Until now, we have been concerned with generating ideas for possible design proposals and we have looked in detail at one or two methods which help us to produce design concepts. It is now time to consider ways in which we can evaluate and so select the best of these solution variants.
Question 2.2.2
List the possible energy sources that might be used to provide the input power for the aileron system described earlier. Also give solution proposals for the possible types of energy provider, that might be used to power the system.
Figure 2.2.2
Figure 2.2.3
The evaluation matrix
One of the most common methods is to produce an evaluation matrix, where each solution concept is set against a list of selection criteria. For each criteria some kind of scoring system is used to indicate how the individual design concept compares with an agreed norm. This process is illustrated in Figure 2.2.4.
Prior to inclusion in the evaluation matrix, if there are a large number of solution proposals, the design engineer should produce some form of pre-selection procedure, in order to reduce the proposals to a manageable size. The pre-selection process being based on fundamental criteria which the design proposal must meet. Such criteria might include: compati-bility with required task, meets the demands of the design specification, and feasibility in respect of performance, etc. meet mandatory safety requirements and expected to be within agreed costs.
The concept proposals should be in the form of sketches together with a short written explanation. Equality with the agreed norm is shown in the skeleton matrix by the letter ‘E’, if the design solution is considered better than the norm, in some way, then a sign is used, conversely a sign is used, if the design solution is worse than the norm, in some way. A score may be obtained by allocating a 1 to the positives, 1 to the negatives, and 0 to the Es. More sophisticated scoring systems may be used involving ‘weightings’, when the selection criteria are not con-sidered to be of equal importance.
Figure 2.2.4
Example 2.2.2
Assume that you are to manufacture a large diameter flywheel, for a heavy pressing machine, at minimum cost:
(i) Write a short list of selection criteria against which the given design solutions can be evaluated.
(ii) Produce an evaluation matrix using the ‘casting method’
of manufacture as your norm, and rank all the remaining design solutions.
Figure 2.2.5 shows the given design solutions, we now need to produce our evaluation criteria. We could use one or more of the previous methods, to generate ideas. However, for the purpose of this example, since cost is of paramount importance, we will just look closely at the manufacturing methods which minimise cost. We will assume that the requirements of the specification have been met and that all design alternatives are compatible for use with the pressing machine under all operating conditions.
Then for each of the design options, we need to consider:
● materials costs;
● skill and amount of labour required;
Figure 2.2.5
● complexity of construction;
● tooling costs;
● machining and finishing costs;
● safety (this will be related to the integrity of the design solution assuming it is chosen);
● amount of waste generated;
● company preference – knowledge, skills, and equipment.
The above list of criteria is not exhaustive, but should enable us to select one or two preferred design alternatives. Further refinements/criteria may be necessary if two or more concepts are closely ranked.
Scoring
Figure 2.2.6 shows the completed evaluation matrix for this problem. You will note that the company preference, immedi-ately skews the scores. The company does not have or does not wish to use foundry facilities; in any case, the production of the mould would be prohibitively expensive for what appears to be a ‘one-off’ job. Obviously you, as the design engineer, would be aware of these facts before evaluating the options.
Note that options 2–6 all involve some form of fabrication, assembly or machining, which we will assume is the company preference.
Proposal 2: Machining parts for a heavy flywheel will require several machining operations and the use of elaborate fixtures, not to mention operator skill
Figure 2.2.6
Costing
The cost of an engineering component or system is of paramount import-ance. Engineering designs require the specification to be met, the artefact to be produced on-time and at the right cost, if the design solution is to be successful. Thus an understanding of costs and costing procedures is something that every design engineer needs to achieve. A detailed expos-ition on costs and costing methods is given in Chapter 1, Business Management Techniques. Set out below are one or two important points concerning costing, directly related to the production of an engineering artefact.
Importance of costing and pricing
The importance of producing a successful tender cannot be overempha-sised, in fact the future of jobs within the company may depend upon it.
for an object of such size; so labour, tooling, and machining costs are relatively high. This process also involves a large amount of material waste.
Proposal 3: The major advantage of this method is that stand-ard stock materials can be used. Difficulties include: the use of jigs and fixtures, weld decay, and possibility of complicated heat treatments.
Proposal 4: Similar advantages and disadvantages to option 3.
Proposal 5: Advantages include use of standard stock materials, little machining required after assem-bly, relatively easy to assemble. Disadvantages include necessity for positive locking of bolts after assembly and outer rim would require skim-ming after spinning.
