I have one like your first illustration, Vic … and the build-quality is dire

… It’s in pieces at the moment, waiting for me to gather enough enthusiasm to ream all the holes to an appropriate size.

Likelihood is that I won’t ever bother

I have a bigger, and much better, table available [see edit] … and a wide assortment of smaller devices.

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I suggest you think carefully about what size of platform you actually need

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Excuse the cheap pun, but: that blue one is “jack of all trades and master of none’

MichaelG.

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Edit: __ have a look here 

https://www.model-engineer.co.uk/forums/postings.asp?th=184518

Edited By Michael Gilligan on 20/09/2023 10:02:47

+1 for what Micheal says about the blue one. Even the good versions are pretty poor and quite unable to lift any sort of load with sensible stability.

For best behaviour the load needs to be in-line with the cross arm centre pivots and no wider than the end arm span at the chosen lift. If it's a pointing or sighting thing the axis should run sideways rather than longitudinally. Used like that the thing is generally adequately stable once set but what happens during elevation tends to be inconsistent.

Realistically precision behaviour is asking too much of the concept. However carefully fitted the holes are. It's basically a load stabilised system but it's not really strong enough to take the load it needs to achieve proper stability.

For DIY best to copy the bottom one. The single piece nature of the pivoting "hinge" arms gives you the best chance of combining positive alignment with nominal zero play in the joints. The ones with a multitude of separate arms with sliding rods for alignment require things to be made to very close tolerances if the end result is not to prove excessively wobbly.

The third variety down looks good and has a minimal parts count but there are issues with the sliding block and plate that stop it from tilting. Had a couple like that made by a respected optical lab equipment maker in the lab at RARDE / DERA / DRA and they just didn't work very well. Certainly the last ones out of the cupboard and first ones on the loan list. As I recall matters the bottom slide assembly was the culprit.

Clive

I do like the second and third idea with the double scissor assembly as this will give the best stability when fully raise unlike the cheap blue one where the scissor assembly ends up to one side