Thread start

A thread start is one continuous helical ridge of a screw thread. A single-start thread has one such ridge; a multiple-start thread has several interleaved ridges. The number of starts controls how screw lead and thread pitch differ:where $n$ is the number of starts, $p$ the axial spacing between adjacent thread crests and $L$ the advance of one ridge in a complete turn.^[ref]

An ideal cylindrical construction explains the relation. At reference radius $R$, represent the $j$th start by the helixAt one fixed angular position, successive complete turns of the $j$th start occur at axial offsets $kL+jp$. When $L=np$, these offsets are $p(nk+j)$. As $k$ ranges over the integers and $j$ over the $n$ starts, they give every integer multiple of $p$. Adjacent crests can therefore be spaced by $p$ even though following one continuous ridge for a turn advances by $np$.

For example, a three-start thread with $p=2\mathrm{mm}$ has $L=6\mathrm{mm}$. At a chosen angular position, crests at axial coordinates $0,2,4,6\mathrm{mm}$ do not identify four starts: the crests at 0 and $6\mathrm{mm}$ can be successive turns of the same start. Following the three ridges gives advances $0\to 6$, $2\to 8$ and $4\to 10\mathrm{mm}$ over one turn. The unwrapped geometry in screw thread displays this construction.

With the screw restrained axially and its matching nut guided against rotation, a quarter-turn in this example advances the nut by $L/4=1.5\mathrm{mm}$. A single-start thread with the same pitch advances only $0.5\mathrm{mm}$. Counting starts is therefore essential when predicting a lead screw's motion from its pitch.