Eli5: in a high pressure water cutting macchine, how does the water jet not cut part of the machine under the object that is being cut?
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The area underneath is a tank full of water. Even in air the jet quickly spreads out, but in a tank full of water it dissipates even faster.
The object to be cut is supported by thin but tall metal beams. These beams are made of metal, so already somewhat difficult to cut, but their size and shape makes it likely that the water stream will just go past them into the tank for most of the cut time.
There’s usually a basin of water beneath the object. Water can’t really be cut, and the pressure dissipates rather quickly when it hits the basin.
The tank below has a heavy metal grate holding up the material to be cut which does get chewed up over time. It’s just thick and in the same orientation as the water so it rarely cuts all the way through.
The lowest part of the cutting area is a water catch basin. So any water and sand from the cutting jet will get slowed down by the water. But you still need something to support the material that is being cut. The first protection is to make sure to have as little material for support as possible. So instead of a sheet there is a grid. That means that most of the time the cutter will be between these grid squares and therefore only the material being cut is bellow the cutter. But when the cutter do cross over one of the ribs in this support material it will wear it down.
The first way to protect it is to make sure the feed rate of the water jet is set correctly for the material it is cutting. You want the cutter to move just fast enough so that as it cuts through the material it moves on. So very little of the water jet actually goes through the material onto the supports. But the support material is still made quite thick, usually over an inch. So when it gets tiny cuts it is not going to compromise its structural integrity. But over time you do have to replace this as it does wear down and become uneven.
And it’s not the water that does the cutting but rather the sand or whatever else they add in the water stream to do the cutting . Like bead blasting , it’s not the air
What hasn’t been mentioned yet, is the water jet loses a lot of its energy as it cuts the work piece, so even though it wears down the support, it receives a less powerful stream, provided the machine is setup optimally.
it does!
the part under the part you are cutting is either metal or plastic, in either case designed to have as little surface on the cut material as possible and still hold it up. and it’s designed to be replaced after every so many hours of use, when it can’t support the material in a nice flat manner.
I own a waterjet and have operated it for 4 years. It has been mentioned but thought I would fill in some gaps.
The waterjet does in fact cut what is beneath it but the further you get away from the nozzle the less “machining” that is occurring. The metal grid that your part sits on is a consumable. It gets eaten up especially in repeatable production runs.
Our grid is 5” deep. It’s suspended in a tank that is about 4 feet deep and full of water. While 4 feet is very far from the nozzle it’s not impossible to poke a hole in the bottom of your tank if you leave the nozzle stationary and the waterjet on. It’s even more likely if the water level is low.
Also keep in mind the shape of the actually jet out of the nozzle is like that of a flashlight. It’s a cone and the further you get away from the source the wider that cone is. This is why thicker parts on a waterjet tend to be less accurate. They call this taper. There are ways around the taper though such as a nozzle/head that can slightly tilt as you’re cutting.
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