Ok, let’s start of with the easy bit - the suggested Big Red Button for the welding area.
Spraying water on someone welding, holding an instrument designed to discharge an arc and use that electrical energy to melt metal, is a seriously bad idea - never mind the effect on equipment should someone set it off by accident, and the mess/damage it could cause. That’s a seriously bad idea, and is why you use buckets of sand and CO2 extinguishers with welding. Which I agree we will definately need in the vicinity if we are doing that level of metalwork.
The cutting off electricity is also not a great idea, due to accidental pushing, and other issues. Any issues electrically should be dealable with the RCBO or other safety equipment that can either plug in to the socket (small circuit breaker etc) or by the equipment is self (otherwise the equipment wouldn’t be safe to use). Any electrical fire would be on the welding desk or in the equipment - the first is solved by having the equipment away from the desk/workspace while running it, while the second is solved by the socket on the wall having a switch, and estop buttons in sensible places on pieces of equipment that we agree need them.
It’s worth noting that these machines are sold for home use, bar the Big One (which is of a type also sold for personal use), and therefore will be relatively safe with a bit of care being taken.
I strongly agree about the need for ventilation along that back wall, for all equipment that needs it - I include the soldering area, CNC bench, maybe a small spray painting area, and definitely welding and smelting. Metal vapours are really quite toxic (and that goes for soldering too), and the ability to remove that hazard is sensible, and is somthing I’ve been thinking about for a while - although I didn’t include the CNC in my thoughts originally.
Firstly, I think that whole room ventilation for the welding area is not a good idea - firstly we have issues with heating the space already, secondly you’d need significantly large equipment to move that much air, and finally it’ll be really noisey.
I suggest piped ventilation going under the proposed cable tray, with a series of in-line fans ( like this http://www.screwfix.com/p/manrose-mf100-100mm-mixed-flow-fan/26867 ) with area;/tool apropreate ventilation fixings, and maybe a larger fan at the final exit point. This would allow for a single easy system to be used, and to be later added to, removed, updated, etc. We can then add air gates and have the ventilation turn on when a gate is open.
I’m thinking 1inch dia flexible/fixable pipes for the soldering area so they can be placed unobtrusively by work when soldering, an attachment for the CNC miller, potentially a connection for the solder oven if it’s close to another ventilation area, and a ventilation setup for the metal area - I’m thinking a duct box similar to the first illustration in this HSE publication http://www.hse.gov.uk/pubns/guidance/wl10.pdf which could also be used or other metalworking fumes. I’d also like to have a position free for potentially adding on a small fume cabinet/paint booth on the extraction system.
Considering the woody workshop would need a similar extraction system in some respects, the air gates, piping, fittings, etc, should all be interchangeable so we only have to effectively design the system and its parts once (and so we can have spares on standby for both systems).
Weldy Grindy Area
The discussion around the lipped table isn’t just for smelting (although that’s why I’d put a good lip on it, to limit spills and reduce risk if we do go down the smelting route), but also as a welding bench, as a steel table helps with connecting up your cables when welding, and is easier than other options to clean welding dross off etc.That and the fact that it poses no fire risk, won’t get damaged by welding or grinding, and will take magnet clamps well if we go that route for fixing parts down for working on them.
Walls or Curtains
I’d first like to state that welding screens and curtains are the industry standard for this kind of work, and it’s normally either that or breezeblock in any welding shop I’ve been in. By screens I mean somthing like this - https://www.machinemart.co.uk/p/clarke-wsc1-heavy-duty-welding-screen-frame/
They come in either the green canvas type, another coated fabric or fibreglass, or in a semi-transparent flame and UV protected plastic designed to protect from both sight damage and sparks (both of which I agree are serious issues for bystanders and the space in general).
If our concern is them potentially being moved in error by a member who isn’t welding, we could attach bolts (http://www.diydoctor.org.uk/project_images/Shedlock/brenton-bolt-and-keep.jpg) and have dedicated holes in the floor to drop the bolts into when the curtains are in use, which should minimise the hoverboard/clutz issue - although I’d argue this is a non-issue as we haven’t had these issues with the wood workshop before, as people are careful around tool users. I’d also argue striking an arc is usually enough of a warning via both hearing and sight to keep people away.
Startling is again, similar for any power tool, and I’d argue that a barrier like a curtain makes people think twice before approaching someone using a welder. Yes, the mask reduces perception while welding, but people other than an instructor or another welder shouldn’t be approaching a welder who happens to be welding - for a start they will happen to be partially blinded by the arc of they get closer than the curtains and don’t have a mask on. Furthermore this ‘situational blindness’ also happens when people have headphones on, are using noisy equipment with their back to you, or are really into their work. And most people understand that, and work on that basis when approaching someone using a dangerous tool.
As for distractions for people working on the tables, as you said, the sound isn’t that bad, and I’d argue the light isn’t any more distracting than the tablesaw and bandsaw. And the welders are likely to be used significantly less than the bandsaw, which our members appear to truly love using (because it’s utterly awesome).
I think un-coated wooden walls in a weldy-grindy area are a large fire risk, and as we saw before, the cost of coating OSB in the paint that we would need is cost prohibitive. The time it took to put up the walls is also an issue, as it took a lot of work, material, and time (we’re upto eight months now, and the original plan was to have them up very rapidly - which didn’t happen for a number of good reasons). Furthermore, with the space as it is, I’m loathed to agree to cut off a chunk of the hackspace to a room that will be this specialised, and that we haven’t got a strong and current user base for. If we used moveable welding curtains, we can keep the space open and in use when people aren’t welding, and keep the space very open and airy. Never mind extra walls cutting off ventilation to the space if we have an accident with a bottle of acetone or worse, perfume.
As for adding the darkroom spec, that’s a totally different specification with different requirements and needs - like a sink, light restriction, etc. I’d argue that not only are they not particularly good bedfellows (flammable chemicals on a welding table… somthing I’d like to avoid), but the spec of both together will make putting the room together prohibitively expensive - I also think there might be ways to do B&W film processing without the need for a full darkroom, but I need to look that up. Then finally, the last, solid, nail in the coffin for the darkroom I think is the fact that again, we don’t really have that many users who want to use it. It’s literally me and one or two others I think, and at this point I don’t think that’s a great way to spend funds (however much I’d like to… I have a canister of B&W film I’ve needed to develop for several years now).
Many of these issues are really about risk management, and we need to look at what the real risks are, how likely they are to occur, and how we can manage those risks in an appropriate way with an appropriate level of funding. Brunelling it is nice (and a hobby of mine if I’m honest), but we don’t have infinite funds, time, or energy. Unless we have a secret billionaire in our midst.