They are essentially the same machine. Capstan is British usage, Turret, US. One old and still technically correct usage of Capstan was the (Turnstile winch) were Bars were extended and seamen walked round and round to weigh the anchor. Almost certainly this inspired the lathe term, as this is a maritime country (UK). The Wheel or circular component on a capstan lathe, however is usually three spoked and Vertical and drives the ram ( containing the usually hexagonal turret- 6 tools) back and forth into the work. They are or were very popular in the automotive sector for fabricating standard parts.