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Accurate plasma cuts start with accurate layout. A plasma cutter can follow a line quickly, but it will not correct a line that is in the wrong place, a marker that disappears under heat, or a workpiece that shifts halfway through the cut. Good marking is especially important when parts must fit together, bolt holes need to align, or you are cutting several identical pieces.

Choose the Right Marking Method

The best marker depends on the material, the finish you need, and whether the line will remain visible near the arc. A standard felt-tip marker is fast and inexpensive, but its ink can burn away several millimeters ahead of the torch. A scribe gives a more permanent line, although a shallow scratch is difficult to see through mill scale and can be hard to follow with a hand-held torch.

For most mild-steel projects, use a bright metal paint marker for the main layout and a scribe or sharp soapstone for critical reference points. On stainless steel or aluminum, avoid ordinary markers if the finished part will be welded or exposed to heat; residue can contaminate the surface. Use a marker intended for metal, then clean the area before welding.

Prepare the Metal First

Measure and mark on a stable, reasonably clean surface. Remove loose rust, dirt, oil, and heavy mill scale where the line will go. You do not need to polish an entire plate, but a quick pass with a wire wheel or abrasive pad makes the line more consistent and helps a square sit flat.

Check the plate for a curled edge before measuring. Thin sheet can lift from the table, changing the position of a square or tape measure. Clamp it flat, or place it on a level cutting table with enough support under both sides of the cut. If the metal moves, even a perfect layout becomes useless.

For repeat work, establish a datum: one known straight edge or centerline from which all dimensions are taken. Do not measure each feature from a different edge. Small errors compound, particularly when cutting panels or parts that must meet at right angles.

Measure and Mark Accurately

Use a steel rule or tape measure for rough dimensions and a combination square for lines that must be square to an edge. Keep the square’s stock firmly against a known straight edge. For long cuts, mark points at both ends and snap a chalk line between them, or use a straightedge long enough to bridge the workpiece without bowing.

For a circle, use a compass, beam compass, or a homemade pivot and arm. Mark the center with a punch before scribing the circle. A center mark helps you find the correct location again after repositioning the plate and reduces the chance of measuring from the wrong side.

Mark the waste side of every line. A simple arrow or “W” prevents confusion when the cut width matters. Plasma cutting removes material through the kerf, so the torch should normally travel on the waste side of the finished dimension. If the finished part must be 100 mm wide, do not place the torch centerline directly on both boundary marks and expect a 100 mm part.

Account for Kerf and Torch Offset

Kerf is the width of metal removed by the plasma arc. It varies with amperage, material thickness, travel speed, air pressure, consumable condition, and the particular machine. As a starting point, many hand-plasma cuts in mild steel produce a kerf roughly 1.5 to 3 mm wide, but measure your own machine rather than relying on a catalog number.

Cut a test slot in the same thickness and measure the removed width with calipers. For an outside part, keep the torch path on the waste side of the layout line. For an inside hole, keep the torch path on the part side of the line. The correct offset is approximately half the kerf when you are programming or guiding a path from a dimensioned edge.

On a hand cut, do not try to visually center the arc on a thick marker line. The line may be 2 mm wide before the kerf is considered. Mark a clear boundary and place the torch so the visible cut edge lands on that boundary. A plasma cutting guide or roller can make this easier on straight cuts, but it adds setup time and may not fit around corners.

Use Guides for Straight Lines and Circles

A hand-held plasma torch rarely produces a truly straight cut over several hundred millimeters without support. Clamp a straightedge or cutting guide next to the line, leaving clearance for the torch cup and your hand. Test the clearance with the machine switched off. The guide must not be so close that the torch contacts it, and it must not be made from a material that can melt or catch fire.

For long cuts, a magnetic guide is convenient on steel, while clamps and a steel square work on most jobs. Magnets are not enough if the plate vibrates or the guide is bumped. Use two clamps when possible, and keep clamps outside the torch’s travel path.

For circles and holes, use a circle guide, a template, or a pivot attachment. A freehand circle often ends with a visible step where the cut starts and stops. Put the pierce point inside the waste area, lead into the layout line, and finish with a short lead-out beyond the point where the cut began.

Protect the Mark and Check Before Cutting

Set the torch height according to the cutter manufacturer’s instructions. Drag cutting with a shielded torch can be suitable for thin steel, but dragging directly on a thick paint line may smear it. A plasma standoff guide helps maintain a consistent gap and can improve cut quality, though it may restrict access in tight corners.

Before starting, verify the finished dimension, waste side, pierce location, travel direction, and where the torch will exit. Make sure the plate is grounded through the work clamp and that the cut path will not run into a clamp. Remove flammable materials from below the cut; sparks and hot slag can travel farther than expected.

Wear a properly rated auto-darkening plasma and welding helmet, gloves, hearing protection, and nonflammable clothing. If the line disappears during cutting, stop rather than guessing. Let the metal cool, clean the surface, and extend the layout from the original reference marks. Guessing is how a small layout error becomes a part that no longer fits.

Inspect and Correct the Cut

After cutting, remove dross and check the dimension from the finished edge, not from the original marker line. Measure both ends of a straight cut and check diagonals on rectangular parts. If the measurements differ, look for plate movement, guide flex, torch tilt, or inconsistent travel speed before changing the layout.

A rough edge does not always mean the marking was wrong. Excessive dross, a bevel, and a narrow or wandering kerf commonly indicate incorrect speed, air supply, torch height, or worn consumables. Fix those cutting variables first, then repeat the test cut and update your kerf allowance.