The picture (which I found on the web without any accompanying explanatory text) shows a home-made z-axis micro-adjust lead-screw add-on to the vertical milling attachment of a vintage hobbyist lathe.

The greenish motor-and-spindle-mount casting rides up and down on the black post, sandwiched between two plates, which ride up and down on two smaller posts.

I can't quite make out whether there is a threaded bushing of some kind where the arrows are pointing. How would that aspect of the design typically be engineered?

I'm thinking that if those plates were simply threaded, without some kind of insert, you could have a somewhat loose and sloppy sandwich.

Z-axis micro-adjust lead-screw


1 Answer 1


It's difficult to determine from the photo which parts are moving and which parts are dependent on another part for specific movement. If you get proportionately smaller travel per revolution of the main knob, the other parts of the assembly could be a differential screw mechanism.

Differential screw mechanisms are used in micrometers, in which the main knob rotates a substantial amount and the spindle moves a fraction of the distance. It is a construction in which an inner rod is threaded to one specification and the outer rod is threaded to a slightly larger or smaller specification.

I've constructed a 3D model in which the travel of the spindle is 0.25 millimeters for each rotation of the main knob, which is threaded to 1.0 mm pitch, while the inner thread is 1.25, a difference of 0.25.

The reason for such a mechanism is that it is easier to build a 1.0 mm and a 1.25 mm pitch thread compared to a 0.25 mm pitch, which is what would be required if one desired to bypass the differential threading option.

differential threading image

Thingiverse provided me the model to learn more of this construction.

differential threading model from Thingiverse

The outer threads of this mechanism would be engaging the brackets you've noted in the photo. Note also the key and matching keyway between the inner threaded rod and outer sleeve. It's critical that the portion does not rotate in order to provide the fractional movement desired.

In your photo, it's logical to expect that only one of the brackets contains a differential mechanism, while the other provides an alignment bushing of sorts.

  • $\begingroup$ Thanks for this explanation, pic, diagram, and link! $\endgroup$
    – Tim
    Nov 17, 2019 at 15:24

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