Measuring thread pitch, angle and diameters

A toolmaker's microscope has a base with a table carrying the work, which can be moved in two perpendicular directions by micrometer heads (least count 0.01 mm or 0.005 mm), a microscope with a cross-hair eyepiece that rotates and has an angle scale, and a light source below the table that throws a sharp shadow of the thread profile.

Major and minor diameters

The thread is held on its axis on the table (between centres). The cross-wire is set on the edge of the crest on one side and the reading of the micrometer head is taken; the table is moved until the cross-wire touches the opposite crest, and the second reading is taken. The difference is the major diameter. The same is done at the roots to get the minor diameter.

Pitch

The cross-wire is set to a definite point (a flank) of a thread and the micrometer reading is noted. The table is moved parallel to the axis until the same point of the next thread (or after $n$ threads) comes in the cross-wire; the difference of readings is the pitch (or $n$ pitches, better for accuracy). Pitch error follows from comparison with the nominal pitch. The table must move parallel to the axis; the head is tilted by the helix angle to bring the flank into focus.

Thread angle

The eyepiece is turned until its cross-wire lies along one flank and the angle scale is read; then it is turned until it lies along the other flank and the scale is read again. The difference of the two readings gives the included angle (for a 60 degree thread, 60 degrees). The half-angles of each flank are measured separately to check symmetry.

Limitations: the microscope only measures the projected profile, so errors arise if the helix angle is not compensated for, and the contrast depends on lighting.