If you've ever cut a part on your plasma table and found it slightly too small or a hole that's a little too big, you've experienced what happens when kerf compensation is missing or misconfigured. Kerf compensation is one of the most important settings in plasma CAM software, and getting it right is the difference between parts that fit and parts that don't.
What is Kerf?
Kerf is the width of material removed by the cutting process. When a plasma arc cuts through steel, it doesn't make an infinitely thin slit — it vaporizes a strip of material roughly 0.03" to 0.08" wide, depending on the amperage, cutting speed, and material type. This strip is the kerf.
Think of it like a saw blade in woodworking. If you cut along a line with a saw, the blade removes material on both sides of the line. The same principle applies to plasma cutting, except instead of a blade, it's a column of superheated ionized gas.
The kerf width is not fixed. It varies with:
- Amperage: Higher amperage creates a wider arc and wider kerf. A 30-amp cut on thin steel might have a 0.035" kerf, while a 60-amp cut on thick plate could be 0.07" or more.
- Cutting speed: Moving too slowly allows the arc to widen the cut. Moving too fast creates a narrow kerf but may not cut through the material.
- Material thickness: Thicker material generally requires more amperage, which increases kerf width.
- Torch height: A torch that's too high spreads the arc, widening the kerf.
- Consumable wear: As your nozzle and electrode wear, the arc becomes less focused and the kerf widens.
Why Kerf Compensation Matters
Without kerf compensation, the plasma arc cuts directly along the design path. Since the arc has width, this means it removes material equally on both sides of the path. The result: outside profiles are smaller than designed, and holes are larger than designed.
For example, if you design a 2" x 2" square and cut it without kerf compensation using a 0.050" kerf width, the finished part will measure approximately 1.950" x 1.950" — one kerf width smaller (0.025" lost from each side). A 0.500" diameter hole in that part will measure about 0.550" — one kerf width larger.
For decorative work like signs, this might not matter. But for parts that need to fit together — brackets, mounting plates, gussets, fixtures — half a kerf width of error on every edge adds up fast.
Inside vs Outside Kerf Compensation
Kerf compensation works by shifting the toolpath away from the design edge by half the kerf width. The direction of the shift depends on whether you're cutting an outside profile or an inside feature:
- Outside profiles (the part boundary): The toolpath shifts outward, so the finished edge lands exactly on the design line. The waste material absorbs the kerf.
- Inside features (holes, cutouts): The toolpath shifts inward, so the hole ends up the correct size. The material removed from inside the hole absorbs the kerf.
Good CAM software handles this automatically. In CutArc, you can click on any profile to toggle between inside and outside kerf. CutArc color-codes the paths so you can visually confirm that outside cuts use outside compensation and holes use inside compensation.
How to Measure Your Kerf Width
Your kerf width depends on your specific machine, consumables, and settings. Here's how to measure it accurately:
- Cut a straight line on a piece of scrap material using your normal cutting amperage and speed.
- Measure the width of the slot using digital calipers. Take measurements at several points along the cut — the kerf may vary slightly.
- Average the measurements. This is your working kerf width.
- Repeat with fresh consumables to establish a baseline. As consumables wear, measure periodically and update your kerf setting.
A more practical method: cut a test square with kerf compensation set to a starting value (like 0.050"), measure the finished part, and adjust the kerf setting until the part dimensions match the design. This accounts for all variables at once.
Kerf Reference Values
These are typical kerf widths for mild steel. Use these as starting points, then fine-tune by measuring your actual cuts.
| Material Thickness | Amperage | Typical Kerf Width |
|---|---|---|
| 16 ga (0.060") | 25-30A | 0.035" - 0.040" |
| 14 ga (0.075") | 30-35A | 0.040" - 0.045" |
| 12 ga (0.105") | 35-40A | 0.045" - 0.050" |
| 10 ga (0.135") | 40-45A | 0.050" - 0.055" |
| 3/16" (0.188") | 45-55A | 0.055" - 0.065" |
| 1/4" (0.250") | 55-65A | 0.060" - 0.070" |
| 3/8" (0.375") | 65-80A | 0.070" - 0.085" |
| 1/2" (0.500") | 80-100A | 0.080" - 0.095" |
Setting Kerf Compensation in CutArc
In CutArc, kerf compensation is configured in the cut settings panel:
- Open your project and navigate to the Cut Settings panel.
- Enter your measured kerf width in the Kerf Width field.
- CutArc automatically applies outside compensation to outer profiles and inside compensation to holes.
- Click on any shape to toggle its kerf direction if the automatic assignment needs overriding.
- The canvas shows the compensated toolpath so you can visually verify the offset direction.
CutArc's kerf display uses color coding: outside compensation paths show in one color, inside compensation in another. This visual feedback makes it easy to catch mistakes before you generate G-code.
Common Kerf Problems and Solutions
- Parts are consistently undersized: Your kerf width setting is too small. The toolpath isn't shifting far enough outward. Increase the kerf value.
- Holes are too tight: Same issue — kerf width is too small for inside compensation. Bolts that should slide through are binding.
- Parts are oversized: Your kerf width is set too large. The toolpath shifts too far outward. Decrease the value.
- Inconsistent dimensions across a sheet: Likely a machine issue rather than kerf setting. Check that your table is level, torch height is consistent (THC working properly), and consumables aren't worn unevenly.
- Kerf changes mid-job: Consumables are wearing during the cut. On long jobs, check your nozzle and electrode between sheets. Replace consumables proactively rather than waiting for visible quality degradation.
Advanced: Kerf and Small Features
Kerf compensation breaks down on features smaller than twice the kerf width. If your kerf is 0.050" and you try to cut a 0.080" slot, the compensated toolpath would need to be only 0.030" wide — smaller than the kerf itself. The cut would collapse into itself.
As a rule of thumb, the smallest inside feature you can reliably cut is about 1.5x to 2x your kerf width. For a 0.050" kerf, that means holes no smaller than about 0.100" diameter and slots no narrower than about 0.100". Below this, you'll get oversized features or incomplete cuts. If your design requires finer features, you'll need a lower-amperage process with a narrower kerf, or a different cutting method entirely (laser, waterjet).