of the object to be manipulated. Becomes a factor when part is too heavy to be grasped and transported with one hand.

Figure 2.10: Extract from the Boothroyd-Dewhurst manual handling tables, showing a number of factors which influence an operator’s completion time during a one handed handling operation ( c 1999 Boothroyd Dewhurst, Inc.).

Figure 2.11: Alpha and beta rotational symmetries for various parts (taken from [29]).

Insertion Factors

1. Ease of Reach: Ease at which the parts and associated tool (including hands) can reach the desired location. Obstructed access means that the space available for the insertion causes a significant increase in the assembly time. Restricted vision means the operator has to rely mainly on tactile sensing during insertion. 2. Insertion Resistance: Resistance encountered during part insertion. Resistance

can be due to small clearances, jamming or wedging, hang-up conditions, or insertion against a large force.

3. Alignment & Positioning: Parts are easy to align and position if the position of the part is established by locating features on the part or its mating part, and if insertion is aided by well-designed chamfers or similar features.

4. Holding Requirement: Holding down required means that the part is unstable after insertion and will require gripping, realignment or holding down in subsequent operations before it is finally secured.

5. Fastening Processes: How the parts are finally secured, which can be done either immediately after insertion or as a separate operation.

Figure 2.12: Extract from the Boothroyd-Dewhurst manual insertion tables, showing a number of factors which influence an operator’s completion time during the joining of parts ( c 1999 Boothroyd Dewhurst, Inc.).

Within the B-D DFA method, manual assembly efficiency (Ema) is calculated using

the equation [29]:

Ema=Nminta/tma (2.3)

where Nmin is the theoretical minimum number of parts, ta is the ideal assembly

time for one part, and tma is the estimated time of the complete assembly process.

The ideal assembly time refers to the minimum time necessary to assemble a part with no handling, insertion or fastening difficulties, and from Figures 2.10 and 2.12 corresponds to a combined assembly time of 3 s [29]. The theoretical minimum number of parts is determined based on the requirement that each added part must either move relative to all other parts assembled, be of a different material, or be separate to permit assembly [29], [37].

The manual handling times represent the time required by an operator to grasp, orient, and move a part to its receptacle, while the manual insertion times represent the time required to begin an insertion, pick up the tool (if required), complete the assembly operation, and replace the tool. When tools are required during an operation, the tabulated times assume that the most suitable tool is selected. This

includes the use of power tools where applicable. The time penalties associated with each individual factor are not necessarily additive. For example, if a part needs to be moved during mating, then it can probably be orientated during the move. This is accounted for within the tabulated times.

As noted, the times presented within the manual handling and insertion tables are based on empirical data. This data was collected over a period of years through experimentation [38] and represents the average time taken by a human to perform each classified operation. The number of parts and scenarios that fall into each operation classification can be quite large, which suggests that the tabulated times have a high variance. A study by MIT students has quantified this varience. While the study does not account for task proficiency, it still reports that the tabulated times are accurate to within about 10% [38]. The variance in operation completion time is acknowledged by Boothroyd and Dewhurst [29], but they note that any errors in using average time tend to cancel when analysing a full assembly process due to equal likelihood of overestimates and underestimates.

The B-D manual handling and insertion tables were generated for small-scale assembly which means that the tabulated times assume that all tools and parts are located within arm’s reach of the operator and that no major body motions are required during the assembly process. Accordingly, the use of these tables is only valid when considering bench and multi-station assemblies. However, supplementary tables can be used when considering larger assembly layouts in order to account for acquisition time [29].

While the B-D DFA method is mainly used within manual assembly, it can also be used to analyse high-speed and general-purpose (robotic) automation assemblies. In these scenarios, the B-D method identifies the overall assembly cost by considering the cost of feeding and orienting individual parts and the cost of automatically inserting those parts [39]. The former is achieved by estimating orienting efficiency and relative feeder cost based on each part’s symmetry and defining features, while the latter is achieved by estimating the relative costs, times and penalties that stem from the base robot, gripper / tool selection, and insertion operation. As before, Boothroyd and Dewhurst develop classification tables to assist in the analysis of these automated

assemblies, however the tabulated values are only relative and are therefore of limited use without full knowledge of the assembly equipment and production strategy. In addition, the tables were first published in 1986 [39] and so were developed with traditional fixed and flexible automation in mind. As a consequence, the generated classification tables assume the presence of dedicated assembly lines and axillary equipment such as feed tracks and part placement mechanisms. These assumptions limit the usefulness of the automated classification tables when considering robotic systems within flexible manufacturing.