Introduction:
In automotive engineering, measuring tools are widely used, for example during an engine inspection. But measuring tools are also used to measure the thickness of the brake linings or the brake disc. In order to perform a measurement, it is important to know what measuring accuracy the tool has. With the inside jaws of a vernier caliper, the cylinder diameter can be measured, but it is not accurate enough (1/20 mm). A dial gauge is much more accurate (1/100 mm).
The most common measuring tools in the workshop and their accuracy are:
- Vernier caliper (0.05 mm, which is the same as 1/20 mm.)
- Micrometer (0.01 mm, i.e. 1/100 mm.)
- Dial gauge (0.01 mm.)
- Feeler gauge (0.05 mm.)
- Plastigage (accuracy depends on the version).
This page explains how to set, read and if necessary calibrate the aforementioned measuring tools, and provides examples of measurements.
Vernier caliper:
The vernier caliper is a widely used measuring tool in automotive engineering. With the vernier caliper, the internal, external and depth dimensions of a component can be measured accurately to one‑twentieth of a millimetre.
Measuring with the fixed jaw:
By clamping the component in the fixed measuring jaw, the size can be read. The scale now shows 20 mm. This is the outside diameter of the ring.
Measuring with the inside jaws:
By clamping the jaws on the inside of the ring, the inside diameter can be read. This is 18 mm. That means that the ring is (20-18) = 2 mm thick.
Measuring with the depth gauge:
For example, for objects that cannot be removed from the surface, or cylinders with a bottom, the height can be measured using the depth gauge. By placing the end of the depth gauge on the surface and the thick part of the caliper on the component, its height can be determined. In this case, the height of the black block is being determined:
To read a vernier caliper, you must also look at the tenths of a millimetre. The position where the next line on the vernier exactly coincides with the line on the main scale indicates the tenths of a millimetre (the digit after the decimal point). In the image, the 0 on the vernier is at 1.1 cm, so 11 mm, on the main scale. The line for the number 10 on the vernier also coincides with the line on the main scale. That means that exactly 11.0 mm is being measured.
In the next measurement, the vernier has shifted slightly to the left and we are dealing with a digit after the decimal point. We look at the position where the next line on the vernier exactly coincides with the line on the main scale. In the image, the 0 on the vernier is at 1.1 cm, so 11 (whole) millimetres. The line for the number 9 on the vernier also coincides with the line on the main scale. That means that exactly 10.9 mm is being measured.
The measurement in the image works according to the same principle. In this case, the 0 on the vernier is halfway between 15 and 16 mm on the main scale. You already know that the decimal value must be around 4, 5 or 6. The lines of the main scale and the vernier coincide at 5; so (15+0.5) = 15.5 mm is being measured.
There are also small lines between the numbers on the vernier. These indicate five‑hundredths of a millimetre. The line between 0 and 1 on the vernier coincides with the line on the main scale. In the image, (10 + 0.05) = 10.05 mm is being measured. Reading to five‑hundredths requires a trained eye.
In this animation, the reading of the vernier is clarified with red arrows.
A vernier caliper can also be digital, as can be seen in the image. The dimensions of the component being measured can be read on the digital display. This can often be set to both inches and millimetres.
There are also calipers with an analogue dial in the place where the digital display is shown in the image above. This type of caliper is not used very often, but it depends on what the user prefers.
Micrometer:
The micrometer (also called screw gauge or outside micrometer) can be used to measure components up to 25 mm in size with an accuracy of one‑hundredth of a millimetre (0.01 mm). With one revolution of the thimble, the spindle moves 0.5 mm.
The micrometer must always be held by the insulated handle, because the heat from the hands affects the measurement result. Local heating in the micrometer can cause the material to expand slightly. Especially for a measurement where the result must be accurate to a hundredth, it is important to follow the instructions.
The component to be measured must be placed between the anvil and the spindle. Turning the thimble moves the spindle back and forth. Before the spindle touches the component, the last part of the distance must be taken up using the ratchet. The ratchet contains a click mechanism that produces a clicking sound when a certain force is applied. At that moment, you know that you must not tighten the micrometer any further. If the micrometer is tightened too much, you may obtain incorrect measuring results. The thimble can be locked with the locking lever to prevent further turning.
Below is an image of a micrometer measuring the size of a ball bearing (the measuring object).
In the image above, the ball bearing has a thickness of 13.43 mm. On the upper scale you see 10, with 3 lines next to it. Each line is one millimetre, so 10+3=13 mm. The digit after the decimal point is read on the thimble. Here, the numbers 40 and 45 are shown. If you look closely, you can see that the line on the scale coincides with 43. Together, this makes 13.43 mm.
