Source: Make: DIY Projects and Ideas for Makers
One of the most complicated aspects to consider when laser-cutting is the delicate balance between translational speed and laser power. We want to cut as fast as possible, but are there times when it is best to slow down? The short answer is: “definitely yes”.
We are going to share with our maker community the effects that speed and power have on cut width, kerf angle and engraving depth. This will be the first edition of an ongoing series of articles, with help from Mako.
So what exactly are we doing?
As any “laser maker” knows, the two most important parameters when calibrating a laser are translation speed and laser power. Other parameters such as resolution, engraving direction, and frequency will also have an impact on your work, but none as much as the big two. We will determine to what extent these two parameters have any predictable behavior on the quality of cut. More precisely, when we cut a piece of material we notice that the kerf, or the thickness of the cut, depends on the overall energy that we are focusing onto a point on a work surface.
For a first approximation in 2D, we have defined this Energy level as the Power of the Laser x the Laser Focal Surface divided by the Translation Speed.
For the sake of our tests:
Our experiment was simple: using “AutoDesk Fusion 360” we drew a series of 5mm by 5mm squares and cut 10 examples of each using different Energy settings. Each individual square was measured with a micrometer for two different characteristics:
1. Average Kerf width: The average kerf width (a.k.w.) is the width of the laser beam that cuts into the manufactured part. This measurement is important as it must be considered into the design for making tight fitting junctions while assembling multiple parts.
Experimentally we determined the a.k.w. by measuring the top and bottom surfaces of our samples in the X-direction and Y-direction. The formula used for average kerf width is:
2. Average Kerf angle: The angle of the kerf channel (a.k.a.) is the angle of the v-shaped burn channel in relation to the laser beam. Experimentally we determined the a.k.a. by measuring the dimensional difference between the top surface and the bottom surfaces for each sample. The formula used for the kerf angle is:
A second test was done where we engraved (rasterization) the 5mm X 5mm squares’ surface while varying the Energy levels and tested to see how much material was removed. This test required us to precisely measure the thickness of the material before and after each test with a high precision micrometer.
What we show is a clear relationship between the thickness of the cut, the angle of the cut and cut depth and the laser energy. As we increase the Energy, the conic laser beam will burn the material from the work-face towards the base in a cylindrical volume and therefore create parallel walls between the kerf. However, as the energy will be much higher, this will burn more material and therefore enlarge the overall tolerance within the cut channel.
Results
Kerf Tolerance:
The Kerf tolerance seems to be fairly predictable and is related to the Energy level that we focus onto the work surface. This is very convenient as it allows us to adjust our cut off-sets based upon the level of tolerance that we seek!
Within certain limits we can predict the tolerance of the Kerf by applying a simple linear equation. However, we must be careful because, as we can see, the more energy we focus onto the cut area, the less accurate our cut becomes. When we start to heavily melt the material within the localized region of the cut, the liquid nature of the cut zone becomes less predictable.
We see that the tolerance is directly related to the power and speed. Equally, the size, focal length of the laser lens, and the users ability to precisely position the Z-height above the work surface play an important role in the accuracy of this measurement. This will be explained in more detail in the “future developments” section of this article.
Kerf Angle:
Within certain limits we can predict the angle of the Kerf by applying a simple linear equation. However, we must be careful because, as we can see, the more energy we focus onto the cut area, the less accurate our cut becomes.
The tests done on the kerf angle are far less precise as the sample group is much smaller, and there is not enough data to verify that there is a linear relation between energy and kerf angle. The following calculations express a linear solution for the data set that we measured.
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