Measuring kerf and tool offset

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All about kerf

Determining the tool offset

All about kerf

“Kerf” is a machining term that refers to the material removed during cutting. For example, if you cut a board with a saw, part of the board is turned into sawdust. If you put the two pieces of the board back together, you would find that they are slightly shorter than they were before, because some of that material (about the width of the saw blade) had been removed during cutting.

The same effect applies with a waterjet machine: part of the material is removed during cutting. Of course, with a waterjet, the amount of material removed is much less than with a saw blade. Typically, the waterjet kerf  will be about 0.020″ to 0.060″ (0.5 mm to 1.5 mm), depending on the nozzle and the horsepower of the pump. There are some specialty nozzles that have a very small kerf on the low end of this range and are useful for cutting jewelry where the material is expensive and fine detail is required. Most systems have kerf widths ranging from 0.030″ to 0.040″ (0.76 mm to 1.0 mm).

It is important to know what the kerf is so that the nozzle can be offset slightly to allow for it. In our saw and board example, if you wanted a board that was exactly 2.0 feet long, you would position the saw so that the part of the board turned into sawdust was outside the 2.0 foot mark. Similarly, with a waterjet, you adjust the software to allow for the kerf size to produce parts that are precise.

Unlike a rigid saw blade, the kerf will have a different width depending on several factors. As the mixing tube wears, the kerf will tend to increase (the width of the cutting stream will increase). Also, as you slow down to make a corner, the kerf grows slightly. The amount it grows is a function of how much you slowed down by, which depends on your material thickness. In other words, the thicker the material, the harder it is to get precision in the corners.

Most controllers compensate for kerf width automatically, but don’t fully compensate for the kerf growing as a function of speed.

How to measure kerf

Measuring the kerf is fairly straightforward. You simple make a part with a known dimension (such as a one-inch square) and then carefully measure the actual width. If your one-inch square is actually 0.96 inches, then your kerf is 0.04 inches. Once you’ve adjusted for this kerf, you should be able to create a one-inch square that is 1.00 inches.

The more carefully you measure the dimensions of your part, the better you will be able to specify the kerf, which will increase the accuracy of all your parts.

For maximum precision

Sometimes you want to get the maximum possible precision with a part. One of the key factors in achieving this precision is kerf measurement. Following are some tips when you want to squeeze that extra 0.1 millimeter of precision out of your waterjet:

  • Temperature matters
    Metals expand and contract with temperature, and even plastic will show some change in size depending on the temperature. Remember that your tank will usually be warmer than the outside air as you cut, so let your material sit in the water until it reaches a constant temperature.
  • Measure carefully
    You’re not going to get 0.1 mm accuracy if you use a yardstick. Invest in a set of high-precision calipers and treat them gently. When you measure the kerf, take several measurements at different locations on the part and average them (remember that the kerf varies with the nozzle speed).
  • Use the actual part and material
    Rather than cutting a square and measuring the kerf, cut the actual part you plan to make and measure the kerf on it. This will let you adjust for variations in kerf between different features of the part. Use the material that you plan to use for the final part as well, if possible.
  • Decide when precision matters
    There may be sections of your part where precision isn’t as important as it is in other areas. In this case, measure the kerf in the areas where it does matter and use that as your kerf measurement.

Determining the tool offset

Because the cutting beam of a waterjet is not infinitely thin, it is necessary to offset the tool slightly from the geometry of the part. For example, a typical kerf width of a nozzle is about 0.030″ (0.76 mm). If you were to trace the exact outline of the part you want to cut, the part would be undersized by 0.015″ (0.38 mm), which is half of the kerf width. Therefore, it is necessary to follow a path that is “offset” by this amount.

The original part as drawn in a CAD system (purple) with the actual path the machine will follow (yellow), which is offset from the original part. The blue dot represents the jet.

In the above figure, notice that by following the yellow path, the purple part will be cut to exact size. If the jet followed the purple path, the part would come out undersized.

Measuring the width of the jet

To determine the offset, you cut a part of known dimensions, then measure the error. For low-precision work, you can just guess that it’s one-half the width of the mixing tube’s inside diameter. For high-precision work, it is necessary to measure the error on a previously machined part.

The part pictured above was designed for this specific purpose. Each “leg” is drawn to be exactly 1.0″ (2.5 cm) wide. By measuring the actual dimension of the finished part, you can determine the error in three axis of motion X, Y, and combined XY. By measuring three legs, instead of just one, you can also get an idea of the roundness of your jet.

All about kerf

Determining the tool offset