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7. EVALUATING DESIGN PROPOSALS

Designers need some method for knowing if the new or redesigned product

will meet its manufacturability and other objectives. The designers general

judgment may be very sound in weighing the designs conformance to

planned design attributes, but an objective measurement almost always will

be better. Every design variation has consequences in the properties of the

product, including its manufacturability.

Evaluation is needed not only so that the design team or designer can know

if the products objectives have been met but also so that alternative designscan be compared and the most effective alternative selected.

Preferably, the design team should be able to carry out an evaluation early in

the design process, ideally at the concept stage. Then the time-consuming

and expensive development and detailed work does not take place unless it is

verified that the proposed approach is really effective. The procedure should

be one that can be applied easily and routinely by the product designers. It

should employ some numerical rating, index, or cost so that as objective a

comparison as possible can be made between alternative designs.

7.1. EVALUATING MANUFACTURABILITY

Manufacturing cost is the most complete measure of manufacturability. It can

be expressed as a total cost for the product or component or can be

approximated with some major cost element such as direct labor time.

Most progress of all has taken place with design for assembly (DFA).

Providing an evaluation of assembly designs is somewhat simpler than

EVALUATING DESIGN PROPOSALS
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evaluating the manufacturability of individual parts, where such factors as

tooling cost, yield, and production quantity all weigh more heavily than they

do with assemblies. Assembly evaluation systems can provide a rapid and

easy comparison between several alternatives. Common measures, in

addition to cost, are parts count, design efficiency, and assembly time.

Direct labor time is a straightforward indicator of manufacturing cost and is

usable by itself in a large number of cases. (Exceptions are those in which

materials costs,

labor rates, and overhead costs also vary significantly with different design

variations.) Therefore, in many cases, manufacturability of a series of design

choices can be evaluated by estimating and comparing the direct labor time

required for production of each design. Eventually, however, a full cost

estimate is the ultimate guide to the designer in knowing how well the

product design has been engineered for manufacturability.

Conventional cost estimates are made by evaluating the materials content of

a design and the labor content of the production operations involved. This is

a valid and accurate way to estimate the manufacturing cost of a particular

design, and hence its manufacturability, although it may be time-consuming.

The time element can be reduced by using computer assistance.

7.2.ASSEMBLY EVALUATION SYSTEMS

What are most interesting and useful are cost-estimating computer programs

developed specifically for DFM use. The longest-standingand in many ways

the most useful for DFMare those applicable to assembly evaluation.

It should be noted that current programs evaluate the labor content of an

assembly design, not the materials costs. Sometimes there are tradeoffs

between materials and labor costs of design alternatives. For example, a

complex part made by combining several simpler parts will reduce assembly

costs, but the cost of the complex part could conceivably be higher than the

cost of several simple parts. Fortunately, however, materials costs are easy

and straightforward to estimate from per-pound or per-square-foot data.

Materials cost differences can be combined with the labor cost differences of

alternative designs to arrive at a more nearly total cost comparison. The

programs may give a design efficiency rating, a ratio comparing the


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calculated assembly time with a theoretical ideal for the number of parts

involved.

There is one other quite useful method to evaluate the manufacturability of

assemblies. This is simply to count the number of parts that the design

entails. Assemblies with fewer parts normally can be assembled in less time

and have higher design efficiency ratings.

One powerful advantage of these computer programs and the parts-count

approach is that they can be used by designers themselves. They provide an

easy way for designers to gauge the effectiveness of their efforts. They help

to eliminate we versus they feelings that can arise when manufacturing

people are doing the evaluation and pressing the designer to simplify the

designs. The programs also have guideline information implicit in their tables

of time data in that the designers, in working to improve the rating of their

designs, will apply guidelines that promote that objective. In this sense, they

are also useful in training designers in principles applicable to better, more

easily assembled products.

7.3. MANUFACTURABILITY EVALUATIONS OF INDIVIDUAL

PARTS

One simple way to compare the manufacturability of alternative designs of a

part is to count the number of process operations that each requires. Other

factors being equal, the part with the fewest number of operations will be

the simplest to manufacture and the lowest in cost. Of course, tooling

complexity and materials cost often must be con-

sidered also. Nonetheless, this metric is often a useful one for comparingparts from a DFM standpoint.

Some companies have developed systems to facilitate the manufacturability

evaluation of piece parts in a product. They are somewhat more complex

than the assembly systems described earlier in that there are separate

methods for each manufacturing process involved. For example, die castings,

injection-molded plastic parts, machine parts, powder-metal parts, and metal

stampings each are evaluated with separate systems, since design principles,

rules of thumb, and manufacturing costs are different for each process.

