How to Measure Kerf

In machining, kerf is the material that is removed when the tool cuts through the material stock. If the tool used is a saw, the kerf left on the material is roughly the width of the saw blade. However, in water jet cutting, the kerf left on the material sometimes is not the width of the nozzle jet. To make things more complicated, the kerf at the top surface is drastically different to that on bottom surface when cutting thicker and tougher material

Each type of material responds differently in water jet cutting. Thus, if you are into precision fabrication, it is helpful to predict what the kerf might be on the material of choice. This guide teaches you how to measure the kerf with common available measuring tools

Tools needed:

A caliper
Filler gauges/needle gauges/ feeler gauge/ pin gauges (if possible)
Spare piece of material of interest.

Procedure

The Slit Method

This method is the most accurate and precise way to measure kerf, since it uses tightly controlled pin gauge to measure a single slit. No systematic error and uncertainties are compounded.

Use your software of choice to draw out a short line, export it into dxf or svg, and then uploaded to WAM. The line does not have to be long

Use the generated cut file to execute the cut on your material. You will see a slit cut out like this. The machine always starts with a pierce (slightly larger in diameter), follow by a slit

Select a pin gauge that can just fit into the slit. The size of the kerf is listed as the diameter of the pin gauge. You can use the same method to measure the pierce hole diameter as well.

The Sqaure method 1

This method is simpler than slit method, and no pin gauges are to be used.
However, this method is less accurate since the uncertainty of a caliper is compounded.

Use your software of choice to draw out a simple square, export it into dxf or svg, and then uploaded to WAM. The square does not have to be in a specific size.

Place at least two tabs on the cut out. The more stably material sits during cutting, the more accurate measurement we can get. Cut path is not important in this method

Use the generated cut file to execute the cut on your material. You will see a cut out like this.

Use the caliper to measure the distance between the two sides of the square (measurement 1), and distance between the two sides of the cut out blank (measurement 2)
The difference between measurement 1 and measurement 2 is twice the kerf

You may repeat measurement and calculation on the bottom surface if the top and bottom kerf are significantly different.

The Square Method 2

This method is slightly harder than square method 1, but we can yield a slightly more accurate reading by eliminating one caliper measurement, thus no caliper uncertainty is compounded.
But this method assumes the gantry motion is as accurate and precise as designed, and the nozzle stream is the same as the preset. Thus, if anything extraneous happens, we can also use this method to check for gantry function and nozzle stream.

Use your software of choice to draw out a simple square. The square have to be in a specific size, e.g. 2” or 50mm on each side.

Cut path is very important in this method. Please make sure you have selected “outside” cut path

Export it into dxf or svg, and then uploaded to WAM. Place at least two tabs on the cut out. The more stably material sits during cutting, the more accurate measurement we can get

Use the generated cut file to execute the cut on your material. You will see a cut out like this

Assuming the middle square is true to its design size, use the caliper to measure the distance between two sides of the cut out blank (measurement 3)

If you are not unsure the gantry motion is good, take another measurement of the two sides of the square (measurement 4).
The difference between measurement 4 and designed dimension an indication of the difference between preset nozzle stream diameter and real nozzle stream diameter.
You can also take measurement 4 and measurement 3, revert to The Square Method 1