Why waterjet? Why not OILjet?

[strantor]

Building a DIY/hobbyist waterjet table has long been a dream of mine. I'll get obsessed with it and stay up late for a week straight looking at high pressure pumps and whatnot, then drop it, and a few months later, back for more. There are some pressure washer pumps out there that would do for a very light duty machine but above about 5,000PSI there isn't a whole lot more available as far as water pumps go, until you get into the 60kPSI pumps that are designed for commercial jet cutting operations.

I've tossed around the idea of using hydraulic pumps though, there would be some modifications required and/or dramatically reduced life expectancy pumping water through them instead of oil. Then for the first time today I asked myself, why try to pump water through them? Why not just pump oil? There are some very low viscosity oils out there now. Many newer cars use 0W-20 oil which feels like water on the skin. I don't see why that couldn't carry garnet and work just as well as water. I googled, could find no evidence to the existence of, or discussion about high pressure oiljet cutting.

Usually when I have a hairbrained idea, I google, and confirmation of the idea manifests itself as such:
- great idea: Someone patented it 75 years ago and it's been for sale ever since; I just never heard of it. I came up the idea on my own, too late.
- good/"ok" idea: someone patented it 75 years ago and it worked as advertised but didn't sell well (or sell for long) because a better solution exists (or came along shortly after).
- bad idea: some dingus tries it every 10 or 20 years and blows his thumbs or testicles off and it makes the news, a paper trail of documentation of just how bad this idea is.
- red flag: google results are nil. Crickets chirping. Either there is in fact at least one new idea under the sun (we all know that's not true) or the idea is so terrible that not even an idiot ever attempted it.

That's where I'm at: red flag. If using oil was feasible then wouldn't waterjet manufacturers be making oiljet machines instead of waterjet machines? everything would be simpler. No stainless/special metals/special seals/etc. A high pressure hydraulic pump is all you need. Too simple. There must be a "gotcha" that's obvious to everyone but me. What is it?

I know 0W oil is expensive. A commercial waterjet tank full to the brim with 0W oil would probably be worth more in oil than in hardware. But there's no reason the entire tank has to be full of oil. The tank could still be full of water, just have a layer of oil that rides on top, and a skimmer/separator to strip it off and feed it back to the pump.

 

[Cheeseking]

Interesting idea to spit ball over but no. Even 0W is way too viscous. The cost for the volume of oil required even for only a top layer so to speak would be huge. Never mind the difficulties trying to keep it from foaming and separating it from the water. The mist generated would be gawd awful also. Oil film on everything including the floor walls, people just everything.

Check out on google a company called Wazer. Better route if you want to WJet on a budget.

(Btw this is 15 yrs being involved with waterjets talking)

 

[pdentrem]

Oil spray containment would be my concern. Fine atomized oil mist will be inflammable!

 

[strantor]

Well, there you go; Aerosolized/atomized oil mist creating an explosive atmosphere and unsanitary environment. Pretty good reason not to do it.

I've seen the Wazer. It has a poly lid that keeps all the water mist contained in the unit. Think a similar approach to an oiljet could work? A 55gal drum of 0w20 oil can be had for under $1k. That seems like plenty for a machine the size of a Wazer ($7k).

 

[Tim9]

Other than fire hazards....which I’m sure is a possibility at higher pressures, kerosene would be a better viscosity. But, I’m guessing that once you get to a certain pressure then there’s a strong possibility of self instant combustion. That’s probably why they use water. Why not just use antifreeze....ethylene glycol. No rust there and somewhat better lubricant characteristics for the pump.

 

[strantor]

(Btw this is 15 yrs being involved with waterjets talking)

I'm going to keep asking questions; it's what I do. Please don't take it as a challenge to your experience.

Even 0W is way too viscous.

I did some looking and some math.
Viscosity of water:


viscosity of 0W30 (couldn't find data for 0w20):




So yeah, even if we heat the oil to the boiling point of water, it's still 10X more viscous than water at room temperature. But does that matter? Since (to my limited knowledge) waterjet cutting has only ever been about water, whose viscosity is well known and understood, do we know what effect viscosity has? I didn't even know water's viscosity varied with temperature, except for when it turns to ice. Do you experience viscosity-related problems with cold water? If so, what's the solution? Could the same solution be extrapolated further to allow oil as transport medium? Could it be as simple as a different sized nozzle/orifice?

 

[strantor]

Other than fire hazards....which I’m sure is a possibility at higher pressures, kerosene would be a better viscosity. But, I’m guessing that once you get to a certain pressure then there’s a strong possibility of self instant combustion. That’s probably why they use water. Why not just use antifreeze....ethylene glycol. No rust there and somewhat better lubricant characteristics for the pump.

Good suggestions. Here they are in some context:
[source]


Kerosene being only twice as viscous as water I think is the better one. I don't think auto ignition would be a problem in the absence of air, would it? I mean diesel auto-ignites from compression in an engine, but only with a very specific air:fuel ration. Kerosene is however quite smelly. I had a leaky bottle behind my shelves once in my garage and just a few drips per hour was enough to make the place feel unhealthy.

 

[Cheeseking]

No worries I didnt mean to sound as if Im the authority by no means just that I do have some background/ experience running one and dealing with the vagaries of the process. It’s definitely harsh and unforgiving even with ideal conditions and maintenance. Im all for out of the box ideas. I’m king of crazy sometimes so no worries I challenge my own ideas to death trying to poke holes in them. I may have missed the reason you’re wanting to use oil vs water. Is it due to corrosion of the parts or machine???

 

[eugene13]

What about super critical CO2? other than it could suffocate you.

 

[Cheeseking]

We have used dry ice as a blasting media to remove mold coatings from permanent molds for aluminum casting. The idea is it “evaporates” and doesn’t leave a mess like shot blasting with sand, walnut shell or steel shot does. Might work but would require a ton of compressed air flow$$$