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1. Simplify the design and reduce the number of parts because
for each part, there is an opportunity for a defective part and an assembly
error. The probability of a perfect product goes down exponentially as the
number of parts increases. As the number of parts goes up, the total cost
of fabricating and assembling the product goes up. Automation becomes more
difficult and more expensive when more parts are handled and processed.
Costs related to purchasing, stocking, and servicing also go down as the
number of parts are reduced. Inventory and work-in-process levels will go
down with fewer parts. As the product structure and required operations
are simplified, fewer fabrication and assembly steps are required, manufacturing
processes can be integrated and leadtimes further reduced. The designer
should go through the assembly part by part and evaluate whether the part
can be eliminated, combined with another part, or the function can be performed
in another way. To determine the theoretical minimum number of parts, ask
the following: Does the part move relative to all other moving parts? Must
the part absolutely be of a different material from the other parts? Must
the part be different to allow possible disassembly?
2. Standardize and use common parts and materials to facilitate design activities,
to minimize the amount of inventory in the system, and to
standardize handling and assembly operations. Common parts will result in lower inventories,
reduced costs and higher quality. Operator learning is simplified
and there is a greater opportunity for automation as the result of higher
production volumes and operation standardization. Limit exotic or unique
components because suppliers are less likely to compete on quality or
cost for these components. The classification and retrieval capabilities of
product data management (PDM) systems and component supplier management (CSM)
systems can be utilized by designers to facilitate
retrieval of similar designs and material catalogs or approved parts lists
can serve as references for common purchased and
stocked parts.
3. Design for ease of fabrication. Select processes compatible with
the materials and production volumes. Select materials compatible with production
processes and that minimize processing time while meeting functional requirements.
Avoid unnecessary part features because they involve extra processing effort
and/or more complex tooling. Apply specific guidelines appropriate
for the fabrication process such as the following guidelines for machinability:
- For higher volume parts, consider castings or stampings to reduce machining
- Use near net shapes for molded and forged parts to minimize machining
and processing effort.
- Design for ease of fixturing by providing large solid mounting surface
& parallel clamping surfaces
- Avoid designs requiring sharp corners or points in cutting tools -
they break easier
- Avoid thin walls, thin webs, deep pockets or deep holes to withstand
clamping & machining without distortion
- Avoid tapers & contours as much as possible in favor of rectangular
shapes
- Avoid undercuts which require special operations & tools
- Avoid hardened or difficult machined materials unless essential to
requirements
- Put machined surfaces on same plane or with same diameter to minimize
number of operations
- Design workpieces to use standard cutters, drill bit sizes or other
tools
- Avoid small holes (drill bit breakage greater)
& length to diameter ratio > 3 (chip clearance & straightness
deviation)
4. Design within process capabilities and avoid unneeded surface finish
requirements. Know the production process capabilities of equipment
and establish controlled processes. Avoid unnecessarily tight tolerances
that are beyond the natural capability of the manufacturing processes. Otherwise,
this will require that parts be inspected or screened for acceptability.
Determine when new production process capabilities are needed early to allow
sufficient time to determine optimal process parameters and establish a
controlled process. Also, avoid tight tolerances on multiple, connected
parts. Tolerances on connected parts will "stack-up" making maintenance
of overall product tolerance difficult. Design in the center of a component's
parameter range to improve reliability and limit the range of variance around
the parameter objective. Surface finish requirements likewise may be established
based on standard practices and may be applied to interior surfaces resulting
in additional costs where these requirements may not be needed.
5. Mistake-proof product design and assembly
(poka-yoke) so that the assembly process is unambiguous. Components should
be designed so that they can only be assembled in one way; they cannot be
reversed. Notches, asymmetrical holes and stops can be used to mistake-proof
the assembly process. Design verifiability into the product and its components.
For mechanical products, verifiability can be achieved with simple go/no-go
tools in the form of notches or natural stopping points. Products should
be designed to avoid or simplify adjustments. Electronic products can be
designed to contain self-test and/or diagnostic capabilities. Of course,
the additional cost of building in diagnostics must be weighed against the
advantages.
6. Design for parts orientation and handling to minimize non-value-added
manual effort and ambiguity in orienting and merging parts. Basic principles
to facilitate parts handling and orienting are:
- Parts must be designed to consistently orient themselves when fed into
a process.
- Product design must avoid parts which can become tangled, wedged or
disoriented. Avoid holes and tabs and designed "closed" parts.
This type of design will allow the use of automation in parts handling
and assembly such as vibratory bowls, tubes, magazines, etc.
- Part design should incorporate symmetry around both axes of insertion
wherever possible. Where parts cannot be symmetrical, the asymmetry should
be emphasized to assure correct insertion or easily identifiable feature
should be provided.
- With hidden features that require a particular orientation, provide
an external feature or guide surface to correctly orient the part.
- Guide surfaces should be provided to facilitate insertion.
- Parts should be designed with surfaces so that they can be easily grasped,
placed and fixtured. Ideally this means flat, parallel surfaces that would
allow a part to picked-up by a person or a gripper with a pick and place
robot and then easily fixtured.