Proposal 6: Labour intensive fabrication and assembly, com-plex assembly, and integrity of construction would raise safety issue. No specialist tooling required, finishing relatively easy and cheap, minimal waste from each machining operation, company preference.
Note that if company preference had been for casting, then option 1 would probably have been preferable, provided it met the cost requirements. Options 5 and 6 appear next to favourite, although option 3 might also be worth looking at again, dependent on the skills of the labour force.
If there was insufficient evidence on which to make a deci-sion, then more selection criteria would need to be considered.
For example, do the options just meet or exceed the design specification, bursting speeds, and other safety criteria might have to be further investigated. This process would need to be adopted no matter what the artefact, an iterative approach being adopted, in an attempt to get ever closer to the optimum design solution.
To ensure that a commercial contract to design, manufacture, and supply on time is won; there must be an effective costing and pricing policy.
Not only must the contract be won, against competition, but a profit margin needs to be shown. Price fixing needs careful planning, clearly too high a price may not result in a successful tender and too low a price may cause financial loss to the company, particularly if there are unforeseen difficulties.
For profit, and as a ‘useful rule of thumb’, we should know our costs to within 2%. The tolerable margin between maximum and minimum prices is small, thus the necessity for design and costing accuracy.
Some important general reasons for costing are to:
● determine the viability of a proposed business venture;
● monitor company performance;
● forecast future prospects of a business deal;
● price, products, and/or services;
● meet legal requirements to produce records of company viability, for public scrutiny as required.
Below are some useful definitions concerned with cost and price:
Price: money paid for products or services.
Value: the amount of money someone is prepared to pay for products or services.
Cost: all money spent by a supplier to produce goods and services.
Material cost: (volume density cost/kg) plus an amount for wastage.
Labour cost: (operational time labour rate) plus wasted labour time which is not directly related to the task.
Standard costing sheets
These are used to ensure that all parameters are considered when costing a product or service. Some standard costing sheet headings together with their definition, are given here:
A: Direct material cost: raw material and bought-in costs.
B: Direct material scrap: materials subsequently scrapped (typically 3–5% of A).
C: Direct labour cost: wages of production operations, including all incentive payments.
D: Direct labour scrap: time spent and paid for on artefacts, which are subsequently scrapped, this would include the costs of machine breakdown or other reasons for stoppages to production (typically 3–5% of C).
E: Prime cost: the sum of all material and labour costs that is A B C D.
F: Variable overheads: cost of overheads which vary with rate of production, these might include: fuels costs, cost of power sup-plied, consumables, etc. (typically 75–80% of C).
G: Manufacturing cost: this is the sum of prime costs and variable overheads (E F).
H, I, J: These are packaging, tooling, and freight costs, respectively.
K: Variable cost (VC) this is the sum of the previous costs, G H I J.
L: Fixed overheads (FO): overheads which do not vary with produc-tion output, these include all indirect personnel not involved with production, marketing costs, research and development costs, equipment depreciation, premises costs (typically 30–40% of K).
M: Total cost (TC): the sum of all direct VCs (K) plus indirect costs (L).
Thus, TC direct VCs indirect costs (FO).
Example 2.2.3
A company has been commissioned to produce 2000 high-quality metal braided shower hoses, complete with fixtures and fittings. Assuming that:
(i) direct material cost per item is £1.50 and material scrap is estimated to be 3% of material costs;
(ii) direct labour costs total £8000 and the labour scrap rate is 4% of direct labour costs;
(iii) variable overheads are 75% of direct labour costs;
(iv) FO are 30% of VCs;
(v) packaging, tooling, and freight costs are 10% of manufac-turing costs.
Estimate the selling price of the shower hose, if the company wish to make a 25% profit.
This problem is easily solved by laying out the costing sheet as shown below, and totalling the amounts.
Cost Amount (£)
Direct material cost (1.5 2000) 3000
Direct material scrap (3% of 3000) 90
Direct labour cost 8000
Direct labour scrap (4% of 8000) 320
Prime cost 11 410
Variable overheads (75% of 8000) 6000
Manufacturing cost (prime variable) 17 410 Packaging, tooling, (10% of 17 410) 1741 and freight cost
VC (manufacturing 19 151
packaging, tooling, and freight)
FO (30% of 19 151) 5745
TC (variable FO) £24 896
Now company is required to make 30% profit.
So selling price per item is 896 7469
£16.18.
24 2000
Here is something to remember when considering costs: always design parts with the over-riding thought of saving money and do not forget that omitting all non-functional features and trimmings from parts saves pro-duction time!