For a 40 amp plasma cutter clean cut, expect mild steel up to about 3/8 to 1/2 inch; the same machine can sever (just separate) metal up to roughly 3/4 to 1 inch, depending on the model. For example, the Thermal Dynamics Cutmaster 40 lists a recommended cut and pierce capacity of 1/2 inch and a 1 inch sever on 230V input. Your own machine’s “recommended” and “severance” ratings in the manual are the numbers that matter.
Quick Answer
- Clean cut: the thickness the machine cuts at a useful speed with a square edge and light dross. Think 3/8 to 1/2 inch on a good 40 A unit.
- Severance cut: the most the arc can separate, at very low speed with a rough, beveled edge. Think 3/4 to 1 inch.
- Input voltage matters: many dual-voltage cutters give up thickness on 120V. The Cutmaster 40 drops to 3/8 inch recommended on 115V.
40 Amp Plasma Cutter Clean Cut vs Severance Cut
Manufacturers publish more than one thickness for the same amperage because the arc can do two different jobs.
| Rating | What it means | Best use |
|---|---|---|
| Recommended / quality cut | Full penetration at a usable speed, light dross, mostly square edge | Parts that will be welded or fitted, daily work |
| Pierce capacity | Thickest plate the torch can start through away from an edge | Holes and interior cutouts; usually lower than edge-start capacity |
| Severance / maximum cut | The arc just separates the metal at very slow travel; heavy dross and bevel | Rough sizing, scrap breakdown, parts that will be ground anyway |
Plan jobs by the finish you need. Use the recommended rating for precision parts and visible edges, and save severance cuts for work where cleanup does not matter.
Thickness Examples for 40 Amp Class Cutters
The table shows published ratings for mild steel from two manufacturers. Treat them as examples of what the class can do, not as a promise for every 40 A machine.
| Machine | Recommended cut | Severance |
|---|---|---|
| Thermal Dynamics Cutmaster 40 (40 A, 230V) | 1/2 inch cut and pierce | Up to 1 inch |
| Cutmaster 40 on 115V | 3/8 inch cut and pierce | Not listed on the page |
| Hypertherm Powermax45 XP (45 A, a step up) | 5/8 inch at 20 IPM and 7/8 inch at 10 IPM; 1/2 inch pierce | 1-1/8 inch at 5 IPM |
Many budget 40 A inverters advertise a lower clean-cut figure than these professional units. Use the figure in your own manual. If a listing gives only one number, check whether it is the quality cut or the severance limit before you rely on it. The Powermax45 XP is listed by Hypertherm as a legacy model last manufactured in March 2024, so it is shown here only as a reference point.
Cutting Speed and Edge Quality
Speed falls quickly as thickness rises. The Powermax45 XP chart goes from 20 IPM at 5/8 inch, to 10 IPM at 7/8 inch, to 5 IPM for the 1-1/8 inch severance cut. A 40 A machine follows the same pattern at slightly lower thicknesses.
- Too slow: heavy, rounded dross along the bottom edge, a wider kerf and more heat in the plate. Speed up in small steps.
- Too fast: sparks trail behind or fly upward, the cut may not go through, and a thin hard bead can form on the bottom edge. Slow down in small steps.
- Worn parts: an oval nozzle orifice or burned electrode causes bevel and dross at any speed. Replace consumables before tuning anything else.
Start with the speed on your machine’s cut chart, make a scrap cut, then change one variable at a time: speed, then standoff, then current.
Products Worth Considering
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Dross, Bevel and Cleanup
On thin to mid-thickness plate, dross is light and usually knocks off with a chipping hammer or wire brush. Near the severance limit expect top spatter, bevel and heavy dross that needs a grinder or flap disc.
To limit cleanup, keep the nozzle and electrode fresh, supply clean dry air at the flow and pressure your manual specifies, and keep the torch square to the plate. Remove only what you need so the part keeps its dimensions.
