Thanks for posting; interesting that they use a diaphragm rather than Bourdon tube. But I suppose the pressures are quite low. And diaphragms in aircraft instruments detect unbelievably small pressure changes.

Last night, at the fourth attempt, I've finally made a working Bourdon tube. It moves about 3/16" at 250psi, which is my intended full scale. It's not quite where I want, but having got one to work I can now fine tune wall thickness and material. I also need to design a better way of bending the flattened tube. Brass tends to kink rather than bend uniformly. I've just knocked up an embryo tube in copper, so we'll see if that is better behaved.

Andrew

Or trying Henry Woods method in the latest ME, looks quite a good article and shows how he bends the tube made from 0.003" shimstock though I have just had a skim over it.

J

I hadn't considered electroforming, although I need a wall thickness of around 8 thou, which I suspect would take some time. At least in an amateur (my) environment.

In due course I will read the article In ME, when I get my copy. I vaguely recall an article in ME using shim for Bourdon tubes (probably in the 1970s) so I made two 'tubes' from 4 thou copper shim. The first one was a total disaster; couldn't remove the mandrel. The second one worked; briefly before the seam split. In its short life it was clear that 4 thou was far too thin a wall by the amount of movement trying to reach 20psi. Of course wrapping up from shim, with a lap joint, puts material just where you don't want it. So I prefer to use tube, thinned, flattened and rolled.

I am aware that Kozo has written on pressure gauges, but there appears to be no information on the internet. Of course I could buy the book(s), but they're expensive. And I suspect may take a different approach to me. I'm not really interested in descriptions of making pressure gauges, that's relatively simple. It's the engineering design of the components that interests me. Having said that, we have a Shay nut at the gliding club, so next time I see him I'll ask if he has any of the Kozo books.

Andrew

Andrew – you really need to read Kozos' pressure gauge articles, as he explains how he rolls the tubes out of bronze, and how he silver solders them. And, how he makes the dead weight tester, and…

Now, I can absolutely understand one wanting to figure it out for oneself, but I can almost – ALMOST – use the name "Kozo Hiraoka" and "Cherry Hill" in the same paragraph.

The books really are not that expensive, and, if you find a used one, you can always sell it on later and you have probably not lost anything, but gained lots.

If you want to know which books contain pressure gauge articles in the back, just ask and I'll dig what books of his I have out, and I'm sure others would, too.

The reason for writing this note is that you do really impressive work, and I want to see you progress faster, so it is a bit selfish on my part…

Another JohnS.

I think we are rather straying off how the best commercial and amateur makers made these gadgets in miniature.

It would help if John Alexander Stewart would provide links for Kozzo he quoted.

Cheers,

Julian

Hi Michael,

The Roy Amesbury type has been described in ME

The Freddie Dinnis type has not. At various times I have mentioned Freddie on here.

Ask any decent miniature loco builder what pressure gauge he wants on his loco and they will say a Freddie Dinnis (FSD) gauge. They were so good they were used by Rolls Royce.

All

All the bottom row in the above pic have corroded bourdon brass tubes. The top row are all Freddie Dinnis (FSD) gauges.

Cheers,

Julian

.

Yes, Julian … you posted that back in November **LINK**

http://www.model-engineer.co.uk/forums/postings.asp?th=122593&p=1

[and appeared to be not amused by my reply]

.

I mean no offence to you, or to Mr. Dinnis; but I do not understand why the fact that he made good things one way should preclude others from considering alternative methods.

That said … I choose to abstain from further debate of the matter.

MichaelG.

.

P.S. for convenient reference, here is Bourdon's patent:

https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=9163A&KC=A&FT=D&ND=&date=18520803&DB=&locale=

The short paragraph commencing line 87 might be of interest to some.

Edited By Michael Gilligan on 14/01/2017 06:34:48

Andrew – You might consider getting gears of that size laser-cut!

. . . and Meadows and Passmore sell them down to 0.2 MOD – not cheap though!

Russell

Andrew – ok, another thought.

A local fellow makes replacement gauges, or "fixes" old gauges for full size steam, including CPR 2839 (which will never move) and CPR 2816, and…

What he does is take commercial gauge innards, and places them in cases he makes, with dials and hands he engraves.

What that gives are gauges that look the part, but actually work.

At first, I was agast that someone would do that, but – these steam locomotives actually work, and need gauges that are accurate and reliable first and foremost, and should look the part as a secondary consideration.

(apologies if this is old news to you, or a path you have decided NOT to go down – the coffee's brewing and not doing its job quite yet!)

JohnS.

With the greatest of respect , the topic is actually the conversion of fire extinguisher gauges, rather than the continued celebration of the work of Fred Dinnis.

I do wonder if Mr Dinnis would have been rather quicker to encourage rather than disparage the efforts of others.

Neil

Fortunately I'm not building a loco, so I can ignore the exhortations of our resident expert.

Thanks for the gear cutter links and ideas. PPThornton list cutters down to 0.1mod, but I'm pretty sure they're all cycloidal form, for use in clocks.

I hadn't considered using the innards of a commercial gauge; I guess the issue might be finding one that is small enough?

I had considered using watch parts for the gears and hands, or even old aircraft instrument parts. The problem is not only finding what I need, but two sets of same. There are jumbo packs of watch parts available but what you get is what you get, not necessarily what you need.

There's another possible issue. In order to get high angular 'gain' through the gear train the pinion needs very few teeth, probably 8 or 10. In theory, even for 20°PA, that means undercut teeth, so you lose the conjugate action for part of each tooth engagement. The problem can be cured by using a different PA, say 30°. But for that one would have to make cutters.

Being a paid up member of the awkward squad part of the fun is to see if I can design the gears, make the small cutters and then cut the gears so that they mesh smoothly with minimal backlash.

Andrew