Differential screw

A differential screw is a mechanism used for making small, precise adjustments to the spacing between two objects (such as in focusing a microscope ^[1]). It uses a "nut" with two screws, one entering each end. The "heads" of the screws are fixed to the objects whose spacing is to be adjusted. The screws have slightly different pitch (threads per inch) so that one rotation of the nut moves one screw into the nut by a small amount, and moves the other screw out of the nut by a slightly larger amount. Thus the total spacing between the objects will be slightly increased, by the difference in travel between the two screws. This allows extremely small adjustments using commonly available screws.

The distance between nuts traveled by one revolution of a bolt threaded differently with a(threads per distance) on one end and b(tpd) on the other end can be calculated simply: 1/a - 1/b = d

For example, a 3/8" bolt with coarse (16tpi) threads on one side and fine (24tpi) on the other would yield about 0.02" per revolution between the nuts: 1/16 - 1/24 = 0.0208

Other arrangements are also possible, such as having two nuts, one on each object to be adjusted, and the two screw heads attached to each other in the middle. The combined screws would be turned to adjust the spacing in that case.

References

1. ^ "A ONE-DOLLAR COMPOUND MICROSCOPE"