Choosing a Machine for Your Thickest Regular Job
Match the clean-cut rating, not the severance rating, to the thickest metal you cut every week. If most of your work is 1/4 inch, a 40 A unit has comfortable headroom. If you often cut 1/2 inch plate, look at 50 A or 60 A machines so you are not running at the limit all day.
- Running at the upper limit slows production and shortens consumable life.
- Check duty cycle at your input voltage, not only at the headline amperage.
- Confirm the air flow your compressor must supply. The Powermax45 XP, for example, calls for about 400 scfh (roughly 6.7 scfm) at 90 psi.
Technique: Drag, Standoff and Piercing
- Drag cutting is only for torches and shield cups designed for it. With the right consumables it steadies the torch on thin and mid-thickness metal.
- Standoff cutting means holding the torch at the height your manual specifies. Too low at speed throws molten metal back onto the nozzle; too high widens the kerf and weakens the cut.
- Torch angle: hold the torch square (about 90 degrees) while cutting for the squarest edge.
- Thick material: start from an edge when you can. Interior pierces on thick plate are hard on consumables and may exceed the pierce rating.
- Work clamp: attach it to clean bare metal close to the cut.
Products Worth Considering
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Safety for Heavy Cuts
Thick cuts throw more sparks, molten slag and fume. Wear a filter lens rated for plasma cutting, flame-resistant clothing, leather gloves and hearing protection, and ventilate the work area. Per OSHA 29 CFR 1910.133, the minimum shade for light plasma arc cutting below 300 A is shade 8, and your machine manual may call for more. Keep combustibles out of the spark path and check below and behind the work before you cut.
Plasma vs Oxy-Fuel for Occasional Heavy Cuts
For occasional cuts in the 1/2 to 1 inch range, a 40 A plasma cutter is generally faster to set up than an oxy-fuel rig because there is no preheat step and no gas cylinders to handle. It also cuts non-ferrous metals that oxy-fuel cannot. Oxy-fuel remains the usual choice for very thick steel plate. See our plasma vs oxy-fuel comparison for details.
Related guides: 40 amp plasma cutter steel thickness guide, 40 amp vs 60 amp plasma cutting, and what thickness a plasma cutter can cut.
Frequently Asked Questions
What gas should I use with a 40 amp plasma cutter?
Clean, dry compressed air is the standard for mild steel on most 40 A machines. Other gases such as nitrogen are used on some systems for stainless or aluminum, but only use a gas your torch and manual approve.
Does 120V reduce what a 40 amp plasma cutter can cut?
Often yes. Dual-voltage machines typically give up output on 120V. The Cutmaster 40, for example, lists 3/8 inch recommended capacity on 115V versus 1/2 inch on 230V.
Can a 40 amp plasma cutter cut 1 inch steel?
Some 40 A class machines can sever 1 inch mild steel, as the Cutmaster 40 page states for 230V input, but it will be very slow with a rough, beveled edge. It is not a clean-cut thickness.
How do I know if a rating is clean cut or severance?
Look for the words recommended, quality or clean for the usable figure, and maximum or severance for the limit. If a listing shows only one thickness, check the manual or manufacturer spec sheet before relying on it.
Can I pierce thick plate with a 40 amp plasma cutter?
Pierce capacity is usually lower than edge-start capacity. Hypertherm lists a 1/2 inch pierce for the 45 A Powermax45 XP. Start from an edge on thicker plate to protect the consumables.
Conclusion
A 40 A plasma cutter gives clean, low-dross cuts on thinner plate and can sever much thicker steel at slow speed with a rough edge. Read your manual to see which number is the recommended cut and which is the severance limit, size the machine to the clean-cut rating of your usual work, and keep consumables and air supply in good shape.
Sources
- Thermal Dynamics Cutmaster 40 product page: cutting capacity on 230V and 115V.
- Hypertherm Powermax45 XP: cut capacity, speeds, air requirement and legacy status.
- OSHA 29 CFR 1910.133: eye and face protection filter shades.
