Practical everyday DFM guidelines: They truly save time and money Part 2: Standard Parts

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 2, we're looking at Standard Parts

Standard parts

SP1: Commercial Off The Shelf (COTS)

Never design a part that you can buy off the shelf.

SP2: Type of fasteners

Use common fasteners. Avoid exotic fasteners, and strive at using the same type of fastener throughout your design.

SP3: fasteners in stock

Always design with fasteners that are readily available from your stock room. Consult the inventory; avoid at all cost specifying fasteners that are not in stock.

SP4: Standardize parts

Standardizing parts will facilitate design activities; minimize the amount of part inventory. Handling and assembly operations will be improved. Common parts will result in lower inventories, thus reducing costs and increasing quality.

SP5: Minimize the number of parts in your design

Strive at design with a minimum number of parts, even standard parts. As the number of parts goes up, the total cost of fabricating and assembling the product goes up as well.

SP6: Design for common tools

Optimize the use of the same type and size of fastener so the same tools can be used throughout the assembly process.

 SP7: Small fasteners

Avoid using small fasteners that cannot be finger grabbed. If the operator has to use tweezers to grasp a fastener, assembly time will increase, therefore cost will increase. The same guideline applies to small components.

SP8: Certified Components

The aerospace industry requires certified components, which are typically more expensive that commercial ones. Imagine the cost of having to certify a component that you designed instead of looking for COTS.

SP9: Application

Use the fastener type that will meet the requirements of you application. However keep in mind all the guidelines in this section. If you feel there are some contradiction; you are right.

SP10: Simple part fallacy

Product designers see some parts as being simple to model, therefore simple to fabricate, therefore inexpensive. This is wrong. Refer to Guideline SP1.

SP11: Inexpensive part fallacy

Product designers see hardware (screws, bolts, nuts, rivets …) as being inexpensive, yet the cost of maintaining an inventory of multiple fasteners to suit everyone's taste can drive production cost up exponentially and drive efficiency down exponentially. Refer to Guidelines SP2 and SP3.

SP12: Fastener installation

Fastener should always be installed for a downward motion.

SP13: Tool Access

Some types of fastener require pneumatic tools that are bulkier than hand tools. Consider the tooling required to install the fasteners; refer to figure 1.

Figure 1

 SP14: Exotic Components

Avoid using Exotic Components. Exotic Components are typically more expensive, and their delivery problematic. If the supplier requires a minimum buy, the cost increases. Unless required on other projects, the unused components will stay in inventory, therefore increasing total cost further.

 SP15: Eliminate fasteners by combining parts 

The interface between two parts must be fastened together and also involves two sets of dimensions and tolerances. Combining part together eliminates the interface; the cost of manufacturing is reduced as an alignment is not required anymore.

Example: Aircraft components have gone from hundreds of sheet metal parts as shown in Figure 2, to one single machined part, thus saving thousands of man-hours in manufacturing.

Figure 2