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Engine Building Fundamentals: Precision And Accuracy

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Precision And Accuracy

03.47

00:00 - Like any specialised task, building engines requires a range of special tools in order to do the job properly.
00:07 Some of the tools will be common to many automotive mechanical tasks, such as good quality sockets, while others, such as a ring file or piston ring compressor, are really only useful in the engine building profession.
00:21 In this module, I'll go over the common tools that you're likely to need.
00:26 I'll make the assumption here that you already have access to a good range of common mechanic's tools, and we'll only focus on the more specialised tools.
00:36 When it comes to measuring engine components and clearances, there are two terms you'll hear used, often interchangeably, by people who don't actually understand the meaning of the terms.
00:48 These terms are precision and accuracy, and before we talk about any measuring processes, we need to properly understand these terms, and how they apply to measuring engine components.
01:01 Let's start by discussing the term accuracy.
01:06 For our application, the definition of accuracy is the extent to which a given measurement agrees with the standard value for that measurement.
01:15 Sounds a little confusing, but in plain English that means, that if we measure the diameter of a piston, and get a result of 85.50 millimetres, but the actual diameter happens to be 90 millimetres, then our measurement is inaccurate.
01:32 On the other hand, the definition of precision, is the extent to which a given set of measurements of the same sample, agree with their mean.
01:42 Again, in plain English, this means that if we made five measurements of the piston skirt, and each independent measurement came out at 85.50 millimetres, then the tool we're using offers a high degree of precision.
01:58 So from these two definitions, we can see that precision and accuracy are actually very different, and completely independent of each other.
02:07 In our example, we had a tool that is highly precise, yet very inaccurate.
02:13 An easy analogy to consider, is that of archery, where we have a target set up, and we shoot several arrows at the target.
02:21 We'll look at four different examples here.
02:24 In the first example, the arrows strike the target randomly, and are spread across the target.
02:30 In this case we have low accuracy, as the mean position is not in the bullseye, and we also have low precision, because there's no repeatability between the different arrows.
02:42 In the next example, we see that now all of the arrows are spread evenly about the bullseye, giving us a high accuracy, however the individual arrows are still spread out, which means our precision is still low.
02:55 In our third example, we now have all of the individual arrows clustered very tightly together, which means we now have high precision, however, the arrows are still a long way from the bullseye, which means that our accuracy is low.
03:11 Finally, in the last example, we have all of our arrows grouped tightly together, and, this time they are also centred on the bullseye, this gives us high accuracy and high precision.
03:25 For our purposes when measuring engine components, we're generally talking about very small clearances, which dictate that we need to work with both high accuracy and high precision.