Motorsport Fabrication Fundamentals: Measuring and Marking
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Measuring and Marking
09.31
| 00:00 | - One of the more common tasks we'll need to perform when designing and fabricating parts is measuring and marking out these parts. |
| 00:07 | There's a lot of different techniques and options at our disposal here depending on exactly what we're trying to do and how precise we need to be. |
| 00:15 | For example, if we're cutting a length of roll cage tubing, then it's probably sufficient to be within 1 mm of our target length. |
| 00:23 | On the other hand, if we're designing a part to be machined with a tight tolerance to a bearing then we may need to be within 100th of a mm or less instead. |
| 00:32 | Let's start by looking at the various methods available to us for measuring our materials and our components. |
| 00:38 | If you're measuring lengths of raw material then it's most likely that you'll start this process using a simple 6 metre or 8 metre tape measure. |
| 00:46 | For this purpose, it's usually sufficient to mark the desired length using a sharpie marker as this will give us a nice obvious mark to cut to. |
| 00:54 | A tape measure is a deceptively simple measuring tool but there's a small trick to understand. |
| 01:00 | The metal tang on the end of the tape is actually slotted which allows a small amount of movement. |
| 01:06 | The reason for this is that the amount of movement corresponds to the thickness of the metal tang. |
| 01:10 | Why this is important is it allows both inside and outside measurements to be made accurately. |
| 01:17 | For example, if you're measuring a length of tube then you would hook the metal tang over the end of the tube and pull the tape tight so that the tang is at full extension. |
| 01:26 | On the other hand, if you wanted to measure the inside length of a component then you would push the tape into the part you're measuring so that the tang becomes fully compressed. |
| 01:36 | If you don't understand this, then it's easy to add a very small amount of error to your measurements. |
| 01:42 | Let's skip ahead and look at steel rulers which we often use for more accurate measurements as well as giving us the benefit of being able to mark out straight lines. |
| 01:51 | The two common lengths we rely on are the 300 mm and 150 mm rule and given the frequent use of the 150 mm ruler in particular most fabricators will keep one of these in their overalls or apron at all times. |
| 02:06 | If you're using the metric version of the steel ruler then it will have markings on one side every 1 mm however on the opposite side it will have markings every half mm which is about as accurate as we're ever going to be able to work to with a steel ruler. |
| 02:21 | One consideration when using a ruler is the effect of parallax error. |
| 02:25 | This is where an error in the perceived measurement can be introduced depending on how the ruler is viewed. |
| 02:32 | Admittedly this is a minor issue with a steel ruler, given the fact it's only around 1 mm thick. |
| 02:38 | However it's always good practice to view the markings on the ruler from directly above. |
| 02:42 | If we view the markings on the ruler from a significant angle then this can give us a false measurement of 1 mm or more. |
| 02:50 | As our required accuracy increases, the next tool we rely on is the vernier calliper. |
| 02:55 | This is another one of those must have tools that we suggest you always have on hand and they're accurate down to 0.01 mm or 100th of a mm. |
| 03:04 | The term vernier calliper actually refers specifically to the non digital version of the calliper which includes a vernier scale for making measurements. |
| 03:13 | These days it's much more common that you'd be using what is technically referred to as a digital calliper, however these are still generically referred to often as verniers or digital verniers. |
| 03:25 | Irrespective which version of the calliper you're using, they're actually a pretty flexible tool. |
| 03:30 | First up we have the outside jaws which can be used to measure the outside diameter, length or thickness of a part. |
| 03:37 | Next we have the inside jaws which can be used to measure the inside of a slot, groove or hole. |
| 03:43 | At the end of the calliper we'll have a depth probe which can be used to measure the depth of a groove, step, slot or hole. |
| 03:50 | There'll be a visual ruler scale on the calliper however with a digital calliper, the ruler markings are only used as a quick reference since the digital read out gives us the accurate reading that we need. |
| 04:01 | Using digital callipers isn't difficult but there are a couple of points to look out for. |
| 04:05 | The most common one which should be the most obvious is to make sure that the read out is zeroed prior to taking a measurement. |
| 04:12 | In most instances we're going to want to zero the calliper with the jaws closed, however if you want to measure the difference between two parts then you can zero the calliper on one part, then measure the second. |
| 04:23 | The readout will now show the difference or delta in measurements. |
| 04:27 | It's also important to take a moment to ensure that the measuring surfaces are clean and free of dust, dirt or debris, prior to making your measurement. |
| 04:35 | This goes for the component that you're measuring too. |
