Metal and Engineering
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Determine work requirements
Systems of measurement
There are two common systems of measurement used in the manufacturing, engineering and related services industries. They are commonly called the metric system and the imperial system. The metric system is sometimes called the SI (Systems International). The imperial system is sometimes called the British system of measurement.
The metric system is used in most parts of the world. The imperial system is still used in the USA. The USA is the biggest economy in the world and at this stage has no intention of “going metric”. As such, you need to be familiar with both systems.
Estimation
It is very important for all people involved in the manufacturing, engineering and related services industries to be able to estimate answers to any calculations they need. They need to know an estimate of the expected answer, before they start plugging values into a calculator. It is very easy to make a simple calculation error, so they need to have an approximate idea of the final answer.
Tolerances and limits
All engineering components are made to specific limits. Components, generally, cannot be made to an exact dimension. For example, if a component is to be made to a length of 100mm, it will almost always never be exactly 100 mm. The engineer will specify the limits applicable to the size. For example, 100mm + 0.5mm. In other words, as long as the component is between 99.5 mm (100 – 0.5) and 100.5mm (100 + 0.5) then the component will be able to do the job it has been designed to do.
Engineering drawings of components should have tolerances included in their dimensions. It is impossible to make any component to an exact size. Tolerances indicate the degree of accuracy allowable in the final component. Often the finish on a component is critical to it having a long service life. Tolerances may need to be within a few microns (thousandths of a millimetre). Otherwise, the component may prematurely wear out its matching component.
Rounding off
“Rounding off” is used when doing estimations. For example, to get an estimation of say, 671 x 792, you would “round off” 671 to 700 and 792 to 800. This would give you an “estimate” of 700 x 800 = 560,000 which is fairly close to the exact answer of 531,432.
Sourcing data
All employees in the manufacturing, engineering and related services industries will need to source data/information when they are asked to manufacture a component. There are many sources of information, but usually, the engineering drawing is the starting point, when making a component. After examining the engineering drawing, you may need to go to diagrams, graphs, charts, reference manuals and specifications so you can make the component.
For example, the engineering drawing may refer to a special type of steel to be used, to make the component. You may need to refer to reference manuals, to get information on the properties of the steel, so you can specify the welding techniques needed, when joining the steel to other components.
Units of measurement
There are many units of measurement that are used to describe quantities.
For example, the length quantity can be measured in millimetres, centimetres, metres, inches, feet, yards etc. The choice of the unit of measurement to use depends on many factors, including the system of measurement (metric or imperial), convention, size of the component and typical units of measurement used by suppliers, etc.
It is accepted practice in Australia for all engineering drawings to use millimetres as their primary unit of measurement. These measurements are given to the nearest tenth of a millimetre, hundredths of a millimetre, micron (thousandths of a millimetre) etc, depending on the degree of accuracy required by the customer. This contrasts with the imperial system, where fractions of an inch are often used for measurements between whole inches (the imperial system does use the decimal system when higher degrees of accuracy are required).
It may be necessary to switch between the two systems of measurement. The engineering drawing may use the imperial system of measurement but the machine/s being used to make the component may have been calibrated using the metric system.
Some common conversions are:
| Imperial measurement | Equivalent metric measurement |
|---|---|
| 1 inch (written as 1”) | 25.4 mm |
| 1 foot (written as 1’) | 3048 mm |
Remember that these are a close approximation, as it is often impossible to get an exact conversion.
Metric and imperial units of measurement
There is a large number of units of measurement used to describe quantities. The table below gives some of the quantities used in the imperial and metric systems, with an example using accepted abbreviations.
| Quantity | Typical metric units | Typical imperial units |
|---|---|---|
| Length | millimetre - 9 mm | inch – 5in. or 5” |
| metre - 12 m | feet – 4ft. or 4’ | |
| Area | square millimetre - 7 mm2* | square inch – 9sq.in or 9 in2 |
| square metre - 11 m2 | square feet – 24sq.ft or 24 ft2 | |
| Volume | cubic millimetre – 4 mm3** | cubic inch – 25cub.in or 25 in3 |
| cubic metre – 45 m3 | cubic feet – 12cub.ft or 12 ft3 | |
| Capacity | millilitre – 32 ml | fluid ounce – 56 fl oz |
| litre – 45 l | pint – 8 pt | |
| kilolitre – 22 kl | gallon – 24 gal | |
| Mass/weight | milligram – 15 mg | ounce – 5 oz |
| gram – 28 g | pound – 45 lb | |
| kilogram – 5 kg | ton – 12 ton | |
| tonne – 27 t | ||
* pronounced square millimetres NOT millimetres squared
** pronounced cubic millimetres NOT millimetres cubed
Need for consistency
When performing calculations with units it is important to remain consistent.
For example, when doing area calculations, such as finding the area of a rectangle, if the length of one side is given as 56 mm and the other side is given as 7.4 cm, the first thing to do is to convert one side to the same unit as the other side.
56 mm = 5.6 cm
7.4 cm = 74 mm
Either the area = 56 mm x 74 mm = 4144 mm2
or
The area = 5.6 cm x 7.4 cm = 41.44 cm2
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