SNO-T Mirror Bracket Repair

17 04 2010

The SNO-T sextant of the former Soviet Union was possibly one of the best sextants ever produced, but it did have one or two drawbacks, one of which was the method used to lock the mirror adjusting screws. These slender, 2 mm metric screws pass through in turn a bush threaded M6 on the outside and M2 on the inside and then through the back of the mirror cell. Once the mirror has been adjusted, turning the bush through less than a quarter of a turn locks the screw. While using the adjusting screw, the bush can be turned finger tight to firm up the adjusting screw. The bush cannot be withdrawn without first removing the screw, and the screw cannot be withdrawn without first unlocking the bush by turning it a little until it feels slack and can be rotated a little either way.

Unfortunately, the bushes were of thin brass, and the slot used to rotate the bush had sharp corners, so that, in the first place, the bush tended to seize solid in the aluminium bracket, and then the sides of the slot cracked and fell apart when pressure was used to unlock the bush. I have now been asked for help with these bushes three times and the latest was the most difficult to sort out.

One of the previous owners appeared to have drilled out one of the bushes and replaced it with a 6BA cheese headed screw. An off-centre hole had been tapped through the 3 mm thickness of the rear aluminium wall of the bracket. He had also managed to twist off the head of the other screw. The present owner, in attempting to unjam the bush without knowing of its structure, had caused both sides of the slot to disintegrate (this is an observation, not a criticism).  Finally, he had attacked what was left with a Dremel tool, leaving an irregular mess of metal in the bottom of the boss on the back of the bracket.

When drilling out a small seized screw, it is next to impossible to start the drill on centre, and if it wanders a little and encounters softer surrounding metal, it takes the path of least resistance. Evidently, then, a twist drill is not the ideal tool to use. However, if you imagine a twist drill with a flat rather than a pointy end, and with one cutting lip extending across the centre line, you have imagined a type of end mill called a slot drill. Fed carefully straight into the work piece, a slot drill will originate a hole and stay on a more-or-less straight line as the hole is made deeper.

Each hole had to be centred beneath the tool before cleaning up the holes. I used a plug of 4.7 mm diameter, the core diameter of the bush, held in the chuck of the machine. This used what was available. I do not claim it is the best method of centring a hole. I replaced the plug with a 3 mm slot drill and fed it straight through the remains of the bushes and back wall of the cell. That took care of the adjusting screw and the bottom of the bush. Maintaining the same centres, I then fed in a 5 mm end mill to remove the rest of the bushes. This left the 6 mm thread in the bosses of the bracket, to be cleaned up with a tap, and 3 mm holes through the back of the bracket, hopefully on the same centres.

I filled up these latter holes by cementing   little shouldered bushes in place with industrial adhesive, The bushes were tapped M2 down the middle and were 3mm diameter on the outside, with  6 mm shoulders. As the next picture shows, I had to file away part of the shoulder of the side-error bush to get it to fit.

The average jobbing engineering workshop, while it may have a lathe, is unlikely to have the small tools needed to complete a repair job like this one. An old-fashioned clock maker or instrument maker very likely will, and many amateur model engineers will have small tools, though those in the USA and UK will quite possibly not have metric screwing tackle. For those of you who find someone able and willing to undertake the job at a reasonable cost (or no cost if you’re lucky), a drawing of the parts follows. The unbotching I will have to leave to their ingenuity…

(If you click on the drawing, you will then be able to print it out. Use the back arrow to get back to the post)


From now on, I would like readers to think of my posts as supplements to my comprehensive book on the structure of the nautical sextant.  By all means also encourage me to continue by leaving comments and suggestions.




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