A 45A plasma cutter cleanly cuts 1/2" to 5/8" mild steel at production speed with good edge quality. That same machine has a maximum severance capacity of 3/4" to 7/8", but edge quality drops dramatically at severance thickness. The charts below give you rated cut, severance cut, and cut speed for every common machine class and material thickness combination.
Always buy based on the rated cut (clean cut) capacity, not the severance rating. The severance number on the box is the absolute maximum the machine can get through, slowly, with heavy dross and significant bevel. It’s useful for emergency or demolition work but not for fabrication.
Rated Cut vs Severance Cut
Rated cut (clean cut): The maximum thickness where the machine produces a quality cut at reasonable speed. Edge characteristics: minimal dross, bevel angle under 3 degrees, smooth cut surface, narrow kerf. This is your daily operating capacity.
Severance cut (maximum cut): The absolute thickest material the machine can penetrate. Edge characteristics: heavy dross requiring grinding, bevel angle of 5-8+ degrees, rough surface, wide kerf, very slow travel speed. Useful for cutting apart scrap or demolition, not for fabrication parts.
The ratio between rated and severance cut is typically 1.5:1 to 1.7:1. A machine with a 1/2" rated cut can sever 3/4" to 7/8".
Machine Class Capacity Chart: Mild Steel
| Machine Class | Rated Clean Cut | Production Cut | Severance Cut |
|---|---|---|---|
| 20-25A | 1/4" (6mm) | 3/16" (5mm) | 3/8" (10mm) |
| 30A | 3/8" (10mm) | 5/16" (8mm) | 1/2" (13mm) |
| 40-45A | 1/2"-5/8" (13-16mm) | 3/8" (10mm) | 3/4"-7/8" (19-22mm) |
| 55-65A | 3/4"-7/8" (19-22mm) | 5/8" (16mm) | 1"-1-1/4" (25-32mm) |
| 80-85A | 1"-1-1/8" (25-29mm) | 3/4" (19mm) | 1-1/4"-1-1/2" (32-38mm) |
| 100-105A | 1-1/4" (32mm) | 1" (25mm) | 1-1/2"-1-3/4" (38-44mm) |
| 125A | 1-1/2" (38mm) | 1-1/4" (32mm) | 2" (51mm) |
Production cut is the thickness where the machine runs comfortably at high speed with extended consumable life. For daily production work, this is the most practical rating.
Cut Speed by Thickness: 45A Machine
These speeds are representative of a quality 45A inverter plasma cutter (like a Hypertherm Powermax45) on mild steel with compressed air:
| Steel Thickness | Cut Speed (IPM) | Cut Quality |
|---|---|---|
| 16 ga (0.060") | 200-300 | Excellent |
| 14 ga (0.075") | 160-240 | Excellent |
| 12 ga (0.105") | 120-180 | Excellent |
| 10 ga (0.135") | 90-140 | Excellent |
| 1/4" (6.4mm) | 55-80 | Excellent |
| 3/8" (9.5mm) | 25-45 | Good |
| 1/2" (12.7mm) | 15-25 | Good (rated cut) |
| 5/8" (16mm) | 8-15 | Acceptable (max rated) |
| 3/4" (19mm) | 4-8 | Poor (severance zone) |
Cut Speed by Thickness: 65A Machine
| Steel Thickness | Cut Speed (IPM) | Cut Quality |
|---|---|---|
| 16 ga (0.060") | 280-400 | Excellent |
| 14 ga (0.075") | 220-320 | Excellent |
| 1/4" (6.4mm) | 80-120 | Excellent |
| 3/8" (9.5mm) | 45-70 | Excellent |
| 1/2" (12.7mm) | 25-40 | Good |
| 5/8" (16mm) | 15-25 | Good |
| 3/4" (19mm) | 10-18 | Good (rated cut) |
| 7/8" (22mm) | 6-12 | Acceptable (max rated) |
| 1" (25.4mm) | 4-8 | Poor (severance zone) |
Cut Speed by Thickness: 85A Machine
| Steel Thickness | Cut Speed (IPM) | Cut Quality |
|---|---|---|
| 1/4" (6.4mm) | 120-170 | Excellent |
| 3/8" (9.5mm) | 65-100 | Excellent |
| 1/2" (12.7mm) | 40-60 | Excellent |
| 5/8" (16mm) | 25-40 | Excellent |
| 3/4" (19mm) | 18-28 | Good |
| 1" (25.4mm) | 10-18 | Good (rated cut) |
| 1-1/4" (32mm) | 5-10 | Acceptable (severance zone) |
| 1-1/2" (38mm) | 3-6 | Poor (max severance) |
Material Comparison at 45A
Different metals cut at different speeds due to their melting points, thermal conductivity, and density:
| Material | 1/4" Speed (IPM) | 1/2" Speed (IPM) | Max Rated Cut |
|---|---|---|---|
| Mild Steel | 55-80 | 15-25 | 5/8" |
| Stainless Steel | 45-65 | 12-20 | 1/2" |
| Aluminum | 65-95 | 18-30 | 5/8" |
| Copper | 30-50 | 8-15 | 3/8" |
Aluminum cuts fastest because of its low melting point (1,220F) and low density. Copper cuts slowest because of its extremely high thermal conductivity, which pulls heat away from the cut zone rapidly.
For material-specific guidance, see plasma cutting aluminum and plasma cutting stainless steel.
