These characteristics and elements create a measurable diametral growth. By subtracting Pitch diameter, from Functional Diameter, we are left with variation only. Taken a step further, we can separate the elements and characteristics that make up the total magnitude of the variation. Variations in thread elements and characteristics are the causes for the diametral additions to Pitch diameter sizes. Lead is the axial advance per unit rotation for a given pitch distance.
Lead equals pitch under ideal conditions.
Machine capability, process, speeds, feeds, approach, programming, and mis-aligned dies can be culprits for lead variations. Flank angle of a Unified National thread design calls for a 60o included angle. Worn tooling and tooling not square to the centerline of the work piece are the usual causes for the flank angle being other than a centered 60o included angle. Taper is caused by part deflection, worn tooling, thread blank configuration. Roundness variations are caused by thread blank configuration, geometry of tooling, such as two flat or roll dies at 180o positions, or improper chucking.