There comes a point at which even the most finely divided ruler isn’t enough, and it’s at this point that fine measuring instruments enter the picture.
The most common of these are vernier calipers. They’re versatile, quick to use and can measure outside and internal sizes, steps and deep holes, and should be part of every tool kit.
While there are more accurate devices, verniers are invariably the first instrument mechanics grab when a quick measurement is needed — even in fine-tolerance engine building. For instance, if you wanted to confirm the basic size of a valve or check if a bore was standard or oversize, a vernier caliper would be the instrument of choice.
Available in manual and electronic versions, the digital type is easiest to use and becoming more common. These can be ‘zeroed’ at any point, which is useful for measuring relative sizes, clearances and the like.
Manual verniers are specified as being accurate to 0.001in but a well-practised eye can read closer than this. Some types have a shaft that telescopes from the end as the jaws opened. This is handy for measuring depths of holes, channels and indents.
We’re supposed to be a metric society but the common use of American parts means the industry still speaks imperial, so most verniers are dual scale. Electronic digital verniers can convert between metric and imperial at the touch of a button.
How To Read Calipers:
- Start with the large divisions on the fixed scale.
- Note that zero on the sliding scale has passed the ‘2’ (red). So that’s 0.2 inches so far.
- Each division on the fixed scale between the ‘2’ and the ‘4’ equates to 0.025 inches (Note that 3 — halfway — is unlabelled).
- The zero on the sliding scale has passed the first division (green) but not reached the second.
- So, it’s 0.2 plus 0.025.
- For the last part of the reading, find the marking on the sliding scale that lines up exactly with any line on the fixed scale — in this case that’s 18 (yellow).
- So, the measurement is 0.2, plus 0.025 plus the final 0.018 which is 0.243 inches.
- On the bottom (metric) scale, the zero is just past 6mm (blue) and the two graduation lines (sliding and fixed) that line up perfectly (orange) are at 1.8 (which is 0.18mm). So it’s 6.18mm.
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