A caliper is less trustworthy than it looks
Where the errors come from in the most common workshop instrument.
A caliper looks like a simple, self-evidently trustworthy tool, two jaws closing on a part and a display reporting the gap between them, and that simplicity hides a list of ways the reading can drift from the truth, from the angle it is held at to the force squeezing its jaws shut, none of which announce themselves on the display the way an obviously broken tool would.
Parallax, and its cousin in a tilted caliper
Reading a ruler from directly above gives an honest measurement, while reading it from off to one side shifts which marking appears to line up with the edge, an effect called parallax that anyone reading a car's speedometer from the passenger seat has met. Some analogue instruments fight it with a mirrored strip in the dial, so the needle hides its own reflection only when the eye is straight on.
A caliper held askew suffers a related problem, and this time the distance itself is wrong. Tilted across two parallel faces, its jaws span a slanted path longer than the true thickness, and a tilt of just three degrees across a 50 mm part adds about seven hundredths of a millimetre, seven times the display's last digit and wider than many tight tolerance bands. Held off-centre across a round bar, the jaws catch a chord shorter than the diameter and the error runs the other way. Either reading looks perfectly ordinary on the display.
How hard the jaws are closed
A rigid metal part barely notices a light touch versus a firm grip, but a soft material, a thin-walled part or a delicate plastic component of the kind earlier sets in this project discussed can compress under force and read smaller than it is, springing back once the jaws open. The same caliper on the same part can give two different honest numbers depending on grip alone. A friction thumbwheel on some calipers lets the jaws close with a small, repeatable force, narrowing that problem without removing it.
A speck in the jaws
A chip of swarf, a smear of cutting fluid or a fleck of dust caught against either jaw adds its thickness straight into the reading, indistinguishable from a real change in the part. Wiping the jaws and part before every measurement prevents it, and forgetting to is easy when one caliper is picked up and set down dozens of times an hour, which is why debris remains among the commonest causes of a reading nobody can later explain. In a comparative check of two similar parts with the debris lodged in the same spot throughout, part of that error cancels.
Used correctly, kept clean, checked periodically against a reference standard and closed with a light, consistent touch, a caliper remains one of the most useful tools in an ordinary workshop. Its trustworthiness has to be earned through technique and maintenance, and trusting a reading means trusting that whole chain behind the glowing number. The final article in this set follows what happens when two people, each trusting their own caliper and technique completely, bring two honest readings of the same part into one room.