|Pictured to the left are 4 common forms of “taper”.
It is also possible to find “combination taper”, where two of the above types of taper may combine.
The biggest causes of taper are:
- Distance of nozzle from material. The closer you can get the nozzle to the material, the less the taper.
- Hardness of material (usually harder materials exhibit the least taper).
- Speed of cut. If the waterjet nozzle moves too quickly, you can get taper in one direction; if it moves too slowly you get taper in the other direction.
- Quality of jet exiting the nozzle. The more focused the nozzle, the less taper exhibited. The quality of the jet is determined by the quality and wear on the mixing tube and the nozzle design.
- Quality of abrasive used.
- Thickness of material (thinner materials tend to exhibit more taper than thicker materials).
Some machines have the ability to remove V and reverse taper by simply tilting the cutting head to compensate, or cutting at a pre-determined speed to minimize the taper. For some more information on such machines and accessories, see Introduction to tilting.
If you are trying to minimize taper, and you don’t have a tilting head, you can minimize the taper using the following suggestions
- Use a small nozzle such as a 0.010″ (0.25 mm) jewel and 0.020″ (0.51 mm) mixing tube.
- Use the best abrasive that you can, and the largest grit size that will fit the nozzle without plugging.
- Use the lowest amount of nozzle stand-off that you can get away with. The lower you can get to the material, the less taper you will have. Consider pre-punching the starting holes with a pass at a higher stand-off, then lower the nozzle to do the rest of the cutting. Some controllers may have an automatic mode for doing this.
- Cut slowly. Generally speaking, the slower you go, the less taper you will get. However, if you go too slowly, you may experience “reverse taper” in some cases.
Taper can also be reduced by tilting the cutting head in the opposite direction as the cutting with an articulated tilting nozzle.
An articulated tilting nozzle for automatic taper removal
Note that it is not possible to completely eliminate “barrel” taper by tilting, so for very thick parts, some barrel will still remain, but there is still some benefit in removing the standard V shaped taper component.
Design for taper
It is possible to design your part to minimize the impact of taper. Parts cut on an abrasivejet will often exhibit a few thousandths of an inch of taper. Usually this is one of the biggest factors in determining the final tolerance of the part when cutting high precision parts.
Knowing this, you can use taper to your advantage. For example, when you are cutting gears, cut every other gear upside down. Then, when they mesh together, the tapers will cancel each other out. This can also be useful when embedding two parts into each other, such as when doing marble inlays.
Some machines can compensate for taper by tilting the cutting head. This is an expensive option, but useful if you need it, and becoming increasingly popular. Another method of reducing taper is to simply slow the cutting down, which reduces but does not always eliminate taper.