Guest Post: 5 Key Waterjet Terms We Should All Understand

I’m pleased to say that we have a guest blogger today.  Colleen Carnagey is a major part of Flow’s marketing group and she would like to introduce to you a new feature on our website that might be of real value to you as you become more educated on waterjet capabilities.

Over the years waterjet has created its own vocabulary. I’m fairly new to Flow (in Flow terms–4 years isn’t much on 20), and one thing I realized almost immediately is to feel confident in your understanding of waterjet technology as a whole, you must feel confident in your understanding of the terms used to explain it.

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Standoff Height: How Close Should You Be?

The standoff height is the distance between the tip of the mixing tube and the material. A standoff too high causes dimensional problems and alters the top edge created by the jet as it gradually widens in open air.  Standoff too low risks scratching material on rapid traverse between cut paths, or snapping the tube causing downtime, cost, and angry operators.  Standoff height should be approximately the thickness of a coin (varies depending on your nationality, obviously) – about 0.100” (2.54 mm).

Standoff Height

I guess I could end the blog right here, but I think there are a few more things you might want to know about standoff height.

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Impact of Grit Size on Abrasive Waterjet Cutting

Garnet abrasive is used on 95% of all waterjet machines. The size of the garnet abrasive typically used today for waterjet cutting ranges from 50 mesh to 220 mesh, with the most common being 80 mesh. Mesh values do not represent particles of an exact dimension, but represent a distribution of particle sizes. An 80 mesh abrasive will have some particles larger and smaller than exactly 80 mesh. Mesh sizes are usually determined by allowing abrasive to fall through a series of screens – each screen smaller in mesh size from top to bottom. A known quantity of abrasive is placed on the top and vibrated for a fixed period of time, and then the amount of abrasive on each screen is weighed to obtain the distribution.

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A child working in the FlowCut program

Anyone Can Program a Waterjet

A waterjet is a beam type cutter, somewhat like a plasma, oxy acetylene, CO2 laser, or fiber laser. Programming a waterjet, and operating one for that matter, is actually simple. In fact, a waterjet is usually considered to be easier to program and operate than most other machine tools. I say this because a waterjet typically does not require a change in parameters when going from one material to another.  The tool is non-contact, pierces material easily to start a cut, and cuts without heat so materials don’t change or warp from the cutting process. If you are newer to this blog, a previous article  on what makes up a waterjet might be a good precursor to reading this post.

Programming Waterjet 101

Fixture Well, And Often

Everyone who cuts parts out of raw stock or a work-piece knows you can’t cut a good part if it isn’t sufficiently held in place. So, what do we have to consider when we’re talking about waterjet cutting? The good news is a waterjet cuts with low force. Where a milling machine might force a rigid cutting tool into a material at 10, 100, 300 pounds of force (4.5, 45, 136 kg), the waterjet head doesn’t touch the part — just the supersonic stream that exits the head touches the part.The machine can’t tell if the jet is cutting material or just shooting into nothingness. The part, however, does feel low forces during cutting.

Cutting-pizza

Although the picture is of pure waterjet cutting pizza, I’m going to focus on abrasive waterjet cutting applications in this post. Fixturing requirements are different in pure waterjet cutting, partially because the material is often very light and the jet forces are an order of magnitude (10x) lower compared to abrasive waterjet.

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So, What Is Stream Lag & Taper?

A waterjet stream acts like a beam when cutting, much like plasma cutting and laser cutting. These types of non-rigid cutting tools have to address the beam flexing and changing within the target material to minimize part cutting errors.

What is taper?

Taper in waterjet cutting is when the entrance width of cut is different than the exit width of cut.

Taper_v2

What is stream lag?
Stream lag causes corner damage when the exit point lags behind the entrance point, shown in the bottom of the part below.

Plasma cut parts often exhibit an upside-down V-shaped taper where the width of cut is wider at the bottom. Laser and waterjet exhibit a normal V-shaped taper (more narrow width of cut at the bottom). Plasma, laser and waterjet can all yield stream lag errors when cutting a part.

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