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Full text of "Modern Mechanical Engineering Vol-I"

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Taking 100 as the index figure for a normal, plain, and straightforward
job, it was found that:

Firm " A " demanded a high finish, the use of brass where the normal
practice was cast iron, and many refinements and specialities. It was
found in this case that almost 50 per cent had to be added to the esti-
mated cost for the normal job.

Firm " B ", on the other hand, insisted not so much on specialities and
finish as on weight. Shafts had to be 20 per cent over Board of Trade
requirements; in approving drawings, thicknesses were generally increased,
resulting usually in a net increase of weight of almost 10 per cent.

Firm " C " got on the whole a very plain job, but the job he finally got
was seldom the job he originally ordered. It was usual for this firm to have
several ships building in different yards, and changes in the job were con-
stantly being made, and were a source of vexation and annoyance and extra
expense. Further, the changes involved extra cost, but it was very difficult
indeed to get these extra expenses paid. As the purchaser was a good
customer it was not considered politic to insist always on payment for these
extras, so that an extra 3 per cent was generally added to the estimated
price to cover these anticipated vagaries. This may not seem to have much
to do with scientific estimating, but it led to the erection of charts based
on pounds per indicated horse-power which looked something like the











3,000      4pOO

6,000     7,000      SjOOO      9,000     10,000     4000    8,000     0,000    MOOO

Fig. i.  Chart of Costs per I.H.P.

This chart shows a method of checking the detailed estimates.   It is
largely based on estimates received from sub-contractors, i.e. prices based