121.6 Threads Per Inch

[rgsparber]

After watching that amazing video on the James Webb mirror drivers, I got thinking about simple, fine movement devices.

Many years ago, I learned, in Guy Lautard' s Bedside Reader, how to work one thread pitch against another to get a finer thread. For a compact unit, the key is to select threads that a very close in pitch yet have different enough diameters so one can fit inside the other.

Now, I am positive this will be offensive to some, but I have no problem looking over Metric and Imperial threads as one spectrum. What if I took a 1/4-28 threaded rod and tapped it 4 mm - 0.7? Looking at the diameters, there is enough metal to do this.

28 TPI is a pitch of 0.0357 inches. 0.7 mm pitch is 0.0275 inches. This means that one revolution screws in the 1/4-28 by 0.0357 inches while it unscrews the 4 mm - 07 by 0.0275 inches. The net effect is a change of .0082 inches per revolution which is 122.6 threads per inch.

Picture the 1/4-28 partially threaded into a block. I hold the head of the 4 mm - 0.7 screw. As I turn the 1/4-28 into the block, it unscrews from the 4 mm - 0.7 screw. Since the Imperial thread is coarser, the head of my metric screw moves towards the block.

This would be useful as the fine adjustment on my finger Dial Test Indictor which can resolve 0.0001 inches. That would be 4 degrees of rotation.

Rick