Monday, August 7, 2017

A New Frontier for The Small Shop

A year ago, I wrote about a Kickstarter campaign for a small-shop waterjet cutting machine called the Wazer.  They're still not on the market (their web site talks about pre-ordering), so they're still not quite vaporware and not quite product.  While the Wazer is a CNC-based cutter, it might be useful to have the technnology for other sorts of cuts.

This month's Make magazine features an out-of-the-box development: a waterjet cutter for a couple of hundred bucks.  The Wazer is a $4500 machine.  Author Ben Krasnow looked around at what he could easily get and started figuring out how to create a waterjet cutter.
Ben does something truly noteworthy. He creates a usable waterjet cutter, capable of cutting through metal, wood, and other material. He accomplishes this using a rig he put together for only a few hundred dollars. The heart of the system is a Sun Joe pressure washer that he bought for $150 on Amazon.
Without buying the magazine, the best source of information on how to replicate his cutter is his 22 minute long video on YouTube.  Information on sources and how to get everything is in the "Show More" tab on YouTube, so you need to go there for that information, but he got most of these parts from two places: AccuStream (a waterjet parts supplier) and McMaster-Carr.

Ben’s cobbled-together rig might not look like much, but it does get the job done. In the video, you see him cutting through 1/16″ aluminum (at ~2″/min with .4 lbs/min of abrasive @3200psi), 1/8″ aluminum (at about 1/2 the cutting speed, abrasive and psi values the same), 3/8″ hardwood, styrofoam (in water-only/no abrasive mode), and bread (yes, bread).
He cuts the styrofoam and bread with only water, but water isn't very useful for cutting.  The agent used is coarse garnet abrasive grains, and the purpose of the water is to accelerate the abrasive to speeds that allow it to work.  It also probably cools the work area, too. Garnet is a moderately hard stone that has historically been used as an abrasive - you can still buy garnet paper.  It's a non-toxic, safe abrasive, just a harder version of the silicon dioxide sand in sandpaper.  The source he links to sells it at about 50cents/lb, so when you look at the 0.4 lbs/minute cuts they describe, you can think 20 cents/minute to cut the aluminum he demonstrated.   
 For the abrasives hopper, after doing research into commercial hoppers, he realized that they’re basically just a gravity-fed tub with a hole in the bottom (and a means of adjusting the amount of abrasive). So, he made his own.
Now, cool as it is to develop a usable waterjet cutter that can cost the builder under $500, I'm not sure where this fits in.  The attraction of a waterjet is that it cuts materials that are difficult to precisely machine: things like carbon fiber or fiberglass laid up panels, or glass.  Industrially, they're also used to cut steel and other hard metals, not just the aluminum he cuts.  I don't think this one has the horsepower to cut harder metals.  Cutting freehand is probably not any more dangerous than cutting with a moving saw blade.  Water at the pressures he's talking about will cut off a finger just as easily as a saw.  At the very end of the video, he suggests turning it into something like a CNC machine that moves the work into the waterjet, something like the way large X/Y Plotters roll paper under the pen, or some wood carving machines will roll a board back and forth while the carving tool moves in the other two axes. 


8 comments:

  1. I've heard of water jet cutters, I just hadn't seen one until now.

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  2. Ben mentions in the video that the project could be done at less cost by simplifying his stainless piping.

    I wondered if Ben's project could be the answer to getting a broken tap out of metal.

    Omax makes water jet cutters and this link covers what can be cut and states that hardened tool steels can be cut.

    https://www.omax.com/learn/what-materials-can-waterjet-cut

    This link goes to a UK company and mentions that carbides can be cut.

    http://onestopsealing.co.uk/water-jet-cutting/capabilities

    If, (large if) you build this with the KISS principle, it might be the answer to removing broken taps at a reasonable cost, and if it will cut a tap, it will cut an Easy Out.

    I also wondered about finer grit abrasives and whether a finer grit would be a better or worse choice for the cutting media. Ben mentions the formula for kinetic energy, and I wonder if a smaller media than his #80 choice would accelerate faster and deliver more energy to the cut.

    This company sells abrasive grits for air powered abrasive blasters.

    http://www.kramerindustriesonline.com/blasting-media.htm

    And they sell grit in a large variety of materials.
    This is the link to their aluminum oxide page, and the page mentions that it will cut sintered carbide.

    http://www.kramerindustriesonline.com/AO.html

    And they also have silicon carbide grit and they state that it is the hardest grit available.

    http://www.kramerindustriesonline.com/SIC.html

    As always, a very interesting and thought provoking post. Thank you.

