Editing Screw thread (section)

===Lead, pitch, and starts===<!-- [[Threads per inch]], [[thread pitch]], [[Pitch (screw)]] & [[Turn per inch]] redirect here -->
{{Redirect|Threads per inch|woven fabrics|Units of textile measurement#Thread count}}
{{Main|Lead (engineering)}}
[[File:Lead and pitch.png|thumb|''Lead'' and ''pitch'' for two screw threads; one with one start and one with two starts]]
[[File:Helical screw single double triple quadruple start.png|thumb|Up to four starts are labeled with different colors in this example.]]
''Lead'' ({{IPAc-en|ˈ|l|iː|d}}) and ''pitch'' are closely related concepts. They can be confused because they are the same for most screws. ''Lead'' is the distance along the screw's axis that is covered by one complete rotation of the screw thread (360°). ''Pitch'' is the distance from the crest of one thread to the next one at the same point.

Because the vast majority of screw threadforms are ''single-start'' threadforms, their lead and pitch are the same. Single-start means that there is only one "ridge" wrapped around the cylinder of the screw's body. Each time that the screw's body rotates one turn (360°), it has advanced axially by the width of one ridge. "Double-start" means that there are two "ridges" wrapped around the cylinder of the screw's body.<ref>Bhandari, p. 205.</ref> Each time that the screw's body rotates one turn (360°), it has advanced axially by the width of two ridges. Another way to express this is that lead and pitch are parametrically related, and the [[parameter]] that relates them, the number of starts, very often has a value of 1, in which case their relationship becomes equality. In general, lead is equal to pitch times the number of starts.

Whereas metric threads are usually defined by their pitch, that is, how much distance per thread, inch-based standards usually use the reverse logic, that is, how many threads occur per a given distance. Thus, inch-based threads are defined in terms of ''threads per inch'' (TPI). Pitch and TPI describe the same underlying physical property—merely in different terms. When the inch is used as the unit of measurement for pitch, TPI is the reciprocal of pitch and vice versa. For example, a {{frac|1|4}}-20 thread has 20 TPI, which means that its pitch is {{frac|1|20}} inch ({{convert|0.050|in|mm|2|abbr=on|disp=or}}).

As the distance from the crest of one thread to the next, pitch can be compared to the [[wavelength]] of a [[wave]]. Another wave analogy is that pitch and TPI are inverses of each other in a similar way that [[Frequency|period and frequency]] are inverses of each other.

====Coarse versus fine====
Coarse threads are those with larger pitch (fewer threads per axial distance), and fine threads are those with smaller pitch (more threads per axial distance). Coarse threads have a larger threadform relative to screw diameter, where fine threads have a smaller threadform relative to screw diameter. This distinction is analogous to that between coarse teeth and fine teeth on a [[saw]] or [[File (tool)|file]], or between coarse grit and fine grit on [[sandpaper]].

[[File:XK engine camshaft cover front stud-0822.jpg |thumb|Camshaft cover stud threaded {{frac|1|4}}-20 UNC (left, for aluminium cylinder head) and {{frac|1|4}}-28 UNF (right, for steel nut; from a 1960s [[Jaguar XK6 engine|Jaguar XK engine]])]]
The common V-thread standards ([[ISO 261]] and [[Unified Thread Standard]]) include a coarse pitch and a fine pitch for each major diameter. For example, {{frac|1|2}}-13 belongs to the UNC series (Unified National Coarse) and {{frac|1|2}}-20 belongs to the UNF series (Unified National Fine). Similarly, M10 (10&nbsp;mm nominal outer diameter) as per ISO 261 has a coarse thread version at 1.5&nbsp;mm pitch and a fine thread version at 1.25&nbsp;mm pitch.

The term ''coarse'' here does not mean lower quality, nor does the term ''fine'' imply higher quality. The terms when used in reference to screw thread pitch have nothing to do with the tolerances used (degree of precision) or the amount of craftsmanship, quality, or cost. They simply refer to the size of the threads relative to the screw diameter.

Coarse threads are more resistant to stripping and cross threading because they have greater flank engagement. Coarse threads install much faster as they require fewer turns per unit length. Finer threads are stronger as they have a larger stress area for the same diameter thread. Fine threads are less likely to vibrate loose as they have a smaller helix angle and allow finer adjustment. Finer threads develop greater preload with less tightening torque.<ref name=katonet>{{cite web |title=Coarse Threads vs. Fine Threads |website=katonet.com |url=http://www.katonet.com/article/coarsevsfine.html}}</ref>