| 04:38 | The last place where we can introduce an error in the measurement is by how firmly we tighten the calliper on our part. |
| 04:45 | For example, on this roll centre adjuster we can achieve a variation of around 300ths of a mm by varying the pressure. |
| 04:52 | With this in mind, the obvious question is how much pressure should we be applying and given the potential to introduce an error, it's a fair question. |
| 05:00 | This is a skill that will come with experience however in general what we want to do is tighten the calliper with light pressure so that there's a small amount of drag on the part as we move it through the jaws of the calliper. |
| 05:12 | If it's difficult to move or rotate the part between the jaws then too much pressure is being applied. |
| 05:19 | If you're measuring the outside diameter of a part then you'll want to make the measurement perpendicular to the part rather than along its length. |
| 05:25 | This ensures that we're measuring the part at its largest diameter. |
| 05:30 | Likewise if you're using the inside jaws to measure the inside diameter of a hole then it's important to move the calliper around a little to ensure that you have in fact settled on the largest diameter of that hole. |
| 05:41 | While the calliper is almost certainly accurate enough for the majority of general fabrication purposes, machining operations will likely require improved accuracy and this can be achieved with the use of a micrometer. |
| 05:53 | These will allow accurate and repeatable measurement down to 1000th of a mm or 10,000th of an inch. |
| 06:00 | As important as their fine resolution is, their ability to remove the human error element of over or under tightening the micrometer onto the part being measured is ideal for accuracy and consistency. |
| 06:13 | This is achieved thanks to a calibrated ratchet on the end of the micrometer, which achieves a consistent amount of clamp each time it's tightened. |
| 06:21 | Given the limited use in general fabrication, it's sufficient for our purposes here to simply know that the micrometer exists and we won't dive deeper into how to properly use and read the mic here. |
| 06:33 | Now that we've dealt with the various measuring tools we'll rely on, we can cover the options available for us marking our materials. |
| 06:40 | The good news here is that we're not going to need to break the bank on the consumables required but we will want to choose specific techniques depending on the task that we're working on. |
| 06:51 | Let's start with our usual tool of choice for basic drawing and design which is the humble pencil. |
| 06:57 | While there's nothing wrong with using a tried and proven wooden pencil, for a modest outlay, better consistency can be achieved with a mechanical or automatic pencil that has a replaceable graphite centre. |
| 07:08 | The advantage of this style of pencil is that they don't require sharpening and the lead or graphite is always the same diameter. |
| 07:16 | For fine detail drawing these are available with 0.3 to 0.5 mm lead. |
| 07:22 | One of the obvious advantages of using a pencil is the ease with which changes can be made to your drawing so the pencil goes hand in hand with a good quality eraser. |
| 07:32 | While the pencil is great for drawing on paper, its use is a little more limited on the material that we're actually going to work with. |
| 07:39 | Depending on the material we're working with, it can be difficult to easily make out a pencil mark and since it's not permanent it can be rubbed away during normal handling. |
| 07:49 | Our usual go to option for marking our material is a sharpie permanent marker with a fine or ultra fine tip. |
| 07:56 | These allow us to easily mark out our material ready for cutting and give a clear visual indicator of where the material needs to be cut. |
| 08:04 | There are two issues with the permanent marker though, firstly the thickness of the mark does introduce potential for error and secondly the ink doesn't handle heat well. |
| 08:13 | Let's deal with accuracy first. |
| 08:15 | Even the fine tip sharpie marker will leave a mark that's around 1.5 mm wide. |
| 08:20 | Understandably if you're working to a tolerance of maybe 0.5 mm then this would be unacceptable. |
| 08:27 | On the other hand, for cutting a raw length of roll cage tubing that's purposefully being left a touch long, then this is likely to be no issue at all. |
| 08:34 | The second problem is more relevant if you're wanting to cut a complex shape out of sheet metal, if you've marked this out with a sharpie then the heat introduced into the work piece during cutting will start to make the sharpie mark fade. |
| 08:47 | This can make it tricky to see the mark you're trying to follow and can introduce errors into the part. |
| 08:53 | In both scenarios the best solution is to switch to a scriber which allows us to scratch a sharp and very thin mark into the workpiece. |
| 09:01 | This reduces the tolerance error when cutting material to length and by combining the scriber with a steel ruler it's possible to mark out complex shapes with a permanent mark that will not be affected by heat. |
| 09:14 | A potential downside of the scribe is that if we're not accurate with our marking and place the scribe mark in the wrong place, this will leave a permanent mark that will affect the surface finish of the material and may remain visible depending on how the part is going to be finished. |
