Machinists rely on their measuring tools to determine whether a machined feature is within tolerance or not, the latter can be costly.
Tools ranging from a simple Caliper, to highly accurate coordinate measuring machines. Many common measuring tools are subjective, relying on human input being repeatable to be reliable. One of the most affordable solutions to this dilemma is a ratcheting mechanism which creates a more repeatable input from user to user, common to many Micrometers.
For this reason, Micrometers are a great entry to precision work. Many Micrometers have a resolution of .0001" or one tenth of a thousandths, but an accuracy of +- .0001" making measurements near the edges of a tolerance risky. Some models go one step further with a resolution of .00005" / .001mm and an actual listed accuracy of .00005" making them much better suited for precision work where every micron (μm) counts.
The rule of thumb is to divide your tolerance range by 10 when selecting the resolution needed from a measuring device. A gauge or measuring device should ideally be 10xs as accurate as the tolerance. This is a widely accepted industry standard.
{Example (+-.0005") .001" total tolerance should be measured with a resolution of .001 ÷ 10 equaling .0001"} Micrometers have a. 0001" resolution and are often used for such measurements as confident readings can be taken without much training. A caliper often has a resolution of .001" meaning a tolerance less than +-.005" may be risky to measure, .001*10=.010"
A tolerance of +-.0001" (one tenth of a thousandths) would require .00001" resolution which common micrometers cannot provide.
At this close of tolerance temperature begins to play a major role. A measurement taken on a climate controlled coordinate measuring machine at 68° F may be vastly different in the field. Parts with tolerances closer than +-.0002" should be designed around the actual operating temperature of the intended use.
For the average shop, guaranteeing tolerances closer than +- .0005" would require investing in the right measuring systems. Aiming for the middle of a tolerance is usually reccomended. Imagine a scenario where micrometers are being used to measure a .001" total tolerance. Is the part scrap if it is measured at the upper or lower end of the tolerance? Impossible to say without a finer resolution measuring device. Surface finish also plays a major role in sizing, a surface should be uniform throughout to guarantee a dimension.