Factors That Affect Real-World Cut Speed
The chart speeds assume new consumables, clean dry air at proper pressure, and a properly functioning machine. Real-world speed varies based on:
Consumable Condition
New consumables produce the tightest, most focused arc and the fastest cuts. As the electrode and nozzle wear, the arc widens, cutting speed drops, and edge quality degrades. A 30% drop in cut speed is typical with consumables at 75% of their life. Replace consumables before they fail catastrophically. For more on this, see plasma cutter consumables guide.
Air Quality and Pressure
Low air pressure (even 5 PSI below specification) reduces the blowing force that clears molten metal from the kerf. The result is slower cutting, heavier dross, and wider kerf. Moisture in the air disrupts the arc and reduces cutting efficiency. See air compressor requirements for proper air supply sizing.
Cut Type: Straight vs Curve vs Pierce
Straight cuts are fastest. Curved cuts require slowing down to maintain accuracy. The tighter the radius, the slower you must go. Sharp corners require significant deceleration or the kerf widens at the turn.
Piercing (starting a cut in the middle of the plate) takes 1-3 seconds depending on thickness and consumes consumables faster than edge starts. A part with 50 pierces per sheet wears consumables 5-10x faster than the same total cutting length done as continuous edge-start cuts.
Torch Height and Angle
Proper standoff (typically 1/16" to 1/8" above the workpiece) gives the best cut. Too high and the arc widens, reducing cutting power. Too low and the torch may contact the workpiece or dross, destabilizing the arc. CNC machines with torch height control (THC) maintain optimal standoff automatically.
A torch that’s not perpendicular to the workpiece produces a beveled cut. The side where the swirl of the plasma exits has a slightly better edge (the “good” side). On CNC machines, the good side is always on the right when cutting clockwise on an outside contour.
How to Read the Spark Stream
When plasma cutting, the sparks exiting the bottom of the cut tell you exactly how your settings and speed relate to the material:
Sparks angle slightly behind the torch (15-20 degrees): Optimal speed. The arc is slightly ahead of the bottom of the cut, which is normal. Good edge quality, minimal dross.
Sparks come straight down: You’re cutting too fast. The arc barely penetrates through. The bottom of the cut may not separate completely. Slow down.
Sparks trail far behind (45+ degrees): Too slow. Excessive heat input. Heavy dross, wide kerf, potential distortion. Speed up.
No sparks from the bottom: The arc isn’t penetrating through the material. Either the thickness exceeds the machine’s capacity, the consumables are shot, or air pressure is too low.
Sparks shoot backward toward you: Way too fast. The cut isn’t penetrating at all. Stop and check your settings.
Using the Charts for CNC Cutting
CNC plasma tables use these same amperage-to-thickness relationships but with tighter speed control. The CNC controller sets a precise inches-per-minute speed that stays constant throughout the cut.
For CNC, start at the middle of the speed range in the charts and adjust based on cut quality:
- Increase speed 5% if dross appears on the bottom edge
- Decrease speed 5% if the cut doesn’t fully penetrate or has heavy bevel
- For curved cuts, reduce speed 15-25% from the straight-cut speed
CNC plasma also uses different consumables (finer nozzles, lower standoff) for precision cutting. The amperage may be the same, but the tip geometry produces a narrower kerf and cleaner edge. For CNC vs handheld details, see handheld vs CNC plasma cutting.
When Plasma Isn’t Enough
If the charts show your material is beyond severance capacity, your options are:
- Larger plasma cutter: Step up to a higher-amperage machine
- Oxy-fuel cutting: For mild steel over 1" thick, oxy-fuel is slower but handles virtually unlimited thickness. See plasma cutting vs oxy-fuel
- Abrasive waterjet: Cuts any material, any thickness, no HAZ. But expensive equipment and slow speeds
- Laser cutting: Precision cutting of thin-to-medium material. Better edge quality than plasma but limited on thick material and very high equipment cost
- Mechanical cutting: Bandsaw, shear, or CNC milling for applications where thermal cutting isn’t suitable
Duty Cycle and Sustained Cutting
Machine amperage determines maximum thickness, but duty cycle determines how long you can cut continuously at that amperage. Duty cycle is the percentage of a 10-minute period the machine can run at rated output.
Typical duty cycles by machine class:
- 30A machines: 35-50% duty cycle at rated output (3.5-5 minutes of cutting per 10 minutes)
- 45A machines: 35-50% duty cycle at rated output
- 65A machines: 50-60% duty cycle at rated output
- 85-105A machines: 60-100% duty cycle at rated output
For heavy cutting at or near the machine’s rated capacity, the duty cycle becomes the bottleneck. A 45A machine at 50% duty cycle can cut for 5 minutes before needing 5 minutes to cool down. On thick material where cut speed is slow, that means stopping in the middle of long cuts.
Workaround: Reduce amperage 10-20% below the machine’s maximum. Most plasma cutters have much higher duty cycles at reduced amperage. A 45A machine might have 50% duty cycle at 45A but 100% duty cycle at 35A. For sustained cutting on thinner material, running at reduced amperage gives you unlimited cutting time with slightly reduced speed.
Selecting a Machine Based on These Charts
Use the charts to select a machine by working backward from your most common cutting thickness:
- Identify your most frequent material thickness
- Find that thickness in the rated clean cut column (not severance)
- That tells you the minimum machine class
- Add one size for margin if you occasionally cut thicker material
For example, if you mostly cut 3/8" steel with occasional 3/4" cuts: a 45A machine handles 3/8" as a comfortable production cut and reaches 3/4" at severance. A 65A machine handles 3/8" effortlessly and gives you a good-quality 3/4" cut. The 65A is the better investment for that use case.
For full purchasing guidance, see plasma cutter buying guide.