A micro-caliper was a small tool.
While attempting to diffuse the Romulan mine that had attached to Enterprise's hull in 2152, Captain Jonathan Archer used a micro-caliper to work on the mine. (ENT: "Minefield")
Also in 2152, Commander Tucker used a micro-caliper to adjust a component on a 31st century communications beacon. (ENT: "Future Tense")
In 2374, Entharan arms dealer Kovin used a micro-caliper to remove the guidance system casing of his particle beam rifle while showing it to Seven of Nine. In Seven's false memory where he assaulted her, Kovin used the micro-caliper to remove her ocular implant. (VOY: "Retrospect")