How to test Kerf

What does laser kerf mean? Kerf is the portion of material that the laser burns away when it cuts.  It is the width of the lasers beam.  It usually ranges from 0.08mm – 1mm depending on the material type and other conditional factors. Some factors that can increase the laser Kerf are:
  • Dirty mirrors or lens
  • Uneven cutting surface
  • Laser not set at correct focus
How can we find our lasers kerf?
There are 2 different ways that we will discuss here.  The rectangle method and the square method.
First the rectangle method.  For this method you start by cutting a rectangle of material and then cutting 20 rectangles within it so that you get 20 cuts.  See picture for example. 
Kerf Test Strip
 
When these rectangles are pushed together at one end of the large rectangle the resulting gap at the other end is the sum of the kerfs. Measure this gap and then divide this gap by 20. This gives the average kerf for that material and material thickness. Keep in mind that this value equals the full width of the laser beam, but because the laser beam cuts from the center of the cut line only half of the laser beam falls on each side of the cut.  Therefore, you will need to divide this number by 2 to get the kerf to add to your design cut lines.  
You can also do this with any number of rectangles instead of 20, just divide by the number of rectangles you use. 
Here is a link to a file so that you can try this method.
Link
The second way that we will discuss is the square method.  This one is the easier of the two.  
Kerf Square Method
Start by cutting a square that is exactly 1 inch with your laser.  Measure the square with calipers after cutting to see what the measurements are.   To calculate the kerf take the width of the square (1 inch) - width of square measured after cutting=kerf
Keep in mind again that this value equals the full width of the laser beam, but because the laser beam cuts from the center of the cut line only half of the laser beam falls on each side of the cut.  Therefore, you will need to divide this number by 2 to get the kerf to add to your design cut lines.  
So for example, 
 1”-.99”=.01”  kerf of the laser beam     .01”/2 = .005” the kerf of the material for each cut line.
To practice more with kerf you can go to makercase.com and choose the basic box generator. Make a small box, like 4”x4”. Fill in the material thickness.  I like to measure my material and put in a custom thickness instead of just choosing one like 1/8”.  Next, click the button that says “download box plans”.  This will make a window come up with the file. There is a button on the bottom of this screen that says, “Kerf and Corner Compensation”. Click on this button and put your materials kerf in here.  This should be the end kerf that has been divided by 2.  So for example, if we used the material from our 1” square test we would put .005” in this box.  Next click the SVG button and download the file to your computer.  Cut it out and see if your kerf was configured right!  If it is too tight then you need to decrease the kerf number and if it is too loose you need to increase the kerf.
When you are making a design that you need to worry about kerf with keep this in mind: 
For edges on the outside of the design that need to be adjusted the offset should be larger than the original.
For edges on the inside of the design, like slots, the offset should be smaller than the original. 
Beach Ball Earrings
Just remember, just like everything, practice makes perfect!
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