Kerf Offset Testing

Well-managed Kerf is the key to perfectly-fitted parts such as intricate marquetry or secure finger-joints in boxes.

Kerf describes the material removed by a tool as it works. In woodworking it's the thickness of a saw, but in laser cutting it's the gap of material removed by the beam. The width of this gap — its Kerf — varies based on the material being cut, the thickness of that material, and the Settings used to cut, such as speed and power.

When Kerf is not taken into account, and a beam is run along the center of a path, it removes material from either side of that line, and so the resulting parts are undersized (or in the case of slots, oversized). However, if you offset the path to compensate for the Kerf, perfect dimensional-accuracy can be achieved.

Applying a Kerf Offset in LightBurn¶

Closed Shapes Only

This feature only works with shapes that are closed. A shape is closed when it is a complete, continuous loop whose start and end points are the same. When a shape's start and end points are different, it is open.

See Open vs. Closed Shapes for more information.

LightBurn makes it easy to apply the correct offset for a particular material through the Kerf Offset setting. You can find this option in the Cut Settings Editor, for layers set to Line Mode.

A correct Kerf Offset expands or shrinks the border so that the edge of the cut matches the edge of the art. Positive Kerf Offset values expand the border, and negative values shrink it — you'll see Inward or Outward appear next to the Kerf Offset field to let you know in which direction your value is adjusting the border.

The Kerf Offset will be applied to all closed shapes assigned to the layer. Shapes that lie within other shapes on the same layer will have the offset direction inverted, to ensure that holes become the correct size.

Kerf Offset doesn't affect the actual geometry of the art — the offset is only applied to the code that gets sent to the machine and shown in the Preview window. This means you can re-use your art with different materials or different lasers by applying different Kerf Offsets, without changing the geometry of your design.

Kerf Offset is different to scaling a design, as scaling increases the size over the whole object, not just at the edges where the material is being lost to the Kerf. Scaling an object usually results in pieces that do not fit together, hence the need for an offset.

It's possible to overcompensate for Kerf by increasing the Kerf Offset value too far. This results in pieces that are too tight to fit together, so it's important to find the "sweet spot" for Kerf Offset.

Because Kerf Offset takes more space, it's important to leave room between the parts.

Kerf Offsets are shown in the Preview window, so it's a good idea to check it before sending the job to the machine.

Shapes with Kerf Offset applied overlapping in the Preview window — These shapes are set too close together, and the cut lines will overlap once the Kerf Offset is applied.

Finding the Correct Kerf Offset Value¶

Some people try to measure the size of their pieces and compare them to the desired size to find their Kerf Offset, but the differences are usually a fraction of a millimeter, and even with calipers measuring this accurately is difficult.

We have created a tool to help you calculate the proper Kerf Offset value much more reliably and easily:

Download Kerf Test

Instructions¶

Set up the Cut (red) and Score (black) layers with your usual settings, and ensure the Kerf Offset is set to 0 mm (off) in the Cut Settings Editor. Position and run the Vernier Kerf Offset Test on your machine as normal.
Remove all pieces from the machine (a piece of painter’s tape can assist), and remove the piece marked DISCARD.
Slide all loose bits to the right →. The horizontal black line can be used to orient the rectangular pieces if they get scattered.
Find the largest number that the line marked D passes on the top scale, and use it as the whole number (D) in our equation (2 in the example).
Find which line on the bottom (Vernier) scale best lines up with any line on the bottom border (3 in our example). This will give us the decimal place (E) in our equation.
Fill in the equation, calculate it, and use the result as your Kerf Offset in the Cut Settings Editor.
```
D . E        2 . 3
-----   =    -----   =  0.0575 mm
  40           40
```
Optional: You may then use the box joints included to test your new Kerf Offset.

Once you have found your Kerf Offset, it's a good idea to save the settings to your Material Library, and the test itself to the Art Library, for future use.

Troubleshooting¶

Dealing With Irregular Beam Shapes¶

Some laser sources have beams that are not circular. For example, some diodes have a rectangular-shaped dot that result in a different Kerf (and power) on the vertical and horizontal axes. There are a couple of ways to manage this:

Find the axis with the larger kerf and use that value, knowing some areas will be loose.
Find the Kerf with the test turned on a 45 degree angle, and then cut all parts on a 45 degree angle, which will give more consistent results but limit the size of objects cut.

Adjusting Tabs For Different Thicknesses of Material¶

Kerf Offset is not the right tool for changing the height of tabs and adjusting slots to suit different thicknesses of material. See the Tab and Slot Resizer for that purpose.

For more help using LightBurn, please visit our forum to talk with LightBurn staff and users, or email support.