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  3. I also wondered about finer grit abrasives and whether a finer grit would be a better or worse choice for the cutting media. Ben mentions the formula for kinetic energy, and I wonder if a smaller media than his #80 choice would accelerate faster and deliver more energy to the cut.

    Fascinating thought. I suppose if the big particles aren't reaching full speed, smaller ones might work better. If they're already going as fast as they can go, I guess changing grit sizes doesn't matter. Abrasives are available in pretty much any grit from small pebbles to powder, so it could be tested.

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  4. I would be extremely unhappy putting my hands anywhere near a water jet. I used to work for a water jet cutting company in the UK and it was not unknown for the water jet to shoot sideways as it started the cut and as it burst through the material. It was traveling at about 2 1/2 times the speed of sound as it hit the material being cut and I doubt it would slow much (or at least enough to make a difference) if it contacted flesh.

    The kerf is determined by the diameter of the nozzle used. Long story short, a drilled sapphire is used to provide the water jet which is moving at somewhere from 2 1/2 to 3 times the speed of sound. The jet causes a venturi effect which draws a mix of air and olivine - garnet is used for extremely fine cuts - which mixes with the water jet and then passes through a tungsten carbide tube to form the cutting jet.

    The sapphire lasts about 48 hours continuous use, the tungsten carbide becomes eroded and the internal diameter increases so that the nozzle lasts about 24 hours continuous use.

    As the tungsten carbide becomes worn the jet becomes less stable and less coherent, leading to a wider kerf and a poorer finish. It is then that you tend to get more random sideways water jets which can cause damage to anything close (i.e. about 3 feet) to the tip of the cutting head.

    One BIG problem is the loud white noise produced by the water jet itself. Wear hearing protection and make sure that you have the OK from the wife and neighbours, otherwise your dinner will be in the dog and you will be sleeping on the sofa as a minimum ...

    Phil B



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    1. I was telling a friend that the pictures of the inventor pushing the metal or wood through the water jet made me squeamish, but it's not like a table saw or bandsaw or lots of other things in my shop can't take off a finger without blinking. It's more than that.

      What's really different about the waterjet is how it's pushing garnet at high energy. They use high pressure syringes to inject through your skin, right? If that water/garnet mixture hits you, does it inject a trail of garnet through your hand or arm or whatever?

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    2. Machinery manuals for stuff with hydraulics at 3K PSI warn that hydraulic leaks cause deep stab wounds, and you need to go get them treated because they will get infected. They say to test for leaks with a piece of cardboard, not your hand.

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  5. The way it was explained to me was that the olivine (or garnet) first acts like shot blasting the surface to form a crater which then catches and concentrates the jet and the "grit". The diameter of the initial penetration is somewhat larger than the kerf due to the grit jumping back out of the crater like depression which grows in diameter until the stream breaks through the other side. Once the penetration is made, think of each particle of grit acting like a miniature saw tooth which will cut away the material.

    The jet loses power and velocity as it cuts through the material and paradoxically, the lower part of the cut is wider than the upper - the swirl as it loses velocity contributes to this. It isn't of significance in most things but if you want a perfectly square cut, you need to dress the cut edge.

    The deflected, sideways jet is still fast enough and grit laden to give a ragged cut through flesh and of course, base material, grit and water deeply injected into the wound will not be recommended from the FDA! Expect a deep and slow to heal wound with significant scarring and loss of flexibility. Not good. If it will cut metal, flesh won't be any problem. Plain water cuts meat neatly and the Norwegians use waterjet cutting to slice up frozen fish.

    I have also seen what happened to one mans foot when he was water blasting the underwater surface of a ship in dry dock. This is again at high pressure and velocity (3 times the speed of sound) and plain water will cut as easily as a chainsaw.

    Phil B

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    1. I have also seen what happened to one mans foot when he was water blasting the underwater surface of a ship in dry dock. This is again at high pressure and velocity (3 times the speed of sound) and plain water will cut as easily as a chainsaw.

      In the mid-80s we knew a guy who worked on the space shuttle's solid rocket boosters. They were coated with some foam that had to be removed to recycle the boosters. The foam was removed with a waterjet system. The scuttlebutt was that it was walnut shells, no one mentioned garnet or olivine, but this guy got hit across one of his boots with the jet. Missed the steel toe cap and cut off the end of his foot.

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