By Pro Steel Solutions, plasma cutting in Ash, Kent. Updated
Profile tolerance
CNC plasma cutting typically holds about ±1 mm on the profile of a part. Thin plate is usually better than that; very thick plate can be slightly worse, mainly because of edge bevel.
Edge bevel
A plasma-cut edge is not perfectly square. One side of the cut usually stays close to square while the other has a bevel of a few degrees. On thick plate this means the top and bottom faces of a part can differ slightly in size. For parts that must fit together precisely, allow for this or machine the faces.
Kerf
The kerf is the width of material removed by the arc, roughly 1.5 to 3 mm depending on thickness and current. The machine offsets its path so the finished part is to size, so you draw the finished outline, not the cut path. Our kerf and hole guide shows typical values.
Minimum hole size
The smallest clean plasma-cut hole is about equal to the plate thickness and never under 3 mm. Holes under 1.5 times the thickness are cut more slowly and taper more. For smaller holes, or holes that need a bolt to be a close fit, we mark them for drilling.
Edge distance and bridges
Keep holes at least about the plate thickness from an edge, and keep thin bridges between cut-outs at least about the plate thickness wide, or heat can distort them. Our edge distance checker works out bolt-hole edge distances for common bolt sizes.
When to drill or machine
Plasma is ideal for outlines and clearance holes. Fitted bolt holes, reamed holes, threads, bearing bores and machined faces should be finished after cutting. A plasma-cut blank is still the cheapest starting point for those parts.
Questions
Can you hold ±0.5 mm?
Not reliably on plasma. For tighter tolerances we cut slightly oversize for machining, or recommend laser cutting for thin material.
Do bolt holes come out round?
Yes, with a slight taper. Holes from about the plate thickness upward are fine for clearance bolts.
Related tools and guides
- Plasma tolerance guide What plasma holds, what needs drilling or machining, and a quick checker.
- Kerf and smallest hole Kerf width, cut speed, amperage and the smallest clean hole for any material and thickness.
- Bolt edge distance checker Minimum end, edge and pitch distances for bolt holes to EN 1993-1-8, checked against your plate.
- Bolt clearance holes ISO 273 close, normal and loose clearance holes, and whether plasma can cut them.
- Plasma vs laser vs waterjet cutting: which to choose
- How to prepare drawings for plasma cutting
- How plasma cutting is priced, and how to pay less