What's inside
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Cutting a clean round hole with a hand plasma cutter is possible, but freehand work rarely produces a truly round result. A plasma circle guide turns the torch into a controlled compass: one end pivots at the center, while the torch travels around a fixed radius. The guide improves accuracy, keeps the torch moving smoothly, and makes repeatable parts much easier.
What a Plasma Circle Guide Does
A circle guide usually consists of a pivot point, an adjustable arm, and a torch holder or torch bracket. The pivot is fixed at the center of the planned circle. The torch then follows the arm around that point, cutting the perimeter at a constant distance.
Most guides are designed for handheld plasma cutters with a standard torch shape. Some attach directly to the torch cup or shield, while others use a separate roller or standoff wheel. Check the guide’s torch compatibility before buying. A loose bracket can let the torch wobble, changing the cut diameter and producing an uneven edge.
For occasional work, a basic plasma cutter circle guide kit is often sufficient. A heavy-duty guide is more useful if you cut thick plate, make many matching parts, or need an adjustable pivot and roller that will survive shop use.
Choose the Right Guide
Measure the range of diameters you expect to cut. Some inexpensive guides handle only small circles, while longer arms can cut larger openings but become harder to control. A guide should also have a stable pivot. A sharp center pin works well on steel plate; a suction-cup pivot is more convenient on clean, flat sheet but can slip on mill scale, oil, or rough surfaces.
| Guide type | Best use | Main advantage | Main drawback |
|---|---|---|---|
| Center-pin guide | Plate and flat sheet | Accurate and hard to move | Leaves a small center mark or hole |
| Suction-cup guide | Clean, painted, or finished surfaces | No drilled center hole | Can release from dirty or rough steel |
| Roller-wheel guide | Frequent cuts and thicker material | Maintains torch height smoothly | Costs more and needs proper adjustment |
| Simple magnetic guide | Light-duty sheet work | Fast setup and low cost | Magnet can attract dross and shift during cutting |
The cheaper option is fine for occasional holes in mild steel if the pivot locks securely and the torch bracket fits tightly. Paying more makes sense when diameter accuracy, repeatability, or operator comfort matters more than the lowest purchase price.
Calculate and Mark the Radius
A circle guide must be set to the distance from the pivot point to the torch’s cutting point, not necessarily to the outside of the torch cup. This detail causes many oversized or undersized holes.
For an inside hole, the torch’s arc cuts along the intended finished edge. Set the guide radius to half the required hole diameter, then account for the torch’s actual cutting path if the guide positions the torch by its cup or outer body. If the guide’s instructions specify a correction, follow that measurement. Otherwise, make a test cut in scrap and measure it.
Example: for a 4-inch hole, the target radius is 2 inches. If your test cut measures 4 1/8 inches, reduce the guide setting by approximately 1/16 inch. Make another test cut before moving to expensive material. Plasma kerf width, torch angle, and starting technique all affect the final size.
Mark the center with a punch. On large plate, use a ruler, combination square, or trammel to verify the distance from the center to several points around the planned circle. Remove heavy scale, paint, and grease near the pivot and cut line so the guide does not slip and the work clamp has a reliable electrical connection.
Set Up the Plasma Cutter
Use the manufacturer’s recommended amperage and consumables for the material thickness. As a general starting point, thin sheet often needs roughly 20 to 40 amps, while thicker mild steel may require 45 to 65 amps or more. The correct setting depends on the cutter’s rating, material, and whether the cut is rated as a clean severance cut or a slower production cut.
Use a clean, undamaged shield, nozzle, and electrode. A worn nozzle can make the arc wander, which shows up as a lopsided circle even when the guide is set correctly. If the torch uses a drag shield, confirm that it is intended for contact cutting. Otherwise, use the guide’s roller or standoff and maintain the specified gap, commonly about 1/16 to 1/8 inch.
Connect the work clamp directly to clean metal. Keep the lead clear of the rotating guide, and arrange the plate so the finished slug can fall safely without catching the torch or guide.
Cut the Circle
Put on a properly rated auto-darkening plasma cutting helmet, gloves, long sleeves, and hearing protection. Plasma cutting produces ultraviolet light, hot slag, fumes, and significant noise. A face shield over safety glasses is useful when positioning the work, but it does not replace the welding lens.
Start the cut at a planned lead-in point rather than directly on the finished line. Many circle guides allow the torch to begin slightly outside the circle, then move onto the cut path. This avoids leaving a heavy start mark on the finished edge. If you must pierce on the line, angle the torch slightly during the pierce and return it to the correct vertical position once the arc has gone through.
Move at a steady speed. The correct speed usually produces a small amount of dross on the underside and a clean, continuous top edge. Moving too slowly widens the kerf, overheats the plate, and leaves heavy slag. Moving too quickly causes an incomplete cut, visible drag lines, or a section that remains attached.
Keep the guide arm under light, even pressure. Do not force it around tight spots. If the torch catches, stop and inspect the consumables rather than twisting the guide. Twisting can bend the bracket, shift the pivot, or produce a tapered circle.
Fix Common Problems
If the circle is oval, the pivot slipped or the guide arm flexed. Clean the pivot area, tighten the hardware, and reduce pressure on the torch. If the diameter is consistently wrong, reset the radius using a test cut rather than sanding the finished part.
A rough edge usually indicates incorrect speed, poor torch height, worn consumables, or insufficient amperage for the material. Excessive bottom dross often means the torch moved too slowly or the air pressure and flow were inadequate. An interrupted cut can result from a weak work connection, water in the compressed-air line, or trying to cut beyond the machine’s rated thickness.
After cutting, let the part cool before checking its size. Knock off dross with a chipping hammer or flap disc, but avoid removing enough metal to change the diameter. For a tight-fitting pipe, flange, or bearing opening, finish with a grinder, carbide burr, or machining operation. A circle guide improves the cut; it does not turn a hand plasma cutter into a precision milling machine.
When to Use Another Method
Use a hole saw for small holes in thin material when the edge must be clean and the material is within the saw’s capacity. Use a magnetic drill with an annular cutter for accurate holes in thicker plate. For many identical parts, a CNC plasma table is faster and more repeatable. A circle guide is the practical middle ground: inexpensive, portable, and accurate enough for brackets, duct fittings, pipe openings, and general fabrication.