On calipers,
I was using mine today. The Efratom FRK oscillator project is progressing slowly. I was looking at the mounting screws. They need to be longer because there is now the rear panel of the case between the oscillator and heatsink. The screws are pretty small. I looked at the manual and there is a defect right through the screw size. It looks like 2.6mm which is a bit of an odd size. I then looke at the Racal standard the uses the FRK. It calls up M2.5. So I measured the screws in the unit that are probably original. They are clearly M2.5 (mjor dia 2.380 to 2.480) not M2.6 (2.480 to 2.980). However they are loose in the tapped holes in the unit.
Both M2.5 and M2.6 are 0.45mm pitch as standard (0.35 option) so I've ordered some M2.6 cap heads. Usefully they are used on some model helicopters so are available.
I have several sets of calipers from cheap innacurate digital ones just used fror marking out through to 12" Mitutoyo and micromoters, height gauges and slip gauges. The calipers in the photo are my everyday mid-range ones from ARC Eurotrade costing about £25 Highly recommended, They even do imperial fractions.
https://www.arceurotrade.co.uk/Catalogue/Measurement/Calipers/Digital-Calipers-150mm-300mm-6-12
M2.5 is a "preferred size", M2.6 is not. The next up preferred size is M3.0. Why, oh why, would someone pick M2.6 as a size? In terms of any property a screw needs (UTS, clamping force, etc.) they're indistinguishable for all practical purposes. In any sensible table of metric screw sizes you won't even find any entries except for preferred sizes.
My suspicion is that it's them folks deeply wedded to the imperial system, but forced to "go metric" for reasons beyond their personal control. Someone sticks a finger in the air and decides that a screw needs to be "about \$\frac{1}{10}\$". They open up Machinery's Handbook, scan down a table of metric thread sizes and pick, by calculation, the first that's greater than 0.1". It has to be something like that because if you picked a #3 screw (basic size 0.0990" = 2.5146mm) when hunting for the closest metric equivalent you'd come up with M2.5 (basic size 0.0984"), and if you'd picked a #4 (basic size 0.1120" = 2.8448mm) you'd either pick a bastard size of M2.8 or M2.9, or the sensible M3.0.
M2.3/2.6/2.8 was a formerly preferred series. It did not make it into the ISO.
Now anything should be 2.0/2.5/3 and that's it. But they survive longer than intended.
Which is a bitch specially with 2.5 and 2.6, as they can't be kept apart visually.
In this case, you need vernier calipers to sort your screws.
That was always a problem with DIN versus Japanese "metric" screws.
Not only that, but the DIN ones all had a finer thread pitch.
Is ISO also the culprit for the disappearance of a bunch of really useful DIN coax connectors?
I have a lot of salvaged cables, & a directional coupler, all with "spinner" connectors.
I managed to swap the output connector on the coupler for an "N" but the others were more difficult.
I have one "N", & a couple of "BNC" to spinner adaptors for the ports, so not so bad there, but there are also detectors to sit on those ports, which I'd like to use.
The demodulated outputs from these appear on what we used to call " Siemens coax sockets" in Telecom Australia.
We used these & the audio ones from the same stable extensively back in the day, but now them & the matching plugs are "unobtainium".
I bought two "Siemens" coax "links" at a hamfest, but they need mods to use as standalone plugs..
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