There is a textured wheel that is attached to a gear train like in a dial indicator, that is spring loaded against the vertical surface of the lathe bed, in front and below the way surfaces; the roller is slightly curved (crowned) so that adjustments can be made to calibrate it for a true reading. They come with a spring brass wiper to keep debris from riding under the measuring wheel and also a foam plastic wiper inboard of the brass wiper to further keep crud from entering. I first saw one back in the mid 1960s, and so far as I know, they are still available. Made by SouthWest Industries.