However, it is possible to cut with the nozzle much further from the material, which can be useful when cutting warped plate, round tubing, or materials with small surface features in the vertical direction. If you need tolerances greater than ±0.005″ (0.13 mm), then you should keep the stand-off as low as you can go. If you don’t need such high tolerances, then you can go up to perhaps an inch or more, if needed for the situation. At high stand-off distances, you will also experience additional “frosting” around the top edge of the cut, and reduced cutting performance.
Having too low a stand-off increases the chances that the nozzle will plug from backwash. Once the nozzle plugs, water will typically back up through the nozzle and possibly into the abrasive line. This will require you to stop machining and clear the plug (and possibly the abrasive line) before resuming.
If you are getting a lot of nozzle plugs, you should consider increasing your stand-off.
Cutting warped material
When working with warped material, the simplest thing to do is to simply raise the cutting head so that it is high enough to clear the highest point on the plate. Since you are using warped plate, you are obviously not doing high precision work, so the slight increase in cutting stand-off will have negligible effect.
Otherwise, some machines have features for “terrain following” or automatically setting the stand-off at each pierce point, or pre-programming the Z heights along the tool path. All of these methods have advantages and disadvantages depending on the particular circumstances. Terrain following has it’s own set of problems because of the water and garnet and tipped parts that can all interfere and make it unreliable. In general, though, simply setting the cutting height to clear the highest point on the plate works fine.