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The difficult thing in water jet cutting, historically, was the development of nozzles that can withstand the abrasive fluid. These parts have to shoot a jet of fluid which can cut through steel and ceramic; but themselves cannot be easily cut by the passage of that fluid.

I'd be concerned about the life of the nozzles and the cost and availability of replacement parts.

All of the hydraulic components in a waterjet are subject to wear: hoses, connectors, etc. The nozzle is considered an item that you simply have to replace regularly, kind of like windshield wipers on a car.

The pump is one of the larger and more expensive components of the waterjet cutting system and also tends to fail at inconvenient times.

How would a failure ever be at a convenient time? :)

Well, as a student that makes martian rover analogues for University Rover Challenge (7th place, woooo) I can attest that there is failure at the most inopportunate time.

Our unis water jet crapped out right before we were to have final parts cut on it before sending them for competition. It was massive PITA

Think about a bomb that ticks down to 0 and doesn't go off. That would be awfully convenient.

That's actually pretty bad because you have no idea whether it's still going to explode. Like a firework that does nothing after the fuse burns into the casing.

The alternative is worse for everyone but the bombers. I very much prefer to have a bomb in the mall that might explode any moment to one that actually just did.

It happens when you turn it on since a sensor or self-test detects it. Failure was inevitable but conveniently prevents you from wasting time and money on that run. I know this kind of thing can happen with some kinds of equipment but can't speak to if these could detect failure early. Works with many types of equipment, including computer software and hardware.

I wonder if you replaced the abrasive material with a ferrous material could you direct the stream with magnets? I guess the material would first have to be charged. And would the water flow follow the [hypothetically] focused stream of ferrous material? I need some coffee.

Like a water-jet rail gun?

Actually I didn't think of that. I was thinking more of a magnetic nozzle. But yeah if you can steer a fluid with embedded magnets you should be able to accelerate a fluid with embedded magnets, right? :) or maybe not. I have no idea.

I don't think this would work because the particles are free to move within the water stream. Say you used something like a magnetic pinch ( https://en.wikipedia.org/wiki/Pinch_(plasma_physics) ) to focus the stream... my hunch is that all you'd achieve is to compress the iron particles with little or no compression of the water since the iron is just floating in the stream.

It's a good idea though.

That would be something of an advantage, no? The metal particles would be mixed with water still, pulling along some of the stream. The tighter the "beam" of metal particles, the better - presumably that allows a more precise cut.

No, because you also need to increase the pressure of the water for it to cut. The nozzle increases the pressure of the water by constricting a constant flow into a smaller area, which increases the kinetic energy of the water and the abrasive particles.

A magnetic pinch would (I expect, I only really covered them a bit in a plasma physics course so I'm not an expert) basically pull all the suspended iron particles out of suspension and compress them into a thin rod, without actually compressing the water very much. My hunch based on semi-informed knowledge is that it just wouldn't do much to actually cut anything, but I could be wrong.

The other problem is that magnetic fields also produce a lot of heat in a conductor. The metal particles that clump up would probably sinter together or even melt. On top of that, it'd take a lot of power to run... pinches aren't super efficient.

Rail guns work by passing a current through the projectile, which would boil your abrasive carrying fluid, unfortunately.

Perhaps he meant Gauss rifle or coilgun. I remember building one when I was younger, I wish I had thought of experimenting with ferrofluid.

Not a bad idea, but ferrous metals aren't very hard. Garnet abrasive is very very hard.

What if we used just enough ferrous metal as a kernel inside a grown crystal of garnet that the metal is sufficient to magnetically levitate the grain of abrasive; perhaps even extremely fine metal dust embedded within the grown crystal's matrix. Then could we pass the resulting metal-enhanced garnet abrasive grains through a Gatling rail gun?

Furthermore, since garnet has specific refractive properties different from other materials, could we pass all ferrous grains through an extremely high-speed discriminating chamber that looks for these properties in each grain, and magnetically directs the garnet grains back to the abrasive holding bin to vastly increase the recycling, while all other ferrous material goes a separate bin (for waste or other recycling purposes)?

I kind of wonder what problem this would be solving? It sounds magnificently cool, but also expensive. In the long run, I have great hope for fiber lasers becoming cheap enough for a home-version. Maybe a home-grade 1kw. Seems more practical than worrying about recycling sand.

I do love the idea of a Rail gun cutter, though - just not sure how it could really help cut things.

