Editing Screw thread (section)

===Classes of fit===
The way in which male and female fit together, including [[Backlash (engineering)|play]] and friction, is classified (categorized) in thread standards. Achieving a certain [[Engineering fit|class of fit]] requires the ability to work within tolerance ranges for dimension (size) and [[surface roughness|surface finish]]. Defining and achieving classes of fit are important for [[Interchangeable parts|interchangeability]]. Classes include 1, 2, 3 (loose to tight); A (external) and B (internal); and various systems such as H and D limits.

====Tolerance classes====

====Thread limit====
'''Thread limit''' or '''pitch diameter limit''' is a standard used for classifying the tolerance of the thread pitch diameter for [[tap and die|taps]]. For imperial, H or L limits are used which designate how many units of 0.0005 inch over or undersized the pitch diameter is from its basic value, respectively. Thus a tap designated with an H limit of 3, denoted ''H3'', would have a pitch diameter 0.0005 × 3 = 0.0015 inch larger than base pitch diameter and would thus result in cutting an internal thread with a looser fit than say an H2 tap. Metric uses D or DU limits which is the same system as imperial, but uses D or DU designators for over and undersized respectively, and goes by units of {{convert|0.013|mm|mil|abbr=on}}.<ref name="MH">{{cite book|editor1-last=Green|editor1-first=Robert|title=Machinery's Handbook|isbn=0-8311-2575-6|page=893|edition= 25|date=1996|publisher=Industrial Press }}</ref>
Generally taps come in the range of H1 to H5 and rarely L1.

The pitch diameter of a thread is measured where the radial cross section of a single thread equals half the pitch, for example: 16 pitch thread = {{frac|1|16}}{{nbsp}}in = 0.0625{{nbsp}}in the pitch actual pitch diameter of the thread is measured at the radial cross section measures 0.03125{{nbsp}}in.