This Blog has spent a lot time talking about the philosophy of Design for Manufacturability (DFM). There are several good books on the subject of DFM and the reader can find in Amazon.com. Just type DFM in the search gadget. We will nonetheless look at some of the very basic Guidelines to DFM, more specifically at Mechanical / Structural and System design guidelines that I picked up in Design For Manufacturability from David M. Anderson as well as other site on the internet. Part 3, we're looking at Selecting Material which requires some serious considerations. Regardless of the industry, Raw Material account for about 30% of the total cost of a component. We'll be also looking at assemblies, Tooling and finally Dimensioning and tolerancing.
Materials
M1: Number of parts that will be fabricated
Part volume can drive the selection of material format, as well as the transformation process.
M2: Availability of the material
Exotic materials are expensive and create delivery problems and are likely to have a minimum buy clause attached. Common raw materials are generally available and delivered within 24 hrs.
M3: Make from
Can existing parts be used as raw material and modified to suit the need? This will save time in design and minimize inventory.
M4: Corrosion
What are the environmental conditions? Are you dealing with dissimilar materials that can generate galvanic corrosion?
M5: Stress
Vibration, creep, fatigue, strength, cracking, etc… The Stress-man always has the final word, but you need to understand the reasoning behind the selection.
M6: Toxicity
Some materials generate toxic fumes when processed, i.e. beryllium copper.
M7: Dimensional Stability
Dimensions can be affected by temperature change, humidity levels, etc.: e.g. wood expands under humidity, Stainless steel warps when welded, etc...
M8: Transformation Process
Machinability, weldability, Formability, e.g.: Some aluminum alloy cannot be welded; Stainless Steel has poor Machinability, etc...
M9: Surface Protective Finish
Can it be avoided? Some metals do not require protection. Stainless steel can replace a painted carbon steel part, and will save the costs associated with painting parts.
M10: Environmental
Chemical transformation processes are harmful to the environment. Think green when you select a material or when specifying a Chemical process; Chem-Milling is very bad in every way.
M11: The Design process
Raw material selection is done at the Design Process level, therefore should be defined as early as possible, and should be kept as much as possible to what is stock or easily available. Designers are solely responsible for material selection, and indirectly define the manufacturing process by design. This is the main reason why manufacturing and Material Management need to be involved early in the design process in order to design manufacturing and material management into the product. This exercise will also help defining a preferred raw material list.
Assemblies
A1: Eliminate over-constraints to minimize tolerance demand
The single most expensive factor in product manufacturing is the tolerances generated by over-constraints. See Figure 1.
Figure 1
A2: Provide unobstructed access for parts and tools
Each part must be designed to fit in its required location, but also must have an assembly path for entry or removal, as well as access to tools. See Figure 2.
Figure 2
A3: Structure the product into sub-assemblies as appropriate
The use of sub-assemblies can streamline manufacturing as sub-assemblies can be built and tested separately. More and more Aircraft manufacturers outsource sub-assemblies to partners. The sub-assemblies are later integrated into the assembly line.
A4: Make insertion of components easy
Avoid having components so small that finger grasp is difficult (refer to Guideline SP7) and become difficult to locate as fingers hide assembly location.
A5: Purchase modules and sub-assemblies assembled and tested
This will eliminate performing incoming inspection, provides quicker feedback to the supplier and allow for repair where the sub-assemblies were built. Modules are typically interchangeable. See Figure 3. These modules are generally purchased via the use of Source Control Dataset (SOCD) or Vendor Item Control Dataset (VICD), commonly known tyoday as Procurement Control Dataset (PCD).
Figure 3
A6: Visibility
Avoid at all cost having to install a part blind. The part location should always be visible during assembly operation. Having full visibility of the working area will ease assembly, and increase safety.
A7: Design parts with symmetry to ease assembly:
Design each part to be symmetrical so that the part does not have to be oriented for assembly. In manual assembly, symmetrical parts cannot be installed backwards, a major potential quality problem associated with manual assembly. If symmetry is not possible, make the part very asymmetrical. Refer to Figure 4.
Figure 4
Tooling
T1: Tool variety
Design for the minimum tool variety. Constantly having to change tool to install fasteners reduces efficiency and increase assembly cost.
T2: Common Tools
Design assemblies so the most common tools can be used. The best way to ensure the most common tools are used, is to go on the shop floor and ask.
T3: Assembly jigs
Design small assemblies to maximize self-alignment. Having to jigs increases cost.
Dimensions and Tolerances
DT1: Cost of Tolerance
Tolerance cost is exponential. Over tolerance increases rejects therefore increases cost. Refer to Figure 5.
DT2: Tolerance Step Function
The type of process depends on the tolerance specified. Each process has a practical limit regarding how close a tolerance can be held for a given skill level on the production line. Refer to Figure 5.
Figure 5
DT3: Tolerancing
Tolerancing is the most significant factor in the cost of a part, yet it is the most neglected factor. Both designers and machinists rely too much on the accuracy of CNC machine tools. Designers must avoid arbitrary decisions when specifying dimensions. Refer to Figure 6.
Figure 6
DT4: Tolerance Accumulation
To avoid accumulation of tolerance adjustment are necessary, make sure they are independent and easy made.
DT5: Manufacturing Process capabilities
Make sure that the tolerances specified on the parts are in line with the Manufacturing Process Capabilities. Specifying ±.005 when the best your Manufacturing Process capability is ±.010 will increase non-conformances.
