Kismet CNC Fabrication Tips · Lesson 4 of 12
You've measured your kerf. Now you need to tell CAM which side of the line gets cut away.
We're going to use the same three terms throughout these lessons: No Offset, Inside Offset and Outside Offset. Some CAM programs call them something else. Don't worry about the button's name until you understand where the torch should travel.
No Offset: follow the line
The torch centerline follows your drawing, with roughly half the kerf falling on either side of the line. That's why we use No Offset when measuring kerf with a single square cut. The removed square comes out smaller and the hole left in the sheet comes out larger.
Inside Offset: keep the material around an opening
Draw a square opening in a piece of metal. You're keeping the sheet around the opening and throwing away the square in the center.
Choose Inside Offset. CAM moves the torch centerline inward, into the square you're removing, by roughly half the kerf. That leaves the edge of your finished opening close to the size you drew.
Outside Offset: keep the square you cut out
Same drawn square, different goal. This time you're keeping the square itself and the surrounding sheet is scrap.
Choose Outside Offset. CAM moves the torch centerline outward, into the surrounding scrap, so the removed square stays close to the size you drew.
How far does the torch move?
For an example kerf of .050", CAM usually shifts the centerline about .025" from the desired edge. That's half the kerf. You normally enter the full kerf width into CAM and select the correct cut side; you don't manually change the finished dimensions in your DXF.
The simplest rule is still: put the kerf in the waste.
Don't trust a label without checking the toolpath
Inside/outside, left/right, center or on contour: CAM programs use different words, and cut direction can matter when they use left/right compensation. Look at the preview before you fire the torch.
The torch path for a cut-out square belongs outside the square. The path for a hole belongs inside the opening. No Offset sits directly on the drawn line. If the preview shows otherwise, fix it there.
Offset can't rescue bad geometry
Correct compensation places the cut. It doesn't make a skinny bridge stronger, create room for a pierce or turn an impossibly small hole into a clean one. That's why the next lessons deal with what happens when the geometry and the physical process run out of room.
See these contour choices on an actual design: browse the Wildlife Scene Panel DXF and identify which contours cut into outside scrap versus internal waste. The final offset selection still belongs in your CAM.
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