Controlling Taper

Types of Taper Inherent to Waterjet Machining
In waterjet machining, taper refers to a difference in the kerf width at the top of a cut and the kerf width at the bottom of a cut. A typical waterjet cut has a slight taper, the most common type being V-shaped taper. This type of taper is generated because the jet stream loses some of its cutting energy as it cuts deeper into the material. Slightly more material is removed at the top of a cut, where the jet stream enters the material, than at the bottom of the cut, where the jet stream exits. V-shaped taper is usually associated with fast cutting. In general, the greater the nozzle speed, the more pronounced the taper will be.

Slow cutting speeds can produce reverse taper where the kerf width is wider at the bottom of the cut than at the top. This is the result of the jet stream removing more material at the bottom of the cut than at the top of the cut. Reverse taper can also occur when cutting very soft materials.

A third type of taper is barrel taper, where the kerf width is widest in the middle of the cut. This type of taper can occur when cutting very thick materials. There are other, less common types of taper that could be produced, but those are caused by calibration or set-up errors rather than the natural physics of water.

Why Taper Matters
For many cutting jobs, reducing the small amount of taper inherent in waterjet cutting is not necessary. You can even use taper to your advantage in certain instances, for example alternating the top side of two parts so the taper in one cancels out the taper in the other for a very tight fit. But for high precision jobs, or when cutting thick parts that must interface with other pieces along the machined edge, eliminating taper is critical. Slowing down machining speeds can usually reduce V-shaped taper, but won’t always reduce it enough to meet specified tolerances. And slow cutting speeds are not optimal for making complete parts with the highest possible accuracy in the least amount of time.

How to Eliminate Taper from Parts
An articulated cutting head can tilt the cutting nozzle in a direction perpendicular to the direction of cut to offset the naturally occurring taper and produce parts with taper-free edges. The taper is moved to the scrap portion of the material. With a tilt-only cutting head and predictive software, you can eliminate taper from parts without slowing down the cutting speed. Predictive software uses an advanced cutting model to calculate the type and amount of taper that will occur at each entity along a tool path. The software calculation takes into account the material type, thickness and desired edge quality. During the cutting process, the software rapidly adjusts the nozzle position so the angle of the jet stream offsets the natural taper.

Methods to Reduce Taper without a Tilting Head

  • To reduce V-shaped taper, slow down the cutting speed.
  • To reduce reverse taper, increase the cutting speed.
  • Stack thin materials. Taper is usually most pronounced in materials less than 1/8” (3 mm) thick. If available, use the stack height calculator in the controller software to determine the optimum number of sheets to stack to make the most parts in the least amount of time.
  • Use a low stand-off. The closer the nozzle is to the material, the less the jet stream will spread and the less taper it will produce.
  • Use the best quality abrasive available. Lower quality abrasives will tend to have less consistent particle sizes. Different particle sizes will increase taper.
  • Ensure the Z-axis is perpendicular to the material in both X and Y directions. If the Z-axis is at a slight angle, it can produce a rhombus taper.