Kismet CNC Fabrication Tips · Lesson 3 of 12
Here's the whole kerf measurement test: draw one square, cut it once, and measure the square and the opening it left behind. A 2.000" square is easy to draw and small enough that you aren't sacrificing a whole sheet to find one number.
1. Draw a 2.000" square
Use your normal drawing software. You don't need a special download. Choose the material, thickness, consumables and machine settings you actually want to check.
2. Choose No Offset in CAM
No Offset means the torch centerline follows your drawn square. Your software might call this center or on contour. Look at the actual toolpath before cutting; if CAM has already compensated the line, these measurements won't give you the intended result.
3. Make one cut and keep both results
Cut the square, allow it to cool, and clean off any dross that stops your calipers from sitting properly. Keep the square that fell out and the sheet with the square opening. That's the whole test.
4. Measure the cut-out square
Use the outside jaws of a digital caliper. Say the 2.000" drawn square measures 1.949" after cutting.
2.000" − 1.949" = .051" kerf.
5. Measure the opening from that same cut
Use the inside jaws to measure the hole left in the sheet. Say that opening measures 2.052".
2.052" − 2.000" = .052" kerf.
Check both X and Y directions. The two measurements won't always agree perfectly; cut bevel, dross and measurement technique can affect the readings. If the results are substantially different, figure out why instead of averaging a problem away.
Also, do not divide either difference by two. Half the kerf was removed from each side of the drawn line. The total change across the piece or opening equals approximately one full kerf.
6. Use that number in CAM
Enter your measured kerf as a starting value. Use Outside Offset when you want to keep the outside piece and Inside Offset when you want the surrounding sheet and a correctly sized opening. Check the toolpath; the torch must move into the waste.
That's the kerf measurement finished. You do not need a second cut to find it. If you're making fitted parts where exact dimensions matter, you can make a separate compensated cut later to confirm your CAM settings. That's verification, not part of the one-cut measurement test.
Write your setup down
Keep the material, thickness, consumables, power, speed, cut height, measured kerf and date. When the machine acts differently later, you'll have an actual baseline rather than trying to remember what the hell you did three months ago.
Apply the measured result: before cutting a 4020 Tractor Farm Sign or any other design, check that your CAM kerf setting reflects your machine and material rather than assuming the file’s reference value fits every setup.
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