TECH TALK
The art of measurement
An engineer will tell you that man has to measure up in order to make mass production possible. Jake Venter explains why
M
ass production of parts requires that millions of so-called identical items match each other as closely as possible – a feat which became possible only when computers were introduced to manufacturing processes a few decades ago. In earlier years, parts such as universal joints, piston and cylinder assemblies, as well as gudgeon pins, to name just a few, were assembled by means of colour, letter or numerical codes. The machining processes of the day produced parts whose dimensions varied in the last decimal. You could not just put any piston into any cylinder. Most motor manufacturers graded piston diameters and cylinder bores from A to F, with diameters increasing as they went down the alphabet. If you fitted an A-piston into an A-bore it would give the correct clearance, but an A-piston in a B-bore would have double the clearance (this was sometimes allowed). However, a B-piston fitted into an A-bore cylinder was never allowed – it would seize up. These days, most modern factories employ machining and quality control techniques that produce parts whose
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dimensions vary so little that selective assembly is no longer needed. The production of a modern engine may require as many as 200 000 separate measurements on the factory floor as well as by metrology technicians in the laboratory. Instruments used in the measuring process are calibrated and serviced either daily, weekly, or at some other period depending on how fragile the instrument is, as well as how critical the accuracy of the readings are. INCORRECT MEASUREMENT PROCEDURES If a metrology technician had to visit the average automotive workshop he would see one or more of the following measuring mistakes being made while using a precision measuring instrument: • Not cleaning the instrument before use. • Not checking it for damage. • Not checking the zero setting. • Measuring incorrectly by not holding the instrument in the correct position. • Taking only one measurement instead of taking at least five and then averaging all the readings. • Misunderstanding the scale and units. • Not being able to read the instrument correctly. MEASUREMENT ACCURACY When one has completed a measurement
one should be able to answer the following questions: • Do you know how accurate your measurement results are? • Is the measurement accurate enough? • How strongly do you trust the result? These factors should be at the back of your mind while you are doing the measuring. The questions can only be properly answered if you understand the difference between precision, accuracy and uncertainty. I’m using a dart-player’s results as an example. • Precision is about how close measurements are to each other. If the results of five throws are close together but far from the target then it is a high precision but low accuracy result. • Accuracy is about how close measurements are to the true answer. If the darts are close to the target but not close together the result is accurate but the precision is low. • If all the darts are close together and near the target the result is accurate and precise. The uncertainty of a measurement can be determined by answering the question; “How wrong could I have been?” A likely answer can be obtained by looking at the measurement mistakes mentioned earlier.
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