I'm thinking in terms of resource-constrained use cases, like on an aircraft carrier or further out, deep space exploration. Until this thread, I had always assumed a laser cutter would be just as good, but didn't realize some of the unique advantages of a waterjet cutter.

Interesting idea. If the water were conductive -- say salt water -- then I don't see why you couldn't steer it with magnets. The trick would be to keep an electrical current running through the jet.

At normal fluid pressures -- the classic example being peeing on an electric fence -- it's hard to get much current flowing since there's so much empty space between droplets. But at thousands of PSI, is that still true?

Water is dipolar (the oxygen atom 'pulls' harder on the electrons it's sharing with the hydrogen atoms, so the middle is negative and the ends are positive). It can be controlled by both electrostatic and magnetic fields, no salt needed.

Electrostatic, sure. You can deflect a stream of water from a faucet by holding a comb near it. But magnetic?

This sort of thing was actually used for water propulsion system:


The hardness of the material is critical. Ferrous material sounds like metal, which could just melt/spatter on impact. Probably not make a good cutting medium.

Is the wear of the nozzle still relevant considering the wear of the pump? I can imagine that the historical problems are due to turbulence in the system and we can simulate that adequately these days. Or do the abrasions inside the system dominate?

The nozzle and hose are easy to exchange, and a replacement part with predictable wear is a plus from the manufacturers/investors perspective. I'd wager that the pump is the problematic part here.

Waterjet operator for about a decade here (OMAX). The nozzle slowly and unevenly wears until such a point as you decide it's hurt precision enough that you want to replace it. Everything else on the machine wears out in sudden and near random fashion. Replacing pump parts is done more just because if anything in there fails, you end up with something catastrophic failing. The regularly replaced pump parts are mostly just little gaskets and fittings, but they're pricey anyway. With 50k PSI going through little stainless pipes, sometimes something gets cracked, at which point you get fog shooting across a room out of a hole so small you can't find it with a microscope. Most failures besides the nozzle end up destroying at least three parts. I'm not sure how much of this would hold true at the much lower pressure used here.

Thank you for your insights.

Going a little off topic here: The failure mode you described reminded me of the horrible accidents that sometimes happen while maintaining high pressure oil systems in industrial machines. The smaller the hole, the worse the incident - human flesh and skin does not pair well with a thin, high pressure stream of fluid. I spare others the details, it's not pretty.

Do you think that a 'personal waterjet' poses more dangers to the operator than machines like a band saw or drill press?

Well, when I see someone using a bandsaw, they're usually running a piece of wood through it with their hands near the blade. Cuts seem relatively likely, severity seems moderate.

With a waterjet like the one I use, you would generally have a computer moving the nozzle and not even have a reason to be near the nozzle. It also sounds like a jet taking off. I seem to remember the water speed being something like mach 3 exiting the nozzle, but don't trust my math on that. Cutting is done under water most of the time, which muffles most of the sound and splashback.

On the other hand, I carry a card that tells medical professionals how to treat me if I arrive with a waterjet injury. Waterjets don't work great on laminated materials, because when they hit a transition between materials, they tend to send their energy horizontally. This is also true if it goes through skin and hits bone. It basically shoots you up with water, air, and abrasive, which travels along or in your bones. I've never even come close to injuring myself with the nozzle, but it would absolutely be more scary than a bandsaw.

The closest I've come to injury was one day when I felt an eerie sensation of wind on my skin. I grabbed a piece of paper and tracked it down to a 1/4" stainless water line. I backed away and ten seconds later it was a white cone of fog from a leak that was visible for about 15 feet without even feeling wet. The moral of the story is don't feel around for leaks.

I heard stories about injuries you can get from high pressure leaks from hydraulics guys. By all accounts they absolutely horrifying. They would have their flesh detached from their bones and oil basically soaking into their flesh so that it becomes necrotic later on. Plus they're said not to hurt immediately, doesn't help with treatment. Makes me glad I don't work on hydraulics that's for sure.

Same danger. A personal water jet will cut your finger off just like a bandsaw. If it can cut metal, it can certainly cut YOU!

I tried to avoid painting a mental picture. I'm not talking about cutting here but rather the things that happen when fluid is injected under your skin and into your body.

Amputation of extremities is not the worst case scenario :-/

Yes, the nozzle wears at mostly a steady rate, while pumps, with their moving parts, seals, etc. can fail suddenly.

Though they have less of a problem pumps pushing 7000-15,000 psi mentioned elsewhere on this thread instead of the 60,000 to 90,000 of traditional waterjet cutting machines.

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