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Cutting steel plate around a bolt pattern is harder than making a straight cut. You need enough power to pierce the plate, enough control to stay close to the layout, and a torch that can restart cleanly as it moves between holes and outside edges. A good plasma cutter can do this quickly, but the cheapest machine is not always the best choice.
What matters when cutting bolt patterns
For most shop work, a 40-amp plasma cutter is the practical starting point. It will usually sever 1/2-inch mild steel and produce a cleaner working cut in 1/4-inch to 3/8-inch plate. A 60-amp machine gives more headroom for rusty material, imperfect technique, and frequent work on 1/2-inch plate. If you regularly cut 3/4-inch steel, step up again rather than forcing a smaller machine at its limit.
Look at three ratings: recommended cut, maximum cut, and severance cut. The recommended rating is the useful number. Maximum cut is normally slower and rougher, while severance means the torch can get through but may leave heavy dross and a tapered edge. For accurately fitting a bolt circle, buy based on the recommended thickness, not the marketing maximum.
For bolt holes, a pilot-arc torch is valuable. It starts the arc without requiring the electrode to touch the work, so it can cross a gap or start on an edge without sticking. This is especially useful when cutting several holes from a template. Machines without pilot arc can still do the job, but they are less forgiving and generally slower to restart.
Best plasma cutter types for the job
| Type | Best use | Trade-off |
|---|---|---|
| 30–40 amp compact cutter | 1/8- to 1/4-inch plate, occasional bolt holes, portable repairs | Limited headroom on thick or rusty steel |
| 40–60 amp pilot-arc cutter | Frequent fabrication, 1/4- to 1/2-inch plate, repeated holes | Higher price, more air demand, heavier machine |
| Dual-voltage cutter | Shops that use both 120V and 240V power | Lower output on 120V and possible duty-cycle limits |
| CNC-ready plasma system | Repeated bolt patterns and production work | Requires a table, height control, software, and setup time |
For a one-off bracket or equipment repair, a compact 30- or 40-amp machine is often enough. If the plate is 3/8 inch or thicker, or you will cut several patterns every week, a 40 amp pilot-arc plasma cutter is a better balance of price and capability.
Recommended buying specifications
Air supply is just as important as amperage. Many small plasma cutters require roughly 4 to 6 CFM at 60 to 90 PSI, though the exact requirement varies. A compressor that technically reaches the pressure but has a small tank may cause the cut to weaken halfway around a large hole. Use a compressor with at least 20 to 30 gallons for intermittent work, and more capacity for long cuts. Drain the tank and use a water separator; wet air quickly damages consumables and creates unstable arcs.
Check the duty cycle at the output you intend to use. A machine rated at 60 percent duty cycle at 40 amps can run for six minutes in a ten-minute period before needing to cool. Cutting bolt patterns involves frequent starts and stops, so duty cycle is less critical than it is for a long straight cut, but undersized machines still overheat when used near maximum output.
A machine with a 20-foot torch lead is easier to use around large plate than one with a short lead. Confirm that replacement electrodes, nozzles, and shields are readily available. Proprietary or difficult-to-find consumables can erase the initial savings.
How to cut bolt patterns cleanly
Lay out the center of the part and the bolt circle with a scribe, paint marker, or transfer punch. Do not rely on a marker line alone if the hole location matters. A punch mark gives the torch operator a visible center and helps a pilot drill or hole template stay in position.
For holes up to about 3/4 inch, a hand plasma cut can work well, but it will not automatically produce a precision hole. Hold the torch perpendicular to the plate and keep a steady travel speed. Start at the edge of the hole or pierce inside the waste area, then move onto the layout line. Starting directly on the final line often leaves a divot or oversized entry point.
For a larger hole, tilt the torch slightly during the initial pierce so molten metal blows away from the nozzle. Once the arc is through, return the torch to square. On 1/4-inch plate, the pierce delay may be only a fraction of a second; on 1/2-inch plate, allow more time for the arc to fully penetrate. Moving too soon leaves a partial hole and heavy dross.
A circle guide, fixed-radius compass, or bolt-pattern template makes a major difference. Some plasma cutter circle guides attach to the torch and maintain a consistent radius. They are inexpensive and worthwhile when several holes must match. For tight tolerance work, cut the rough opening undersize and finish it with a drill, annular cutter, or boring tool.
Consumables and common failures
Heavy dross on the underside usually means the torch traveled too slowly, the amperage was too low, the air pressure was wrong, or the plate exceeded the machine’s comfortable capacity. A wide, angled kerf can indicate excessive torch tilt or worn consumables. If the arc sputters, inspect the nozzle and electrode before blaming the machine.
Keep the torch standoff consistent. Drag shields are convenient on thin plate, but dragging directly on the material can wear the shield and pull the torch off line. A worn nozzle enlarges the arc and makes bolt holes less round. Replace consumables when the center hole becomes visibly distorted or the electrode develops a deep pit.
Use a replacement plasma cutter consumables kit that matches the torch, not just the amperage. Keep spare nozzles and electrodes in the toolbox; a single damaged consumable can ruin an otherwise good cut.
Safety and final fit
Plasma cutting produces intense ultraviolet light, hot slag, fumes, and considerable noise. Wear a properly shaded welding helmet, flame-resistant clothing, gloves, hearing protection, and closed leather footwear. Ventilate galvanized, painted, or coated steel aggressively, and remove coatings near the cut when possible. Never cut a sealed tank, pipe, or container unless it has been professionally cleaned and made safe.
For most owners, the best value is a 40-amp pilot-arc machine with a decent air supply and a circle guide. Choose a cheaper 30-amp cutter when the work is occasional and limited to thin plate. Choose a 60-amp unit when 1/2-inch steel is common or downtime costs more than the extra purchase price. No handheld plasma cutter replaces a drill press or boring tool when the bolt holes must be truly round and accurately sized.