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).
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.
If the standoff height is lifted up high in abrasive waterjet cutting (and often in pure waterjet too) you get a rounding of the top edge of your cut. In some applications I’ve dealt with over the years we did this on purpose, but usually the slight frosting and rounded top edge created by a standoff too high is not desired. Why do we get a rounded edge when we cut with a higher standoff? It is because the supersonic jet is shooting through air and the outer layers of the jet are slowing down. The outer layers not only slow, they spread out making the jet wider. When you cut with a high standoff the wider portions of the jet don’t cut as well (the abrasive isn’t traveling as fast), but they will cut well enough to round the top edge.
Now here’s an interesting trick. If you cut under water the jet is not exposed to open air, and the jet tends to stay together for a greater distance allowing for a higher standoff without rounding the top edge. When cutting underwater with a head fixed so that it is normal (perpendicular) to the material, you can double or triple the standoff height with little or no dimensional change to the part. An added bonus is the jet is very quiet.
You may be wondering: why doesn’t everyone cut underwater all the time? My customers say it is because the operators like to see the jet cutting. Operators of all types of machine tools like to see the actual cutting process to keep tabs on where the rubber meets the road. Also, buoyancy can make it more likely to have thin, small parts tilt up upon being cut free. In another blog I’ll talk about tabbing and other means of getting around this issue.
Lastly, some systems do not have adjustable water level control. Still, those customers often still cut under water; they add water via a garden hose and remove it via the overflow drain to raise and lower the level (albeit slowly).
Another question I get is do we every want standoff lower than the recommended height? In some cases in abrasive waterjet cutting where you’re cutting glass or polished material or very small parts you can reduce surface frosting to nearly nothing by dropping the head down to ½ or 1/3 the recommended standoff height. Operators need to really pay attention now. When standoff is really low there is no benefit to reducing frosting by cutting under water.
In summary, the tip of the cutting head should be kept approximately 0.100 (2.54mm) from the material. There are exceptions to every rule, of course, but this is the general recommendation for waterjet cutting.
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