More Kerf Questions

Two things: 1. How can i put connection tabs on the drawing so the cut doesn’t fall completely out of the sheet and still use the kerf function?

2.If the upload works and you can see the picture, take note of the slot on one end. When I ran my test cut, the kerf function made this slot bigger instead of smaller. I have an outward kerf adjustment set, and I am guessing LB is thinking the outside is going the other way. Should I just put that slot on the blue layer? (The blue layer is set to adjust inward to keep the tab slots snug.)

This won’t help with your kerf question but rather than tabs I use application tape (used in the sign business to transfer cut vinyl) on the underside of my cuts in cardstock and wood veneer. My diode laser cuts cleanly through the material but not the tape. All the small bits remain in place until I remove the tape.


For #1, read this thread:


Do you have this shape enclosed by other shapes? LightBurn looks at whether something is contained by something else to decide whether it’s inside / outside, and which direction to kerf.

For example, look at the shapes below:
The outer ring is obviously the “outside” shape, and if I told it to kerf outward, it would push the laser outward from that circle. The black ring is inside, so it would be kerfed in the opposite direction. The inner red ring is back to being an “outside” cut again, and would kerf outward.

LB, that is kind of how I thought it would work. However if you look at the pic I posted and note the slot on the left side. This slot got bigger, not smaller as I had hoped. The red line is all one polyline.

Can you post (or email) the original so I can try it? Kerf just uses the built-in offset functions, so if offset works, kerf should work the same.

Does it have something like this proven old algoritm for deciding inside/outside:

  • Draw an imaginary line from the point of interest to a point you know is outside the shapes (usually slightly smaller than minimum X and Y).

  • Count how many lines this line crosses. Odd=insine, even=outside.

If we know how it figures it, it’s easier to understand the effects…

Something may be happening here, highlighted in yellow…

Any chance I could see the original lbrn file?

It won’t let me attach the file, if you can shoot me an email, i will email the LB file to you.


I would have to email it. I drew this in TurboCad and the segments you have highlighted are joined at the vertex. Shoot me an email, I can send a dxf, a pdf and the lightburn file.

You can email directly to developer at lightburnsoftware dot com and reference this thread.

Sent… and now I need to come up with 20 characters.

The summary comments in the code:

// Do an initial test of bounding areas
  // a shape cannot contain another with larger bounds.

// to decide if another shape is inside this one:
// for all vertices in this shape
  // scan horizontally for segment intersections with the other's prims
  // if there are an odd number of intersections, this point is inside
  // If this holds true for ALL vertices, we're probably inside
  // If multiple verts are on the same horizontal, don't count them

// After that, test all segment to segment intersections
// If no segs between the shapes intersect, the in/out test succeeds
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I’ve repaired and sent the file back. It looks like a couple things happened:

  • Based on the scale of the files you sent, I’m guessing the DXF was modeled in inches, and you imported it in mm, then scaled up. If you have distance-based point merging enabled, importing the file at 1/25th scale and then sizing up means that the point merge distance is effectively multiplied by 25. I’m guessing this is what made the wing curve so jagged in your version.

  • The source DXF had a couple pieces that didn’t quite line up, so I’m guessing it simply wasn’t a closed shape, and kerf offsetting only works on closed shapes. A minor node edit to connect the two pieces that were the most “off”, then using auto-join on the rest fixed it.

Hmmmm, that’s weird. I don’t think I did any scaling. I went back and checked and it seems the object shrunk by .04 inches overall. I was also not trying to draw a wing curve, its the side of a fuselage.

I did find the extended line and fixed. I am going to run another test this morning and I will post my results. Thanks for all your help! You guys are great!