Six Sigma and Beyond: Design for Six Sigma, Volume VI

The core of the DFM/DFA process is to make sure that the design and assembly are planned in terms of:

1. Simplicity (as opposed to complexity)

2. Standardization ( commonality )

3. Flexibility

4. Capability

5. Suitability

6. Carryover

So, a designer designing a product should be cognizant of the effects on product design. Some of these are:

• Materials selection is based on the targeted manufacturing process.

• The forms/ shapes of parts are based on the targeted transportation, handling, and parts feeding system.

• Field environment can affect the production durability, which contributes variation to the components /parts as well as the manufacturing process.

• Shelf life.

• Operating life.

• Product MTBF and MTBR.

In the development of the primary design, consideration must be given to whether to start with a basic process or to start with secondary process with purchased raw or semi-raw materials. If the decision is to start with a basic process, then the next question will be ” what kind of materials to start with? There are three options:

1. Start with materials in liquid state, i.e., casting.

2. Start with materials in plastic state, i.e., forging .

3. Start with materials in solid state, i.e., roll forming (sheet), extrusion (rod, sheet), electroforming (powder), automatic screw machine work (rod).

If a secondary process is needed, either as a sequential operation of a basic process or a fresh starting point, consideration must be given to the selection of the most favorable forming and sizing operations. A number of factors relating to a given design that need to be considered include:

1. The shape desired

2. The characteristics of the materials

3. The tolerance required

4. The surface finish

5. The quantity to be produced

6. The average run size

7. The cost

The focus then of a product design is to:

1. Minimize parts/components: The fewer parts/components and the fewer manufacturing/assembly operations, the better, i.e.,

   • Combine mating parts, unless isolation is needed.

   • Eliminate screws and loose pieces. Replace screws with snap-on parts or fasten rivet , if practical. If screws are a necessary evil, try to make them all the same type and size.

   • Do not use a screw to locate. Remember that a screw is a fastener.

2. Use common/popular components/parts: Off-the-shelf type components/parts usually are user friendly and less expensive. Tooling/setup charges also can be avoidable beyond the pilot try headache , i.e.,

   • Use fasteners with common/popular/standard length and diameter.

   • Use common values of resistors, capacitors, diodes, etc.

   • Use standard color chip of paints and coatings, if possible.

3. Design the parts to be symmetrical: If you must use customized unique parts, try to design the parts to be symmetrical, and use a jigless assembly method, if at all possible, i.e.,

   • Avoid internal orientations.

   • Design an external accentuated locating feature, if it cannot be internally symmetrical.

4. Design the parts to be self-aligned, self-locating, and self-locking, i.e.,

   • Design locating pins and small snap protrusions on mating parts.

   • Chamfers and tapers.

   • Use mechanical entrapments and snap-on approach.

   • Connect necessary wires/harnesses directly and use locking connectors.

   • Make sure that parts are easy to grip.

   • Avoid flexible parts ” the more rigid the part, the more easily handled and assembled .

   • Avoid cables, if practical.

   • Avoid complicated fastening process, if practical.

     (Special note: If screws must be used, remember these rules:

     • Shank to head ratio: 1 greater than or equal to 1.5; 1 greater than or equal to 1.8 if tube feed

     • Head design

     • Thread consideration:

       Tapped holes?

       Thread cutting screws?

       Thread forming screws?

     • Quality screws)

5. Design for simple or no adjustment at all:

   • Remember, adjustment is a non-value added operation. Minimum adjustment ” if necessary ” with one-hand operation should be at most.

6. Modularize sub-assembly design:

   • Modularize sub-assemblies. Assemble and test them prior to final assembly.