Current systems simplify and ease the task of making an estimate of the


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manufacturing cost of a part. They consider tooling cost and amortization,

process labor, and materials costs. As in the case of assembly evaluation

systems, comparisons can be made for different design concepts. Some

systems are computerized and are programmed to request the input data

needed to develop a cost estimate.

7.4. EVALUATING DFX ATTRIBUTES

All the systems described earlier deal with manufacturability and not the

other DFX attributes that are discussed in Chap. 9.2. Evaluating designs for

DFX requires different, more complex methods such as the following:

1. The attribute being evaluated can be expressed in terms of cost or some

other monetary factor. This is difficult because of the intangible nature ofmany of the DFX attributes. The most intangible factor is the financial benefit

that can accrue when a company incorporates desirable attributes into a

product and gains additional sales, larger profit margins, or both. For

instance, speed to market is emphasized in order to enhance product sales.

How much additional profit margin can be generated if the product

realization time is reduced by 3 months? Providing an accurate estimate for

such a not easily predicted factor is difficult but remains a possible avenuefor someone establishing an evaluation system for improved speed to market.

Similarly, estimated cost or profit amounts could be related to different

design concepts when evaluating a design for such attributes as safety,

serviceability, reliability, etc.

2. Use a scoring system that rates or ranks design alternatives against some

criterion. Ideally, such a system should provide a design efficiency or other

numerical rating of the extent of the attribute. Normally, systems of this type

must be somewhat generalized and rely quite heavily on the experience,

knowledge, and judgment of the person making the evaluation.

3. Test the design. This involves making at least one prototype of the

proposed product and subjecting it to whatever tests are appropriate for the

objectives being evaluated. For example, if reliability is one of the design

objectives, life tests are called for; if user friendliness is an objective, anumber of user tests of the product with feedback information from the test

users is needed.
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Almost all DFM and DFX guidelines are still qualitative in nature and often

conflicting. It would be ideal if the effect of any one design alternative,

considered with respect to some DFX recommendation, could be evaluated.

There is one way that the DFX guidelines can be related to cost and thereby

given quantitative evaluation. This is through use of the life-cycle cost

conceptTaguchis concept of overall product quality (see definition in Chap.

9.3). The lower the life-cycle cost for such factors as service, safety, the repair

of quality defects, and so on, as well as the initial cost, the better is the DFX

performance. As noted earlier, however,

many of the life-cycle cost factors are highly intangible and not well suited to

quantification. How, for example, can one predict the cost (or profit effect) of

sales that are lost due to a poor reliability reputation? How can one predict

the cost of product liability lawsuits resulting from safety defects? Broad

overall projections of such costs may be possible, but relating them to

specific design changes such as changing a sharp corner in a part to a

radiused corner (sometimes a safety and sometimes a product reliability

factor) is not really feasible. Even calculating the manufacturing cost effect of

such a change may be somewhat lengthy and uncertain.

7.5. WHO SHOULD MAKE THE EVALUATION?

The best approach, when possible, is to have the designers themselves make

the evaluation. This certainly has speed and accuracy advantages in that

there is no need to transfer information about the design from one person to

another. The time to prepare documentation and to explain the design

concept to the specialist is avoided. One of the advantages of some of the

current evaluation systems, particularly those involving assembly or other

aspects of manufacturability, is that they make it relatively easy for the

designers themselves to carry out the evaluation. In addition to the

convenience and time advantages of this, there is also the learning factor

that benefits the designer. Designers who conduct an evaluation with a

prepared system tend to learn the design principles that underlie the

system.

7.6. TESTING DESIGN PROPOSALS

Prototypes should be subjected to a variety of tests:
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1. Use tests to verify that the product functions as it was designed to.

2. Life testing is important to determine the reliability and useful life of the

product and its components.

3. Environmental testing, usually at various temperatures, humidities, and

other conditions, confirms the products performance under any extreme

conditions that the product may face in use.

4. Field tests help to confirm the successful operation of the product under

customeruse conditions. Another purpose of field tests is to verify that

customers will understand and be able to use the product easily and as

intended.

5. Shipping tests help to verify the effectiveness of packaging and the

sturdiness of the product.

Good testing is a powerful and essential step in perfecting the design and in

ensuring that it meets the varied objectives of the program.

Citation

James G.Bralla: Design for Manufacturability Handbook, Second Edition. EVALUATING

DESIGN PROPOSALS, Chapter (McGraw-Hill Professional, 1999, 1986),

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