Question

What are the general principles of DFA? What are the steps to minimize the number of parts for an assembly?

Answer 1

The general principles of Design for Assembly are -

1) Minimize number of components.

Assembly costs are reduced. The final product is more reliable because there are fewer connections. Disassembly for maintenance and field service is easier. Reduced part count usually means automation is easier to implement. Work-in-process is reduced, and there are fewer inventory control problems. Fewer parts need to be purchased, which reduces ordering costs.

2) Use standard commercially available components.

Design time and effort are reduced. Design of custom-engineered components is avoided. There are fewer part numbers. Inventory control is facilitated. Quantity discounts may be possible.

3) Use common parts across product lines.

There is an opportunity to apply group technology. Implementation of manufacturing cells may be possible. Quantity discounts may be possible.

4) Design for ease of part fabrication.

Net shape and near net shape processes may be feasible. Part geometry is simplified, and unnecessary features are avoided. Unnecessary surface finish requirements should be avoided; otherwise, additional processing may be needed.

5) Design parts with tolerances that are within process capability.

Tolerances tighter than the process capability should be avoided; otherwise, additional processing or sortation will be required. Bilateral tolerances should be specified.

6) Design the product to be foolproof during assembly.

Assembly should be unambiguous. Components should be designed so they can be assembled only one way. Special geometric features must sometimes be added to components to achieve foolproof assembly.

7) Minimize use of flexible components.

Flexible components include parts made of rubber, belts, gaskets, cables, etc. Flexible components are generally more difficult to handle and assemble.

8) Design for ease of assembly.

Part features such as chamfers and tapers should be designed on mating parts. Design the assembly using base parts to which other components are added. The assembly should be designed so that components are added from one direction, usually vertically. Threaded fasteners (screws, bolts, nuts) should be avoided where possible, especially when automated assembly is used; instead, fast assembly techniques such as snap fits and adhesive bonding should be employed. The number of distinct fasteners should be minimized.

9) Use modular design.

Each subassembly should consist of five to fifteen parts. Maintenance and repair are facilitated. Automated and manual assembly are implemented more readily. Inventory requirements are reduced. Final assembly time is minimized.

10) Shape parts and products for ease of packaging. The product should be designed so that standard packaging cartons can be used, which are compatible with automated packaging equipment. Shipment to customer is facilitated.

11. Eliminate or reduce adjustment required.

Adjustments are time-consuming in assembly. Designing adjustments into the product means more opportunities for out-of-adjustment conditions to arise.

Steps for minimizing the Number of Parts are -

The basic approach in design for assembly is to reduce the number of parts without disturbing the functionality of the part.

Some important principles to reduce the number of parts are as follows:

1. Combine parts.

This includes integrating the functions of several parts into one part. Integration of several parts has been demonstrated below through various examples.

Additional guidelines for combining specific parts are provided below.

2. Make an outright reduction in the number of parts.

The fasteners can be reduced by using smaller number of heavier fasteners.

3. Make a major or full-product redesign.

When the assembly is redesigned, it is sometimes possible to achieve the function supplied by a separate component by another method. One example would be the replacement of a flanged and bolted pipe system with a threaded pipe system.

4. Use a different technology.

Often with drastic change in design, it is possible to achieve the same function by some other methods. For example, when a mechanical device is replaced by an electronic micro circuit.