The thimble has a scale from 0.0 to 0.49 mm. This is because the main scale with the whole millimetres (to the left of the thimble) also contains half millimetres; the lower lines indicate the half millimetres. Several examples are given below.
On the horizontal line, the whole millimetres are shown. In this case, that is 13 mm. The 16 mark on the thimble coincides with the horizontal line on the sleeve. The size indicated in this image is (13 + 0.16) = 13.16 mm.
In the image, the line below the line of the scale on the sleeve is visible. This line below the horizontal line indicates that it is a half millimetre. According to the scale, it is therefore at least 5.5 millimetres (without including the thimble). The scale on the thimble shows 36. The total size now indicated is (5.5 + 0.36) = 5.86 mm.
In this image, the line at the bottom of the scale is again closest to the thimble. According to the horizontal scale, it is therefore at least 12.5 mm again. Then we add the reading from the thimble; this value is 0.35 mm. Next, we add 12.5 and 0.35 together.
This gives a total of (12.5 + 0.35) = 12.85 mm.
In this image, the indicated size is (16 + 0.355) = 16.355 mm.
The image shows the micrometer reading a value of 75.235 mm. The scale on the thimble is between 23 and 24 mm. Because the gauge block is 75 mm, the micrometer deviates by 0.235 mm. Every measurement taken will therefore be too high. Using a special adjustment wrench, the sleeve must be rotated relative to the handle. The adjustment wrench can be seen in the image above.
Before measurements are taken with the micrometer, it must first be calibrated. Incorrect calibration leads to measuring errors! Calibrating the micrometer is performed using a special gauge block. The gauge block in the image below is exactly 75.00 mm. That means that when the micrometer measures the gauge block, the micrometer must indicate this value exactly. If the reading is incorrect, we must first calibrate the micrometer by turning the inner sleeve with the wrench.
Dial gauge:
With a dial gauge, extremely accurate depth measurements can be carried out. The small hand on the inside indicates the whole millimetres and the large hand indicates the digit after the decimal point. When the dial gauge is standing on a flat surface, it must indicate 0.00 mm, as shown in the image below. The outer ring can be rotated to allow calibration. If, for example, it reads 0.3 mm when it is on a flat surface, the outer ring must be rotated so that the large hand indicates 0.
The dial gauge in the image shows 5.00 mm. The small hand is on 5 and the large hand on 0. If the large hand were on 81 and the small hand between 5 and 6, the gauge would show a value of 5.81 mm. The further the plunger at the bottom is pushed upwards, the smaller the value that will be read.
The dial on the micrometer reads: 0.01 – 10 mm. That means that the micrometer can indicate a value between 0.01 and 10 mm. A depth measurement where the depth is 12 mm cannot be carried out, because the plunger is too short and the hands cannot indicate that. To still be able to measure values greater than 10 mm, various extension pieces are supplied with the micrometer. An example of this can be seen in the image. The extension piece is being measured here with a micrometer. It shows a value of 10.0 mm.
Only the barrel-shaped part is measured here, not the thread. By mounting this extension piece on the micrometer, the plunger is no longer too short. A value of, for example, 12 mm can now still be measured. Care must be taken to add the size of the extension piece to the measured value. Here is an example: when the micrometer indicates a value of 5.19 mm, the actual size is the measured value + the length of the plunger, so 5.19 + 10.00 = 15.19 mm.
On these pages, measurements with the dial gauge are carried out:
Feeler gauge:
The feeler gauge is used to measure the clearance between two components. The feeler gauge consists of a number of metal strips, each with a different thickness. The thickness is indicated on each metal strip. On the lowest strip of the feeler gauge in the image below, it says “30”. That means that the metal strip is 0.30 mm thick.
To measure the space between two parts, any metal strip should be unfolded and inserted between the parts. If the strip can be moved through very easily or even without any resistance, then the clearance is greater than the thickness of the strip. A thicker metal strip must then be unfolded. If the strip no longer fits in between, then this strip is too thick. When the strip can be slid between the components with some resistance, that is the correct size.
In the following image, the piston ring end gap is being measured.
On these pages, measurements are taken with the feeler gauge:
Plastigage:
Plastigage can be used to check the clearance between plain bearings. Plastigage is a special plastic wire that must be applied to the part between which the clearance is to be measured. The bearing cap must then be tightened so that the plastigage is squashed flat. The deformation of the plastigage is a measure of the clearance.
There are different colours of plastigage. Each colour represents a different size.
- Green: for a bearing clearance of 0.025 to 0.076 mm.
- Red: 0.050 – 0.150 mm.
- Blue: 0.102 – 0.229 mm.
- Yellow: 0.23 – 0.51 mm.
On this page, a measurement is taken with plastigage:
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