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Design for Assembly Guidelines

This section looks at a number of design guidelines which if followed will generally lead to an assembly friendly design. Many of the guidelines are common sense guidelines with which many of the readers will be familiar, however, they often get forgotten or pushed into the background during the design process, especially if the designers are time constrained. The opportunity is taken here to remind the reader of these important guidelines. It is particularly important to bear these guidelines in mind when performing a design for manufacture and assembly analysis.

Minimise the number of parts

There are obvious advantages to be gained by keeping the number of parts down to a minimum. It may sound equally obvious but non-existent parts never jam. Fewer parts should lead to a reduced assembly time, less inventory, and you should only consider increasing the number of parts if it leads to a simplified assembly process.

Clearly high strength is required at one end of the above part and it would be better from the point of view of assembly and function to make it as one part out of stainless steel.

Avoid parts that are difficult to handle

The Boothroyd-Dewhurst DFA system assumes that parts are supplied in bulk and may be randomly oriented. This has obvious consequences regarding the handling of these parts. For example, parts of a certain design may nest or tangle when supplied in bulk. Parts that nest are cup shaped parts; a typical part that will tangle is a spring. If nesting or tangling occurs then additional time will be required to separate the parts, before they can be assembled, this increases the overall assembly time. Likewise, thin flexible parts are also difficult to handle because of the physical difficulty of picking them up. Often only small changes are necessary to prevent these difficulties and it is surprising how much assembly time and money can be saved.

The above diagrams show parts that present handling difficulties together with the modifications suggested to improve handlability. Of course it will not always be possible to to make these changes because of other constraints, but we must try if at all possible.

Avoid parts that need to be reorientated during assembly

Parts that require manipulation in order to achieve the correct orientation for assembly will increase the assembly time and cost; it will almost certainly lead to operator fatigue and mistakes. In order to minimise this problem, design the parts with maximum symmetry. This will sometimes mean adding non-functional features to improve symmetry. These additions may increase the manufacture cost, however, the reduction in assembly cost is usually greater.

Some non-functional features have been added to the above parts to make the more symmetrical requiring less reorientation during assembly.

Design parts for ease of insertion  

Once a part has been picked up and manipulated to get its correct orientation it needs to be inserted into an assembly fixture, if it is the first part, or the actual assembly for subsequent parts. Design parts for ease of insertion by adding features such as tapers and leads and make them as generous as possible. Where parts are not secured immediately, attempt to design self-locking/locating features, so that a clamping tool or another pair of hands is not required to maintain their correct orientation, during subsequent assembly processes.

The above diagrams illustrate some insertion problems that are been redesigned to improve assembleability.

Minimise the use of fasteners

The addition of a screw is a time-consuming operation which does not add value to a product. Try to eliminate screws by using snap together designs. If screws need to be used, minimise their number and reduce fastener variations to minimise complexity. To aid the placement of the screws, specify dog or cone point. If screws are to be blow fed, as is often the case with automatic assembly, ensure the correct length to head ratio (L 1.3D) to prevent jamming.

Avoid reorientation of assembly during assembly and assemble from above

Every time you reorient an assembly during the assembly process this will add time and cost to the product. Similarly, if parts are added along more than one axis this may be difficult to automate. Remember robots are not as dexterous as their human counterpart. The aim should be unidirectional assembly from above. In addition, try to use what is often referred to as the pancake concept where parts become ‘pancaked’ between others as they come together. This often eliminates screw fasteners if the outermost parts can be snapped together.

Unidirectional Assembly

Pancake Assembly

Specify the widest possible tolerance finish

Tight tolerances and tight surface finish lead to more expensive parts and will often lead to greater scrap and rework. You will probably need to employ more skilled labour and invest in more expensive precision equipment.

 

Develop your product using Cross Functional Teams

Promote teamwork through the use of Cross Functional Teams and employ Concurrent Engineering philosophy with the concurrent design of both product and process. Only in this way do you stand the chance of getting it ‘right first time’.

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