The causes of taper and ways to minimize it

CNC WaterJets use a stream of water to cut material. Taper is caused when this stream of water, erodes the material unevenly so that the top of the cut is not the same as the bottom. An example would be a cut that is wider at the top of the material, and thinner at the bottom.  Some machines have the ability to offset taper by tilting their cutting head, not all machines have this ability. For this reason, it is helpful to know how taper is caused and ways to minimize its effects.

The causes of taper

There are several different reasons why you might be getting a tapered cut. One reason might be the distance of the nozzle from the material you are cutting. Normally, the closer the nozzle is to the material, the less taper there is. The hardness and thickness of the material can also have an effect on taper. Harder, thicker materials such as steel typically exhibit less taper than thinner, softer materials. Another cause of taper is the speed of your cut. If the nozzle moves too fast, the cut could be tapered in one direction and if it moves too slowly you could get it in the opposite direction. The other big cause of taper is the quality of the jet stream exiting the nozzle. The more focused the jet of water is, the less taper will be produced. This jet stream of water is directly impacted by the nozzle’s design.

Ways to minimize taper


If you have a machine that does not have the ability to tilt its head to offset taper, there are still several techniques you can use to help minimize it. One such technique is to use a smaller nozzle, such as a nozzle with a 0.010” jewel and 0.020” mixing tube. You will also probably want to use the lowest amount of nozzle stand-off that you can. The lower you can get to the material, the less taper you will have. Perhaps the most important and best way to minimize taper is to cut slower.  Although cutting too slow can cause reverse taper at the bottom of the cut, it is generally a good idea to cut slowly to minimize taper. If you utilize these methods, you should be able to effectively compensate for taper even without the ability to tilt the WaterJet’s head.

How to design the part you are cutting to prevent taper
It is possible to actually design the part you are cutting to reduce the impacts of taper. For instance, parts cut using an abrasive jet will often have a few thousandths of an inch of taper. You can use this knowledge to combat taper. An example would be if you are cutting gears, try cutting every other one upside down. By doing this, when the gears mesh together the taper will essentially cancel each other out.


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