Design for manufacture & assembly (DFMA) is based on the radical notion that rather than just designing a product to meet what the customer wants, it would be a rather good idea if it could actually be made!

Of course it has always be possible to make products, but over time the design engineers and the manufacturing engineers had become separated which ended up in designs being ‘thrown over the wall’ into manufacturing, without being optimised for the manufacturing process. This resulted in a lot of redesign to get it right, or an inefficient manufacturing process. This situation was brought into focus in the West in the 1980’s, following publications describing Japanese industrial practices that gave them competitive advantage, leading to the onset of simultaneous engineering and also design for manufacture and assembly.

There are actually two components to this; design for manufacture of the individual parts and the design for assembly so that they can be assembled efficiently and particularly in a mass production environment by automated machines.

Various people studied how to come up with rules that would help the designer to do a better job. In the USA the push for DFMA was led by two British engineers, Professor Geoffrey Boothroyd and his partner Dr. Peter Dewhurst. They devised methodologies for assessing both manufacturability and assemblability and founded Boothroyd Dewhurst Inc..

In Europe Lucas came up with something similar, as did Hitachi in Japan. These days Boothroyd Dewhurst probably remain the leaders in this field, but there are others, for instance.Munro & Associates.

In the Boothroyd Dewhurst system on calculates a DFA (Design for Assembly) Index, which is a measure of the efficiency of the design as it relates to the assembly process.

The DFA Index (Ema) is calculated by dividing the theoretical minimum assembly time by the actual assembly time (tma). The theoretical assembly time is given be assessing the theoretical minimum number of parts that are needed (Nmin), and the basic assembly time for 1 part (ta), which is the time to assemble without difficulty (typically about 3 seconds).

Thus the formula for the DFA Index is

Ema = (Nmin ta ) / tma

In all cases we are trying to maximise the value of Ema

For example if and assembly comprises 15 parts, but the theoretical minimum number of parts for that assembly is 10, and if each part takes 5 sec to assemble on average, the actual assembly time is 15 x 4 = 60 sec. If the basic assembly time (what it could be if there were no difficulties in assembly) is 3 sec, then the theoretical minimum assembly time = 10 x 3 = 30 sec.

Thus the DFA Index, Em1 = (10 x 3) / (15 x 4) = 30 / 60 = 0.5

If the design was changed to eliminate some of the parts, say reducing the part count to 12, and the assembly process was made a little bit easier, say reducing the average assembly time per part to 3.5 sec, the actual assembly time would come down to 42 sec, and the DFA Index would increase to

Revised DFA Index, Em2 = (10 x 3) / (12 x 3.5) = 30 / 42 = 0.714

The target is always to try to get the DFA Index to be 1.0, i.e. 100% efficiency

Thus in all cases the key is to reduce the number of parts to a minimum, much like in Value Engineering, and the first way to attack this is almost always to reduce or eliminate the number of mechanical fasteners, i.e. get rid of nuts and bolts and especially washers, and either combine the two connected parts into one, or find a different way of attaching them, e,g. a snap fit.

Of course this is not always possible, so there are some rules or questions to ask to determine if the parts really have to be separate.

  1. Is there relative movement between the parts, e.g. wheels & ?
  2. Do they have to be different materials, e.g. transparent?
  3. Do they have to be separate for assembly or disassembly reasons , for instance replaced or serviced in the product’s life, e.g. batteries?

The take up of DFMA procedures, whether formal or informal, and simultaneous engineering has led to improvements in manufacturing efficiency. However there are some downsides.

In spite of rule 3 above, serviceability often suffers, i.e. cost is saved in manufacturing and hence the initial price of the product is lower, but serviceability becomes more expensive, but of course the owner does not know that until he has bought it and it is too late by then.

Optimising manufacturability for one product tends to mitigate against modular design for a family of products, so a balance has to be found between these two drivers of efficiency.

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