Back to basics in the workshop
1 Calibration (zeroing)
You’ll need the ‘C’ adjusting spanner, supplied in the case. If you look at the micrometer’s sleeve, you’ll find a small hole into which the pin is engaged. Rotate the sleeve until the 0 on the thimble aligns with the datum line.
2 Larger micrometers
Most micrometers measure within the 25mm or 1” range so when zeroing larger micrometers it’s necessary to use a setting gauge, which will be in multiples of 25mm or 1”. Measure the gauge as you would a component and zero.
3 How tight is tight?
A micrometer is not a G-clamp. You only need a light pressure. Most micrometers have a ratchet or friction drive to standardise clamping pressure to that when zeroing, but if you get a feel for it, directly turning the thimble is fine.
4 Using the lock
It’s not always necessary to use the lock, either via a lever or a ring that’s rotated, but it will prevent accidental movement of the thimble after taking a measurement. After locking, check that the reading hasn’t moved from before.
5 Imperial micrometers
Each thimble division is 0.001”, a thousandth of an inch, or a thou. One whole turn from zero, therefore, is 0.025” or 25 thou, thus marked. After 4 of these divisions, there’s a 1, denoting 0.10” or a hundredth of an inch, or 100 thou.
6 Measuring the shim
The shim we measured at 2.68mm turns out to be quite a simple reading. There are four divisions of 0.025”, or 25 thou, visible and it’s also possible to see the 1, indicating that we have 100 thou. Add the 5 on the thimble and we get 0.105”.
7 Metric vernier mic
A standard metric mic is accurate down to 0.01mm; a vernier mic is accurate to 0.001 or 0.002mm. The mic is read as a standard one, but whichever numbered line aligns with a mark on the thimble provides the extra decimal place.
8 Imperial vernier mic
An imperial vernier mic measures down to 0.0001” or a ten-thousandth of an inch instead of 0.001” (one-thou). The sleeve scale is read first, 0.125”, then the thimble, 0.006”, giving 0.131”, plus 0.0006” from the vernier scale: 0.1316”.
9 Other mic types
Once you’ve mastered metric and/or imperial micrometer reading, you can translate that to other types of measuring tool, such as this depth gauge. Practice by measuring things at random to build up muscle-memory and reading skills.
10 Calipers vs micrometers
A standard mic is able to measure down to 0.01 or even 0.001mm whereas a vernier caliper is only accurate to 0.02 or 0.05mm. Calipers are great for quickly measuring bolt diameters and lengths, internal diameters, hole depths, etc.
11 Know your calipers
The outside jaws A are for external measurements, like bolts, and the internal jaws B are for internal measurements, like holes. The moveable jaws C are moved by the thumb grip/wheel D, which also extends the depth rod/blade E.
12 How to use calipers
Make sure any locking screws are loose so the jaws move easily. Use the thumb grip to open and close the jaws to touch the item to be measured. Use the lock if necessary. When using the depth rod, ensure the scalloped end is bottoming out fully.
13 Reading a vernier caliper
This caliper has metric (top) and imperial (bottom) scales. The main metric scale is accurate to 1mm, the 0 mark on the top sliding jaw aligning with whole mm. You can see here that the 0 mark is aligned with something over 2.5mm but when reading the vernier (or Nonius) scale we can be more accurate. The only line on the vernier scale that lines up with a line on the main scale is after the 6, so the shim is at least 2.6mm. Each interval is 0.02mm so three makes 0.06mm, making this shim 2.66mm
15 Feeler gauges
These are thin blades of different thicknesses, marked on each one. Ideal for use when setting valve clearances, piston ring gaps and contactbreaker points. Correct measurements are taken when a gentle drag of the blade is felt.
16 Dial test indicator (DTI)
DTIS are used to measure movement, such as end-float (back and forth) in shafts, and deflection (wobble) in brake discs, etc. As the plunger is depressed, the needle moves. Each graduation corresponds to either 0.01mm or 0.001”.
14 Imperial vernier caliper
The imperial scale readings are similar to the metric’s. Each graduation on the lower main scale is 0.025”, the 0 aligning just after 0.100”. Looking at the vernier scale, it’s clear that only the 5 line aligns with a main scale line, so it’s 0.105”.
17 Accessibility & Digital
Digital mics, calipers and DTIS are getting cheaper and allow swapping between metric and imperial readouts, and may be more accessible for people with restricted vision. Left-hand mics and calipers are available, but at a premium.