Compass cutting warped material

The Importance of Standoff Height, Especially with Bevel Cutting

Standoff height, the distance between the tip of the mixing tube and the material you are cutting, is important when cutting parts on a waterjet. In a previous blog I provided recommendations for proper standoff height. In general, stand off height should be about 0.100” (2.5 mm), or as thick as a dime. When your jet is perpendicular to your part, straight up and down, then raising the standoff will increase noise, mess and round the top edge of the part. You will lose a little cut power as well.

It’s important to maintain stand off under conventional waterjet cutting.

How Waterjets Work

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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Geologic versus Supersonic Erosion

The Colorado River versus Waterjet

Waterjet gives the best edge. The surface is unaltered, exhibiting no heat or stress damage.  This outstanding edge quality it is created by supersonic erosion.

Erosion is an extremely gentle form of material removal, however it achieves amazing results.

For example: the Colorado river can erode the one mile deep Grand Canyon in 35 million years, and the abrasive waterjet can erode over one foot thick of granite at 0.2 inch per minute (5mm/min).

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Why Not Ultrahigh-Pressure Slurry Jets?

A reader asked about Slurryjet, and why there are no ultrahigh-pressure units out there in the world today cutting in production.

First of all, let’s make sure everyone who has not studied the subject understands what we’re talking about here.  Abrasive waterjets today are created by pressurizing water, forcing it through a small jewel orifice where the pressure is converted to velocity, and then the abrasive particles are metered into a mixing chamber and accelerated like a bullet out of a rifle down the mixing tube.  Abrasive slurry jet is where a water/abrasive slurry is pressurized and pushed through an orifice.  It is inherently more efficient because the water and abrasive are going the same speed, and no momentum transfer from the water to the abrasive is taking place.

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The Waterjet Control: Inside the Brain Behind a Waterjet

In 1984 the abrasive waterjet was released as a commercially available product. It was powerful, it was incredibly versatile; but it took a special operator to run it.  Someone who was into it – who had caught the waterjet bug. Why? Because at the time, waterjet wasn’t all that easy to run.

I had the unique opportunity to be such a waterjet operator by getting into abrasive waterjet just one year after it was released.  While in undergrad and grad school running the waterjet laboratory and demo center I could reference basic cut speed tables for what was believed (at that time) to be the best operating parameters. The table included water pressure, orifice size, mixing tube size, abrasive size, abrasive flow rate and the maximum cut speed for a dozen common materials at one or two thickness levels.

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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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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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