Rule 1 – Minimize the number of parts
Having fewer parts is beneficial to the manufacturer from a number of standpoints. It reduces overhead by eliminating documentation, (drawings, material specifications, purchase orders, inspection reports, routing and inventory), speeds assembly and improves quality.
To determine the theoretical minimum number of parts required in an assembly, the need for each part should be
Productivity Gains
Multifunctional parts reduce number of parts, and ease assembly and handling.
Design For Manufacturing & Assembly
challenged. A part can be identified as a candidate for elimination if all three questions are found to be negative.
• Does the part move relative to the mating part? • Does the function require a different material? • Is the part required for disassembly or service?
Figure 4-1. Parts consolidation.
Typical parts to be eliminated: – Fasteners – Spacers – Labels To be replaced with:
Snap fits ultrasonic Press fits thermal stake Interlocks
Bosses
Embossed lettering
Rule 2 – Minimize Assembly Surfaces
Multiple assembly surfaces typically add time and motion to the assembly sequence. In addition to the time required to rotate a partially assembled product, parts which are not fully secured can dislodge requiring rework or worse, a quality problem.
If automating, multiple assembly surfaces can increase fixture costs and equipment costs. If plastic parts are involved, the mul- tiple assembly surfaces will likely complicate the mold with side pulls or lifters.
Rule 3 – Design for Z-Axis Assembly
The simplest and most preferred assembly motion is a straight down (Z-axis) stroke. This design utilizes gravity to assist the assembly. It also tends to result in the most automatable assemblies.
Alignment features such as depressions, locating pins, slots, or ribs can guide mating parts together and facilitate assembly.
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Rule 4 – Improve Assembly Access
Provide a “clear view” for assembly operations. This is important for manual assembly and, in most cases, essential for automatic assembly. Avoid parts or assembly sequences that require tactile sensing for installation. Such “blind” assembly exposes the manufacturing process to significant quality risks.
Rule 5 – Maximize Part Compliance
Part mating is a major challenge to automatic assembly. Part misalignment and tolerance stack-up is the form in assembly where different processes such as stamping, injection molding, casting, and machining — all from different vendors — are to be mated. Compliance, the ability of one part to move so that it can mate with another, must be designed into both the product and the production process. Product design compliance tech- niques include generous chamfers on both mating parts, adequate guidance surfaces and specifications requiring part consistency. One simple example of design for compliance is the use of oblong holes rather than round holes for post mating or screw assembly. This allows for slight misalignments due to tolerance issues. Part tolerance specification should be reasonable as they can drastically increase per part cost.
Rule 6 – Maximize Part Symmetry
The more symmetrical a part, the easier it is to handle and orient, both manually and automatically. Symmetry also reduces assembly quality risks. The need for symmetry increases significantly with high rate automation. If symmetry is impossible, existing asym- metry should be “identified” on the outer surfaces of the part in order to provide easy recognition of parts for feeding devices or to avoid component mislocation.
Rule 7 – Optimize Part Handling
Avoid flexible parts, such as wiring for parts that require two- handed manipulation, whenever possible. Flexible parts are difficult to automate economically. If possible, retain part posi- tioning from the point of manufacture to the point of assembly. For example, retain position of plastic parts by automatically unloading parts to palletized trays. Avoid part designs that nest, tangle, stick together, are slippery or require careful handling. Figure 4-2. Nut & Bolt vs.
Stud & Nut.
Figure 4-3. Part Mating.
Figure 4-4. Nut & Bolt vs.
Stud & Nut.
Design For Manufacturing & Assembly
For robotic or automatic handling, provide symmetrical vertical surfaces to simplify gripper design. Simple surfaces or towers can often be molded in to allow for neat and orderly stacking of parts without sticking.
Rule 8 – Avoid Separate Fasteners Wherever Possible
Fasteners are a major barrier to efficient assembly. They are difficult to feed, can cause jamming due to poor quality, and normally require monitoring for presence and fastening torque. In manual assembly, the cost of driving a screw can be six to ten times the cost of the fastener. The best design approach is to incorporate the fastening function into a major component. In plastic design, threaded fasteners are particularly poor due to the high levels of hoop stress they can generate and the notch effects they can induce. See the Product Assembly (Section 5) for molded-in fastening techniques.
Rule 9 – Provide Parts with Integral “Self-Locking” Features
Design parts to “nest” so that no further repositioning is required. Provide projections, indentations or other surface features that maintain the orientation and position of the parts already in place. Self-locking features are especially important for automation if the difficult task of handling and orienting parts can be uncoupled from the easier task of securing parts in the assembly sequence.
Rule 10 – Drive Toward Modular Design
Modular design simplifies final assembly because there are fewer parts to assemble. Automation system downtime is reduced, since experience has shown that total system performance is directly related to the number of parts being assembled. Try to limit subassemblies, including the final assembly, to no more than 15 components. Modules lend themselves to easier quality inspec- tion prior to their insertion into the final assembly.
Reference
The ten “rules” listed above are taken from “Product Design for Manu- facturing and Assembly” by Bart Huthwaite. The actual rules appearing in that source have been modified to apply specifically to thermoplastics.
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