You say that level to a thou or so can you not turn two pillars of identical length (hollow) and bolt together through these?

Differential screws?

To be clear, I meant a screw which has different threads at each end, one screws into one part while the other screws out of the other. So if the pitches were 1mm at one end and 0.9mm at the other, 1 turn would move the parts relatively by 0.1mm. I've seen these use a lot on (electron) microscopes. The trick could be to find two standard pitches that are very close, quite possibly one metric and the other imperial so their pitch difference is very small.

If you are equipped to tap and thread your own screws the ME range of threads includes sizes such as 1/4 " x 40 TPI, so one whole turn will move the screw 0.025" ( like an Imperial micrometer.) If you need finer than this then John's suggestion of differential screws is worth further investigation. Some combinations will give very fine adjustments,

I am not completely sure from your post that you have entirely understood the working of a differential system, Apologies if I am wrong.

Rod

I think you need the same thread handedness at each end? You rotate the screw, it moves into one part and out of the other but different amounts.

The MK3 Quorn uses differential threads for the rocking arm adjustment by having a thread within a thread arrangement. A threaded adjuster fits in a threaded hole and has a knurled collar to turn it. Another threaded rod passes through the centre of the adjuster but with a slightly smaller TPI. If the central threaded rod is constrained and the hollow adjuster turned then if it is screwed in the central rod will be screwed out the movement being the difference of the pitches. All threads are right handed.
So if the central thread was constrained in a lower plate and the hollow adjuster threaded into an upper plate the separation could be adjusted to a fine degree. 3 such adjusters would give you what you require.
Alternatively you could just purchase a theodolite tri bracket, a device that has been doing the job for years.

regards Martin

Edited By Martin Kyte on 01/02/2023 21:16:28

1/20 – 1/28 is 1/70, how fine can you want it.

What is wrong with just a fine thread screw? Differential screws are all well and good but they have a woefully small range of adjustment. A fine thread screw say 10mm x 1mm pitch will give a range of adjustment which is the effective length of the screw itself and easily adjust to better than the 1 thou you require. In fact, I use a sensitive level on parts supported on 1mm pitch thread screw jacks and find no difficulty adjusting to tenths with a 4" long spanner on the hex head of the jack.

It has been done before, but my books mention it mostly in terms of disadvantages, and don't give any practical constructional details. From memory, the bearing is prone to wear and easily disturbed.

The closest analogue I know of is the old-fashioned Chemical Balance of the beam type, in which the fulcrum is an Agate edge. My school had four, studied briefly because the history of accurate weighing is important in chemistry, but we weren't taught how to use them in anger – chemical technique had moved on. Agate is harder than steel, and I don't know what the edge rested on – maybe a 'V' in another hard gemstone. Each scale lived in a front-opening glass case, I think levelled by four ordinary screw feet; my memory could be faulty!

I don't think chemical balance construction helps SK much because the scale's fulcrum edge was only put in contact with a lever whilst actually weighing. Most of the time it was lifted off. As there are over 30,000,000 seconds in a year, I don't know how long a sharp edge would last swinging a heavy pendulum bob.

On the subject of levelling, the support can be pretty crude as long as it's "good enough". A mechanically sophisticated way of levelling my clock has been on the TO DO list forever, but this is how it's done at the moment:

Though this abomination is levelled within the accuracy of a Wixey, about 0.2°, balancing a pendulum clock on a stack of coins isn't Horological best practice! I only get away with it because my bob is tiny and swings well below the clock's centre-of-gravity, close to the base.

Dave

Edited By SillyOldDuffer on 02/02/2023 10:39:03

I do wonder if on a clock with active impulse compensation reducing frictional losses is actually helpful. In order to exert control over the pendulum you actually need a certain amount of amplitude decay as impulsing can only work to increase amplitude. If you imagine the limiting condition where there are no losses there is nothing for the amplitude control to work against.

Very small impulses require driving currents with hard to achieve signal to noise ratios so the frictional losses so long as they are not huge and are constant help to keep a decent signal to noise ratio on the impulsing cuts.

I recently designed a Cryostat to maintain liquid ethane at a constant temperature. The device was a copper cup in a bath of liquid nitrogen with insulation between the two. Two copper ‘ears’ of 2mm wire were attached between the cup and the nitrogen to ensure a reasonable cooling of the cup. The cup has a heater and a temperature sensor and the whole is driven by a PWM PID controller. Without the extra losses provided by the ears the controller had little ‘command’ of the ethane temperature.

I can understand the desire to minimise friction in a totally mechanical timepiece but with impulse compensation I don’t think it’s relevant and may well be counter productive.

regards Martin