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. |
| 03:37 | To achieve this, we need specialised measuring tools, that tend to be relatively expensive. |
