Re: [CAD_CAM_EDM_DRO] OT! Air compression, reply off-list

John wrote:

> [Please reply to me off list]
>
> I apologise for this being off topic but I noticed a couple of you guys seemed to understand what is required for a pump to achieve the high pressures of a water jet table and some where interested in the idea of machining a pump out. I am interested in how a high pressure air compressor differs from a standard one you see on car spray guns etc. The type I mean are the ones used for recompressing scuba air tanks. They achieve a pressure up to around 3kpsi and I was wondering if there's a tiny possibility I could machine such a 'lower' (compared to water jet) pump head out myself.

These generally are 4-stage compressors with intercoolers between each stage. There are other
concerns in this, because this is air you are going to BREATHE! If there's any contamination of
the air, you could die! Oil mist contamination of breathing air is an old problem, but probably
would not be such a problem for air stored for several hours in a tank. But, oil mist getting into
the tank could cause a fire and explosion of the tank, or possibly low-rate decomposition and
poisoning of the air. So, you'd probably need an oilless compressor, or at least find an oil
that would not contaminate the air.

> I have been offered the opportunity to go scuba diving with some of my metal working amigos from work but the gear is expensive and I'd like to be able to recharge my own cylinders. If you like, this is a CNC project more than problem as it'd probably go on some CNC machinery, like me lathe.

The cost of recharging the tanks seems minimal compared to the cost of the tanks, themselves.
Any, you have to periodically test the tanks. Aluminum tanks have a short lifespan, and then
must be retired, or the risk of them bursting becomes serious.

> So, is it possible? What do the pumps look like inside? Do you have any reference I could look at? It's got to be cheaper than buying them. They're about £4k new and they aren't running at the monster pressures a water jet table does.

Although high-pressure water is fairly dangerous, high pressure air, especially with large
tanks involved, is REALLY dangerous! I really wouldn't even think of fooling with this.

I DO know that there was a popular military surplus item called a strut pump that was used
on certain bombers from the WW-II to early Vietnam war era, that could be used to
refill SCUBA tanks. I don't think it could reach 3000 PSI, but could do well over 1000 PSI.
These would probably be hard to find now, but were plentiful when I was a kid, and sold
for under US $100.

Jon