TECH TIPS: The impact of grit size on abrasive waterjet cutting

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There are several fewfactors you should consider when selecting the abrasive for your waterjet cutting applications. PHOTO courtesy Flow.

This TECH TIPS instalment is provided by Flow, a member of the Shape Technologies Group

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 to 220 mesh (grit-size), with the most common being 80. Mesh or grit values represent a distribution of particle sizes, and each size is typically 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 before the amount of abrasive on each screen is weighed to obtain the distribution.

In general, we cut with mixing tubes with internal diameters of 0.030, 0.040, and 0.050 inch (.76, 1.0, 1.27 mm).

The abrasives used in each are typically 100 to 120 mesh, 50 to 80 mesh, and 50 to 60 mesh. In very special applications, mixing tubes of .015 to 0.020 inches (.38 to .51mm) are used, requiring 220 to 150 mesh abrasives.

Here are a few factors you should consider when selecting the abrasive for your cutting applications:  

Surface Finish
The edge produced by abrasive waterjet cutting is sandblasted. This is because the garnet sand particles are removing the material rather than the water. A larger mesh size (a.k.a., grit size) will produce a slightly rougher surface than a smaller grit size. An 80-mesh abrasive will produce approximately 125 Ra surface finish on steel as long as the cut speed is 40% or less of the maximum cut speed. It’s important to note that surface finish and cut quality/edge a quality are two different variables in waterjet cutting, so be mindful to not confuse the two.

Cut Speed
The larger the abrasive particle, the faster the cut speed. 50-mesh will cut slightly faster (4 to 8%) than 80-mesh at the same abrasive flow rate. Very fine abrasives, such as 150 or 220 mesh, are typically used to cut slower for special cutting when a very smooth edge or very small sized mixing tube is needed.

Root causes for clogging are spawned by these primary four issues:

Moisture can cause abrasive to clump and then clog.

Oversized Particles
The abrasive particle distribution must be such that the largest grain is no larger than 1/3 of the mixing tube ID (internal diameter). If you are using a 0.030” tube, the largest particle must be smaller than 0.010” or the mixing tube will likely clog over time as 3 grains try to exit the mixing tube at the same time.

Foreign Debris
Debris in the garnet delivery system is usually caused by carelessly cutting the bag of garnet open, or by not using a trash screen atop the garnet storage hopper.

Very small particles like dust increase static electricity and can cause rough abrasive flow to the head. Dust-free abrasives ensure a smooth flow.

Keep your abrasives clean and dry to prevent moisture, oversized particles, debris, and dust from interfering with your flow.

Cost is reflected by not only the cost of the garnet but the cut speed and overall time to cut your part (slowing in corners versus linear areas). When possible, cut with the largest abrasive that is recommended with that mixing tube, and evaluate cutting speed along with garnet cost. Some abrasives may cost more but are tougher and more angular, thereby producing higher-speed cutting.

Mines across the world naturally produce garnet of a certain size. For example, if a mine naturally produced mostly 36 mesh, then the abrasive must be ground to obtain 50, 80, etc. Different abrasive suppliers have varying costs per mesh size. All garnet abrasives will cut differently, as well, as some garnets fracture more easily or are more rounded.

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