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.
In general, we cut with mixing tubes with inside diameters of 0.030, 0.040, and 0.050 inch (.76, 1.0, 1.27 mm).
The abrasives used in each are typically 100-120 mesh, 50 to 80 mesh, and 50-60 mesh, respectively. In very special applications, mixing tubes of .015 to 0.020 inch (.38 to .51mm) are used, requiring 220 to 150 mesh abrasives, respectively.
Here are a few factors you should consider when selecting the abrasive for your cutting applications.
The edge produced by abrasive waterjet cutting is sand blasted. That is because, of course, the garnet sand particles are actually removing the material, not the water. Larger mesh size (a.k.a., grit size) will produce a slightly rougher surface than smaller grit size. An 80 mesh abrasive will produce approximately 125 Ra surface finish on steel as long as cut speed is 40% or less maximum cut speed.
The larger the abrasive particle the faster the cut speed. We will use 80 mesh as our starting point as it is the most common. 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, cut significantly slower and are used for special cutting when a very smooth edge or very small sized mixing tube is needed.
Root causes for clogging are spawned by four primary issues: moisture, over-sized particles, foreign debris, and dust. All abrasives must be kept clean and dry.
- Moisture can cause abrasive to clump and then clog.
- The abrasive particle distribution must be such that the largest grain is no larger than 1/3 the mixing tube ID (internal diameter). So, if you are using a 0.030” tube, the largest particle must be smaller than 0.010” or you will very likely get clogging of the mixing tube over time as 3 grains try to exit the mixing tube at the same time.
- Debris in the garnet delivery system is usually caused by carelessly cutting open the bag of garnet or by not using a trash screen atop the garnet storage hopper. Make sure debris isn’t in with your abrasive or you can cause stoppage of cutting by clogging.
- Lastly, very small particles, dust, increases static electricity and can cause non-smooth abrasive flow to the head. Dust free abrasives flow better.
Cost per inch of cut is reflected by not only the cost of the garnet, but also the cut speed. If possible, cut with the largest abrasive that is recommended to be used with that particular mixing tube. And make sure to evaluate cut speed along with garnet cost. Some abrasives might cost more but are tougher and more angular, thereby producing higher speed cutting.
Garnet 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. Therefore, different abrasive suppliers have different costs per mesh size. All garnet abrasives do not cut the same. Some garnets fracture more easily, or are more rounded. Talk to your waterjet manufacturer for a list of recommended or certified abrasives that will cut properly, reliably, and offer a good cut speed.
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