The lead time for product development is the period of time between the go ahead signal to develop the product is given to the time when the product is ready to be released in the market. Previous sections dealt with the various stages through which the development cycle has to go through. This product development lead time varies from product to product. Increase in global competition has compelled the manufacturers to progressively reduce the product development time. A set of tools have been developed to examine the design from different points of view like convenience in manufacture, assessment of risks in design, assembly, testing, servicing etc. These tools are generally labeled as “design for X”, where X stands for cost, manufacture, assembly, testing, servicing etc. During as well as at the end of the process of design it is now customary to examine the designs based on certain specific considerations. They are listed below:
1. Is the design of parts such that they can be easily manufactured on fabricated? This is referred to as design for manufacturing.
2. Is the product design such that the product can be assembled fast, easily and economically? Does the part design lend itself for automation of assembly?
3. Is the product design is such that the product can be tested easily? Many of the IC and microchip designs should facilitate easy testing.
4. Is the product design such that the product can be easily serviced? One of the plus points of a good design is its easy serviceability regardless whether it is a computer or a washing machine.
5. Is the design carried out such that the cost is globally competitive? Today cost of the product is a very important consideration. Most of the designs start from a targeted selling price and the corresponding production cost.
Five important considerations are mentioned above – manufacture, assembly, testing, service and cost. These can be a few other considerations too depending upon the product.
Current Approach to “Design for X”
There are some common guidelines which can be followed to satisfy the above five considerations. They are discussed below.
Design with less number of parts and sub assemblies
Cost and time to market can be considerably reduced if the number of parts is reduced. Figure 2.10 shows a clamping lever assembly which can be cited and example of reduction in the number of parts. The clamp assembly has 3 parts (two steel and 1 plastic), each requiring several machining operations. Finally two parts are to be chrome plated. This assembly can be replaced by a simple die cast part shown in Fig. 2.11or a plastic clamp, thereby reducing the design and manufacturing cost. The alternative parts are available off the shelf. The new design is superior from the point of view of manufacturability. It is also cheaper and aesthetically superior. This trend is particularly seen in aircraft and automotive industry. Increased use of sophisticated CNC machines like 5 axis machines and multitasking machines enables us to integrate several parts into a single monolithic part. This approach not only drastically reduces set up times but also improves the accuracy and makes available a part for assembly much earlier. A typical example where cost, quality and performance could be improved through integration in ICs and VLSI chips. The concept of a system on a chip further reduced cost and improved reliability.
Introduction of high power series motors in spindle drives of CNC machines eliminated the need for a gear box, thereby not only improving performance but also reducing cost. The integral rotor spindles used in high speed CNC machines further reduced the number of kinematic elements in main drive. The use of linear motors in reciprocating drives eliminated not only ball screws and nuts but also paved way for increasing linear traverse speeds.
Another example of improvement of design is shown in Fig. 2.12. Fig. 2.12 (A) shows the assembly which require the use of a screw driver. The number of parts required in 3. The assembly required aligning the screw and a screwdriver. The manufacture can be simplified using a rivet, thereby eliminating the threading operation (B). Making the rivet integral with one part reduces the total number of parts to two (C). The design can be further improved using a snap fit approach (D). An alternative design can be joining the two parts by a spot weld which may be cheaper than all the previous designs.
A product may be made in many variations to meet specific customer needs or end use
It is advisable to keep as many parts as standard to minimize product variations. Automotive manufacturers adopt this approach very effectively. They create many designs from the same platform and components often by adding options only. This keeps the cost of the product variations to a minimum.
Fastener is a critical factor which makes easy assembly
A sound principle that could be followed is that minimum variety and number only should be used. If there are socket head cap screws used, then use only one type so that a single Allen key is necessary. This will reduce the assembly time by eliminating the need to change Allen keys and eliminate the need to keep inventory of a variety of screws while assemblling as well as the need to supply many tools for servicing. If possible, fastener could be altogether eliminated. Adhesive bonding and snap fit are examples. Both lend themselves to easy automation. The reader could realize the advantages of a snap fit assembly if a cell phone is examined.
Ease of fabrication and is very critical
Take the case of an end cover shown in Fig. 2.13. The provision of a locating spigot will make the assembly easy. (Fig. 1.13 (B)) Making the thickness of the casting of the cover uniform [Note the change in the thickness of the end cover from Fig. 2.13(B) to Fig. 2.13 (C)] makes casting defect free. Provision of a chamfer at the mouth of the tapped hole facilitates easy entry and alignment of screw while fastening. Chamfers in the bore and at the end of the socket make assembly easier. A few examples of how careful detailed design will improve assembly are discussed here.
Designing for easy manufacture
Designers have to think about easiness in manufacture. Take for example part A in Fig. 2.10 which is shown separately in Fig. 2.14. There is an angular core hole to be drilled to make the tapped hole. Since the drilling is to be done on a conical face, the drill is likely to wander. Creating a flat surface by spot facing prior to drilling makes the drilling process easy.
Specify proper tolerances
Designers often have a tendency to play safe. This results in specifying tighter tolerances. For example, a simple drilled hole through which a bolt has to pass through need not have a 7th grade tolerance. A 10th or 12th grade or general tolerance will do for this purpose. Care must be taken when specifying surface finish for such a hole. N8 finish will be adequate here which requires only a drilling operation. If N7 is indicated, the process engineer will be required to introduce a reaming operation, which is not needed here. Not only such designs increase time and cost but also will result in increase in the inventory of tools.
Specifying tolerances should take into consideration the process capability of the machines. Mismatch of tolerance specified on the component and the machine’s process capability will result in avoidable rejections.
Rework is another offshoot of specifying tighter or incompatible tolerances. Rework will result in delay in assembly and has to be avoided as much as possible.
Even though the designer may specify a bilateral tolerance, the manufacturing engineer has to understand the end use. For example, if a part length is specified as 100 +/- 0.1, it may be advisable to keep the dimension close to 99.9 in the case of an aircraft structural part. This will result in reduction in over all weight. If the tolerances are maintained on all components near the higher limit, there will be considerable increase in weight affecting the payload capacity.
Standardization is another important issue
Standardization not only reduces design effort but also cost. Let us take the example of modular fixtures. Before the advent of modular fixtures aircraft industry had to make several thousand new fixtures for each aircraft project. Once the model is scrapped, most of the fixtures also may be scrapped. Further, it is necessary to have a large storage area and an efficient system to retrieve the fixtures. With modular fixturing, the need to maintain such large inventory is eliminated. Once the use is over, the fixture can be dismantled and the fixture components can be used for another fixture. The design is also easy as all the suppliers of modular fixtures supply also a matching CAD library. Thus both the design and tool realization time are substantially reduced with modular fixtures.
Standardization will make maintenance and replacement of parts easy. The design lead time will also be reduced.
Increase in the number of set ups has the risk of stack up of tolerances apart from delay and cost increase. 5 axis machines, multitasking machines, machines capable of 5 side machining, etc. reduce the number of setups. Reduction in the number of setups reduces handling as well as the number of fixtures required.
Avoid frequent design changes
Design changes are often unavoidable. However, frequent design changes will create confusion and attendant wastage. It is recommended that design changes are to be taken up at specified time intervals. A typical example is software. This policy is good for engineering goods too.