Small cotter

The outside thermometer at my lake cabin is mounted on an ell bent from 1/4” dia aluminum rod. It fits in a sleeve with a nylon grub screw that doesn’t grip it quite tightly enough to keep the wind from rotating it. I decided to make a little “cotter”, like the quill lock on a Bridgeport mill. I didn’t invent the name or the device; it was described in one of Guy Lautard’s “Machinist’s Bedside Reader” books.

They have two intersecting but slightly offset perpendicular bores. In one bore goes the cylinder to be locked. The other bore contains two slugs, one threaded and one not, with partial cylindrical cutouts. In operation, when a screw pulls the slugs together they grip the cylinder to be locked very tightly without marring it as a grub screw might.

This one is way smaller than the one on a Bridgeport quill, fun to make. It goes from having the aluminum rod completely free to rotate, move axially or remove, to locked up tight with half a turn of the screw. Here are the various parts:

Attachment 45350

The intersecting bores were made of 3/8” dia brass rod, drilled and reamed to .251” and silver brazed together. The “fishmouths” where the bores intersect were made with two-flute center-cutting end mills: the fishmouth in the horizontal bore is 3/8” dia while that in the vertical bore is ¼” dia to clear the slugs. This assembly was then silver soldered to the base plate. The slugs are made of ¼” dia brass rod, one of which (the one on the right) is tapped with #6-32 threads.

Here it is with a “keeper” aluminum rod in place:

Attachment 45351

The “keeper” is just to get the slugs lined up correctly when this thing is mounted to the cabin. Then the keeper can then be removed and the thermometer mounting rod inserted.