Design for manufacturing and assembly (DFMA)

Differences between design for manufacturing (DFM) and design for assembly (DFA)

Design for Manufacturing (DFM) is a design method to reduce the complexity of manufacturing operations and the overall cost of production including the cost of raw materials.

Design for Assembly (DFA) is a design method to facilitate or reduce the assembly operations of parts or components of a product.

The difference is that one focuses on parts (DFM) and the other on part assembly (DFA).

Why using design for manufacturing or design assembly

It is usually accepted (demonstrated?) that 70 to 80% of production costs are determined by design decisions.

Hence the importance of using design for manufacturing or design for assembly.

Design for manufacturing and assembly (DFMA), the right balance

Design For Manufacturing and Design For Assembly can be contradictory. At the extreme, the DFA approach can naturally lead to having only one part: the assembly cost becomes zero.

On the other hand, it can lead to an extremely complicated part to produce, with a high cost.

It is therefore appropriate to use a combined Design for Manufacturing and Assembly (DFMA) approach, which has become the standard today.

Main levers to reduce manufacturing costs through DFMA

Reduce the number of parts

This is probably the most important lever. It directly reduces the assembly cost, and a significant number of other indirect costs: administrative and logistical costs (ordering, transport, storage, etc.), test costs (for each part) and reliability costs (more parts = more risk of failure)

Use standard parts

Standard parts are cheaper, more available and often more reliable than custom parts.

Adopt a modular design

The use of modules allows several benefits depending on the case:

• Reduce the overall number of components or parts. This is the case if a product exists in several variants (each of which may differ by a module) or if several products may use the same module. This reduces the associated indirect costs (administrative and logistical)
• Reduce redesign or test costs by using existing modules rather than new parts
• Take advantage of production scale effects for modules common to several parts

On the other hand, modularity is limited by the potential increase in assembly costs with more connections.

Design multifunctional parts

A multifunctional part reduces the total number of parts because instead of using two parts for two functions, only one part is needed. For example, a heat dissipator can be used as a separation as well.

Design error-proof parts

It is probably the most famous lever of DFMA, sometimes called Poka-Yoke. It consists in designing parts that cannot be assembled in the wrong way or assembled on other products.

Design parts to facilitate assembly

Some parts can be designed to facilitate assembly or testing. For example, in addition to their "product" function, a part can provide an assembly guide, or be reflective to facilitate inspection.

Design to facilitate manufacturing

This is the counterpart of the previous lever, focused on the production of one part rather than the assembly of two parts.

This applies particularly to finishing operations such as machining and painting, which must be limited. We can also mention complex moulding parts.

Limit tolerance constraints

It is also necessary to reduce binding tolerances or in general any "over-specification" that is not a customer requirement because they will increase production costs by imposing more robust processes, more sophisticated equipment or more extensive quality control operations.

Define the right fastening means

In general, separate and complicated fasteners should be limited and simple automatic fasteners preferred.

Separate fasteners increase the cost due to additional handling and indirect management costs. Moreover, they are often sources of quality defects.

It is necessary to design fasteners that have the lowest cost, for example:

• prefer automatic fasteners such as clips
• prefer rivets to screws
• prefer self-tapping screws
• avoid separate washers
• avoid screws with tapped holes or that are too long or too short,

Reduce or facilitate handling

The positioning, orientation and fixing of parts must be limited, for example by using symmetrical parts to avoid orientation.

The parts must be designed to be easily and safely grasped by the operator: not too small, not too large, not too heavy, not with sharp edges or tips.

Reduce assembly directions

All parts should be assembled in one direction only and if possible, from top to bottom to take advantage of the effects of gravity.

Design For Manufacturing and Assembly methods

There are several detailed methods and tools for DFMA. We focus here on the fundamental principles of these methods:

• collaborative work between design and manufacturing teams. Many stakeholders must participate in this process: marketing or sales (definition of the initial specifications), R&D or engineering, purchasing, manufacturing methods, quality and even logistics teams (logistical constraints) or customer service teams
• an upstream work; it starts at the marketing phase, which can therefore be several months or even years before the arrival in production
• practice feedback on products already in production. It is not only a question of production teams being involved in R&D, but also of R&D teams coming into production to see for themselves

Other methods associated with Design For Manufacturing and Assembly (DFMA)

Some other design methods are associated or similar to the design for manufacturing and assembly:

• The Design for six sigma (DFSS), which seeks to reduce manufacturing defects and variations by ensuring a part manufacturing capability greater than 4.5 sigma
• The Design For Testability (DFT), which aims to make it easier to test products in production. It is often included in the design for manufacturing
• The Design For Maintainability (DFM), planned for the design of manufacturing equipment and seeking to allow easier, less costly maintenance that can be carried out without production stoppage.