Plasma cut speed is the single biggest lever on edge quality. Run too slow and you build top dross and a wide kerf; run too fast and the arc lags, leaving an incomplete, beveled cut. This calculator estimates a target inches-per-minute speed for mild steel, stainless, and aluminum from thickness and amperage using manufacturer-style cut-speed curves, and warns when the amperage is simply too low for the plate.
How it works
Cut speed falls steeply as thickness rises and climbs with amperage. The tool models speed as a base capacity for the selected amperage, divided by a thickness factor and scaled by a material conductivity factor:
capacity = k × amperage (energy available)
speed (IPM) = capacity / (thickness^p × materialFactor)where the exponent p is greater than 1 because thicker plate needs disproportionately more dwell, and materialFactor is 1.0 for mild steel, higher for stainless, and higher still for aluminum, which sheds heat fastest. A minimum amperage is required to pierce a given thickness; below it the tool flags an incomplete cut.
Example and notes
At 45 A on 1/4 in mild steel the tool suggests roughly 40 to 60 IPM, a typical hand or CNC range for that combination. The same 45 A on 1/2 in plate drops to a slow, marginal speed and the tool warns you are near the torch’s limit, where edge quality suffers. Aluminum of the same thickness returns a lower speed because it pulls heat away from the cut. Treat the number as a starting point: run a test coupon, watch where the sparks exit, and adjust for a square top edge and a clean, dross-free bottom.
Reading the signs: what the sparks tell you
The stream of sparks exiting the bottom of the cut is one of the most reliable real-time quality indicators:
- Sparks trail straight down — speed and amperage are well matched; the arc is pushing molten metal cleanly out the bottom.
- Sparks blow back up at the torch — you are moving too fast or your amperage is too low; the arc is lagging and not fully penetrating.
- Sparks fan wide below the plate — you are moving too slowly; excess energy is spreading the molten pool and broadening the kerf.
Once you see what straight-down sparks look like, diagnosing a bad cut becomes quick.
Material differences at a glance
| Material | Thermal conductivity | Typical effect on speed vs mild steel |
|---|---|---|
| Mild steel | Moderate | Baseline reference |
| Stainless steel | Low (roughly 1/3 of mild steel) | Needs lower speed or reduced amperage to avoid excessive heat buildup and edge oxidation |
| Aluminum | High (roughly 5× mild steel) | Needs lower speed because heat escapes the cut zone quickly and the arc must dwell longer |
Stainless also benefits from a secondary gas (argon-hydrogen or nitrogen) to minimise the chrome-oxide layer on the cut face, which can impede welding.
CNC versus hand-held cutting
CNC plasma tables run at precise, repeatable speeds, so the calculator output maps directly to your feed-rate setting. Hand-held cutting is harder to keep consistent: even a small waver in travel speed shifts you in and out of the dross-free window. For hand cutting, aim for the lower third of the recommended range — it gives you margin against the slight slowdowns that happen at corners and when repositioning.
Common mistakes and how to avoid them
Starting on cold steel after a long break. Steel at room temperature vs steel that has been running for 30 minutes behaves slightly differently because residual heat in the plate reduces the energy the arc must supply. On a cold plate at borderline amperage you may see more dross; a brief warm-up run at a slow edge helps.
Treating the minimum pierce amperage as a safe operating point. The minimum amperage to pierce a plate is not the same as an amperage that produces a clean, repeatable cut. It is the floor below which penetration fails entirely. For quality work, stay well above the minimum — usually at least 20-30% headroom.
Ignoring consumable wear. A worn nozzle or electrode shifts the arc characteristics and can make a correct speed suddenly produce bad cuts. If your edge quality degrades without changing material or settings, inspect the consumables first before adjusting speed.