Kismet CNC Fabrication Tips · Lesson 2 of 12
Your drawing is a line. Your plasma torch isn't. Every cut removes a strip of metal, and that missing metal has a name: kerf.
It is a small measurement with a big job. Get it wrong and your outside parts can come out undersized, your openings can come out oversized, and parts that should fit together won't.
That line on the screen has no thickness
Drawn lines tell CAM where a contour belongs. The torch follows a path, but its arc removes a strip of metal along the way. That missing strip is kerf.
Picture a 2.000" square. If the torch follows its outline with No Offset, the square that falls out gets smaller, while the square opening left in the sheet gets larger. It's the same cut, viewed from opposite sides.
With an example kerf of .050", that drawn 2.000" square gives you approximately a 1.950" cut-out piece and a 2.050" opening. The difference across either feature is one full kerf, not half of one.
Kerf and offset aren't interchangeable
Kerf is the full width the arc removes. Offset moves the torch centerline so the kerf falls on the scrap side of the finished edge. With .050" kerf, CAM typically shifts the centerline about .025"—half the kerf.
No Offset follows the drawing line. Inside Offset shifts into an internal opening, and Outside Offset shifts into the scrap around an outside part. We'll walk through the three options after the measurement lesson. You keep the artwork at the intended finished size.
Why you can't copy someone else's number
Your kerf depends on the cutting process. Material and thickness, consumables, amperage, speed, cut height and consumable wear can all change the cut. Manufacturer cut charts are useful starting points, but your finished dimensions are what count.
The next lesson shows exactly how to find a starting kerf value on your own machine. All you need is a square, your normal cutting setup and a caliper. No downloadable test file and no separate internal hole.
Does a good kerf number guarantee a good cut?
Nope. Kerf compensation moves the cut where it belongs; it can't repair an open contour, give a tiny hole more room for a lead-in or strengthen a skinny bridge. That's why we measure kerf on a simple square instead of trying to learn everything from one complicated piece of artwork.
Get your initial kerf from a simple, known-size cut. After that, check how your CAM compensates for it. The measurement, offset choice and actual cut quality are related, but they aren't the same job.
See the concepts in an actual listing: the Firepit Panel DXF is a project to use when thinking about the difference between your drawing dimensions and the finished cut. Check the final toolpaths in your own CAM.
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