How to choose the right abrasive for your CNC waterjet

January 15, 2019

Want to get better performance out of your CNC waterjet, and more profitability too?

You can do lots of things. For today, we’re going to focus on choosing the right abrasive type and size.

How to Start

First, you want to examine your material and its cutting specs. You’ll also have to look at how difficult your material will be to cut and the finish you want for your abrasive.

Regardless of the abrasive you choose, it must have hardness, toughness, and a certain shape. When you get right down to it, virtually every abrasive substance has been considered for use as a CNC abrasive.

Almandine garnet, however, has emerged as a great all-around abrasive. Other minerals can be harder, heavier, or lower in cost, but none has all of these qualities in the proportions you want like almandine garnet. So if you’re not sure what else to choose, it’s definitely a good starting point.

What Your Abrasive Needs

Abrasives need certain qualities more than others. Here are those qualities, and why your abrasive needs them:


Waterjet cutters need to balance cutting speed and component wear. A soft abrasive extends nozzle life, but slows cutting. A hard abrasive cuts faster, but also erodes your nozzle faster too. Almandine garnet balances the two well, as it falls to 7-8 on the Mohs scale.


Mass multiplied by velocity gives you the primary cutting force of CNC waterjet machining. Simple logic dictates the ideal abrasive has the heaviest particle the water stream can accelerate to maximum velocity. Since Almandine garnet has a specific gravity of 4.0, it’s the ideal abrasive in this regard.


Weak material breaks down in the focusing tube and end up too fine to make the cuts you want. An abrasive that’s too tough gets rounded in the mixing process and winds up too dull to cut well. The ideal abrasive has a measured rate of breakdown and creates sharp, angular cutting edges.

Particle Shape

You know the constant balancing act that must be played among acceleration, wear, and cutting. The ideal particle shape depends on the material you need to cut and the type of edge your customer wants.

Typically, rounded grains get used for general purpose cutting. Angular grains get used when you need a faster cut or superior edge finish.

You can find more detail on this from Randolph Rapple. But for now, you have an idea of what you need to consider to choose the right abrasive for your CNC waterjet.

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