Boothroyd & Dewhurst: Who Were They and Why Should You Care if You’re in DFMA?

by | Feb 19, 2020

When it comes to manufacturing with assembly in mind (DFA), Geoffrey Boothroyd and Peter Dewhurst are credited (rightfully) with changing the name of the game and writing the textbook (literally, see “Product Design for Manufacturing and Assembly”) on how to efficiently tackle manufacturing. In Dragon Innovation’s DFM course, Scott Miller dives into the operating thesis behind the Boothroyd Dewhurst (BD) method and explains why it is important to take a step back, not rush, and think about assembly from a higher level. I have outlined some of the main points here.

The BD method is the most widespread DFA methodology. The operating thesis is that the product cost is directly related to the assembly time. Assembly time, and quality for that matter, are directly related to the number of parts and the ease of assembly.

By analyzing part count and part geometries, you can calculate theoretical assembly time and compare that to minimum assembly times to determine design efficiency. The BD process is as follows:
  1. Design the product
  2. Create an Assembly Matrix (organized parts list with below part metrics)
    • Handling: size, weight, orientation (symmetry)
    • Insertion: Alignment & Securing Method
  3. Calculate Theoretical Assembly Time (TAT)
  4. Calculate Design Efficiency Index: 3s*Min_Parts / TAT
  5. Iterate to improve the design
To populate your product’s assembly matrix you will need to use the below definitions of symmetry angle, thickness, and size and then reference the B-D Manual Handling and B-D Manual Insertion charts. Here is a link provided by Dragon Innovation for a template Assembly Matrix (along with a filled out template for this assembly as reference).
  • Symmetry is all about alignment and orientation. There are two angles that are noteworthy to assembly, alpha (align) and beta (orient). The alpha angle refers to the max angle the part must be rotated perpendicular to the axis of insertion to repeat its orientation, as beta is the max angle rotated about the axis of insertion to repeat its orientation. For instance, a rectangular part sliding into a rectangular hole would have an alpha of 180 degrees and a beta of 180 degrees; the total angle of symmetry = alpha + beta (the example would be 360 degrees). In general, the greater the angle of symmetry, the more time consuming it will be to assemble and orient the part.

  • Thickness is the length of the shortest side of the bounding box (an imaginary box around your part complete geometry). If your part is a cylinder with D<L, thickness = enclosing radius.

  • Size is the length of the longest side of the bounding box.
With a completed Assembly Matrix you can calculate your full assemblies Theoretical Assembly Time (TAT) by summing the individual part assembly times. This is used to calculate the design efficiency with the above formula (4.).

To summarize, the BD method is immensely useful for identifying independent inefficiencies in your product’s design. It is great practice to perform an analysis such as this so that you can iterate and look for opportunities to increase efficiencies, decrease assembly time, and save money!
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