By Bob Gates

The requirement was to make the square threaded reversing screw and nut for a 5” gauge Merchant Navy class Pacific. Cutting the external thread on the reversing screw was fairly straight forward and only necessitated grinding up a suitable cutting tool which was similar to a parting off tool but with an offset scroll angle.

Producing the internal thread in the nut was rather more complex.

The usual way of making square thread taps is to cut the thread on the tap in the lathe from silver steel and then harden and temper them. Tempering is difficult to get right and the tap either ends up too soft or too brittle.

Having a Quorn tool grinder it was decided to manufacture a tap from HSS. The photos tell the story.

It was difficult to get the tap started and to cut the thread from scratch so he another tool was ground up to cut a pilot thread in the nut in the lathe, then finished to size with the tap.

Strangely on the drawings the nut is named as a ‘gnut’.

Has anyone heard of this naming convention before?

Cutting the thread using a slitting wheel.


Cutting the longitudinal slots.


Adding the taper to the end of the tap


Grinding the square end.


The finished tap.


Lathe tool ground to cut the reversing screw thread.


The finished product on the reversing screw bracket.

This article is based on a Zoom meeting published in The Bristol Model Engineer.

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Editor: David Carpenter