To understand what taper is, you will first need to understand the meaning of kerf. Kerf refers to the amount of material the waterjet removes as it cuts through the material. Kerf width depends on various factors such as material type, material thickness, condition of the waterjet nozzle, and more. If a cut has taper, it means there is a difference in kerf width at the top of the cut versus the kerf width at the bottom of the cut. Taper can cause parts to fall outside of tight tolerances and design specifications, so it is important to understand what causes it and how it can be eliminated.
In addition to the kerf width, the abrasive waterjet cutting process also introduces a kerf angle. The kerf angle refers to the taper or bevelled edge that occurs on the cut surface due to the nature of the waterjet stream. The kerf angle can vary depending on factors such as the type of material, material thickness, the waterjet pressure, the waterjet nozzle design, and the cutting speed. It is important to consider the kerf angle, especially when precise vertical cuts are required.
Different Types of Taper
Depending on the speed of the waterjet and the materials used, there can be different kinds of taper. Four of the types include:
V-Shaped Taper. This is the most common kind of taper, with more kerf width at the top of the cut than at the bottom. Usually associated with faster cutting speeds, V-shaped tapers occur because the waterjet stream loses some of its cutting power as it cuts through the material.
Reverse Taper. Reverse taper has a wider kerf at the bottom of the cut rather than the top. This type of taper can be produced when cutting too slowly or when cutting soft materials.
Asymmetrical Taper. If a part cut by a waterjet has different taper angles on different sides, the likely cause is a faulty cutting head setup or calibration errors.
Barrel Taper. This type of taper is shaped like a barrel, with a wider kerf in the middle of the cut rather than the top or bottom. This kind of taper is due to waterjet orifice failure or extreme nozzle wear.
Some of the many factors that affect taper :
Speed of cutting. Higher cut speeds result in a V-shaped taper, whereas lower cut speeds result in less taper. Very slow cut speeds can even result in reverse taper. However, some waterjets with taper compensation can eliminate taper even while cutting quickly.
Amount of nozzle standoff. If the nozzle is too far away from the material, the jet stream can spread and cause tapered cut faces with excessive erosion on the top edge of the cut.
Quality of cutting stream. More focused nozzles produce less taper. If the orifice and/or nozzle are worn or damaged, the cutting stream loses its symmetry and coherence.