- Minimize thin, flat parts that are more difficult to pick up. Avoid
very small parts that are difficult to pick-up or require a tool such as
a tweezers to pick-up. This will increase handling and orientation time.
- Avoid parts with sharp edges, burrs or points. These parts can injure
workers or customers, they require more careful handling, they can damage
product finishes, and they may be more susceptible to damage themselves
if the sharp edge is an intended feature.
- Avoid parts that can be easily damaged or broken.
- Avoid parts that are sticky or slippery (thin oily plates, oily parts,
adhesive backed parts, small plastic parts with smooth surfaces, etc.).
- Avoid heavy parts that will increase worker fatigue, increase risk
of worker injury, and slow the assembly process.
- Design the work station area to minimize the distance to access and
move a part.
- When purchasing components, consider acquiring
materials already oriented in magazines, bands, tape, or strips.
7. Minimize flexible parts and interconnections. Avoid flexible
and flimsy parts such as belts, gaskets, tubing, cables and wire harnesses.
Their flexibility makes material handling and assembly more difficult and
these parts are more susceptible to damage. Use plug-in boards and backplanes
to minimize wire harnesses. Where harnesses are used, consider foolproofing
electrical connectors by using unique connectors to avoid connectors being
mis-connected. Interconnections such as wire harnesses, hydraulic lines,
piping, etc. are expensive to fabricate, assemble and service. Partition
the product to minimize interconnections between modules and co-locate related
modules to minimize routing of interconnections.
8. Design for ease of assembly by utilizing simple patterns of movement
and minimizing the axes of assembly. Complex orientation and assembly movements
in various directions should be avoided. Part features should be provided
such as chamfers and tapers. The product's design should enable assembly
to begin with a base component with a large relative mass and a low center
of gravity upon which other parts are added. Assembly should proceed vertically
with other parts added on top and positioned with the aid of gravity. This
will minimize the need to re-orient the assembly and reduce the need for
temporary fastening and more complex fixturing. A product that is easy to
assemble manually will be easily assembled with automation. Assembly that
is automated will be more uniform, more reliable, and of a higher quality.
9. Design for efficient joining and fastening. Threaded fasteners (screws, bolts, nuts
and washers) are time-consuming to assemble and difficult to automate. Where
they must be used, standardize to minimize variety and use fasteners such
as self threading screws and captured washers. Consider the use of
integral attachment methods (snap-fit). Evaluate other bonding techniques with adhesives.
Match fastening techniques to materials, product functional requirements,
and disassembly/servicing requirements.
10. Design modular products to facilitate assembly with building
block components and subassemblies. This modular or building block design
should minimize the number of part or assembly variants early in the manufacturing
process while allowing for greater product variation late in the process
during final assembly. This approach minimizes the total number of items
to be manufactured, thereby reducing inventory and improving quality. Modules
can be manufactured and tested before final assembly. The short final assembly
leadtime can result in a wide variety of products being made to a customer's
order in a short period of time without having to stock a significant level
of inventory. Production of standard modules can be leveled and repetitive
schedules established.
11. Design for automated production. Automated production involves
less flexibility than manual production. The product must be designed in
a way that can be more handled with automation. There are two automation
approaches: flexible robotic assembly and high speed automated assembly.
Considerations with flexible robotic assembly are: design parts to utilize
standard gripper and avoid gripper / tool change, use self-locating parts,
use simple parts presentation devices, and avoid the need to secure or clamp
parts. Considerations with high speed automated assembly are: use a minimum
of parts or standard parts for minimum of feeding bowls, etc., use closed
parts (no projections, holes or slots) to avoid tangling, consider the potential
for multi-axis assembly to speed the assembly cycle time, and use pre-oriented
parts.
12. Design printed circuit boards for assembly. With printed circuit
boards (PCB's), guidelines include: minimizing component variety, standardizing
component packaging, using auto-insertable or placeable components, using
a common component orientation and component placement to minimize soldering
"shadows", selecting component and trace width that is within
the process capability, using appropriate pad and trace configuration and
spacing to assure good solder joints and avoid bridging, using standard board
and panel sizes, using tooling holes, establishing minimum borders, and
avoiding or minimizing adjustments.
ABOUT THE AUTHOR
Kenneth A. Crow is President of DRM Associates,
a management consulting and education firm focusing on integrated product
development practices. He is a distinguished speaker and recognized expert
in the field of integrated product development. He has over twenty years
of experience consulting with major companies internationally in aerospace,
capital equipment, defense, high technology, medical equipment, and transportation
industries. He has provided guidance to executive management in formulating
a integrated product development program and reengineering the development
process as well as assisted product development teams applying IPD to specific
development projects.
He has written papers, contributed to books, and given many presentations
and seminars for professional associations, conferences, and manufacturing
clients on integrated product development, design for manufacturability,
design to cost, product development teams, QFD, and team building. Among
many professional affiliations, he is past President and currently on the
Board of the Society of Concurrent Engineering and is a member of the Product
Development Management Association and the Engineering Management Society.
For further information, contact the author at DRM Associates, 2613 Via
Olivera, Palos Verdes, CA 90274, telephone (310) 377-5569, fax (310) 377-1315,
or email at kcrow@aol.com.
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