Understanding Taper in Waterjet Cutting

Water is not a solid wall, it will not stop you.

But water always goes where it wants to go,

and nothing, in the end, can stand against it.’

― Margaret Atwood

You may be familiar with waterjet cutting and justifiably blown away by the power of its directed supersonic jets of water.

Sorry to have to cut the daydreaming short, but it behoves us to remind you that you can lose a finger and seriously damage a hand or your shins! So, as enticing as it may be, leaving this kind of complex, high-end steel wizardry to the pros makes a great deal of sense.

If you’re reading this, the first thought in your head on this subject might be, ‘Ok, but can it cut steel?’ This is an excellent question with a great answer and fascinating mitigating factors.

Waterjet Cutting Requires Skill

A waterjet cutter uses a high-pressure stream of water that creates submission in something as strong as steel. A skilled waterjet operator can cut intricate designs into the steel and also cut clear through nine inches of it. Some waterjet systems can be configured to cut clean through 8-inch steel blocks – we kid you not.

Waterjet cutting is an extremely versatile cutting method that utilizes a high-pressure stream of water to cut into a wide variety of materials accurately. Waterjets are complex machines, and the kind of cut possible is dependent on a few factors, which we’ll tell you about below.

Two Critical Waterjet Cutting Factors

First, get your head around these two mitigating factors:

  • Taper – is inherent to waterjet cutting and affects the quality and accuracy of your cut. It gets pretty exacting. Some taper is caused by problems either in the calibration or the cutting head, which can be eliminated. But great skill is needed to compensate for other kinds of taper so that the cut part is not affected. It is possible for the taper to cause the parts you’re fashioning to fall outside of tight tolerances and design specs.
  • Kerf – the Anglo-Saxon word related to the English word carve and references the amount of material the waterjet removes as it cuts. Kerf width will depend on the thickness of the steel and the state of the nozzle. The taper indicates the difference in kerf width at the top of the cut versus kerf width at the bottom of the cut.

The Four Different Types of Taper

Different kinds of taper each tell a story, which a skilled operator uses to assess performance:

  • V-Shaped Taper – the most popular, usually associated with faster cutting speeds. V-shaped tapers have more kerf width at the top of the cut than at the bottom, and they happen as the power of the waterjet fluctuates.
  • Asymmetrical Taper – when you see different taper angles on different sides of your waterjet-cut part, the likely cause is a calibration error or a faulty cutting head setup.
  • Reverse Taper – this happens easily in soft materials, so no panic if you’re working with steel, but it can also happen if you cut too slowly.
  • Barrel Taper – this shaping is due to a wider kerf in the middle of the cut rather than the top or bottom but can be due to hectic nozzle wear and tear or waterjet orifice failure.

Three Common Taper Culprits

We can’t cover all the bases but look out for the following:

  1. Nozzle Standoff –  If the nozzle is too far away from the material, the jet stream spread creates tapered cut faces with excessive erosion at the top edge.
  1. Waterjet Cutting Speed – you know this, but to recap; higher cut speeds give the V-shaped taper, lower cut speeds result in less taper and cut speeds that are too slow together deliver a reverse taper.
  1. Cutting Stream Quality – worn or damaged nozzles through which your water must pass result in loss of symmetry and coherence.

Waterjet Cutting Fixes

Just so you appreciate what your skilled waterjet cutting operator is doing to give you the cutting edge you require, here are the fixes for the various issues  we explored together:

Nozzle Standoff – the rule of thumb here is; the lower the nozzle is to the material, the less taper you get. A  0.100”  nozzle standoff is ideal.

Cutting speed and taper compensation – by controlling the angle of the cutting head, high-level waterjets can compensate for taper. Even at high cutting speeds, a properly calibrated waterjet with taper compensation is the best solution to taper. However, the taper can be eliminated by slowing down the cut speed if compensation is not possible. Conversely, if the problem is reverse taper, increasing the cutting speed should solve the issue.

High-quality abrasive  – waterjets cut by mixing an abrasive with the supersonic water stream. For better cut quality and less taper at comparable speeds, a higher-quality abrasive such as crushed garnet cuts more aggressively.

Steelmor is a leading manufacturer and supplier of stainless steel. Contact us today to talk about your specific requirements.