CNC Plasma Cutting Design Guide for Steel Plate

Plasma is the workhorse for heavy steel plate: base plates, gussets, brackets, skid plates and the big flat parts of frames and structures. CNC plasma cutting quotes instantly online at The Job Shop, the same way the fiber laser does — upload a file, choose the plate and see the price. This guide covers choosing between the two, then designing plate parts that cut cleanly on plasma.

Plasma or fiber laser?

Start with what the laser stocks. Its catalog covers 0.032–1″, depending on the metal, and the heaviest of it is plate: mild steel up to 1″, 304 stainless up to 3/4″ and 6061 aluminum up to 1″. The plasma quote carries the same mild steel, stainless and aluminum plate, while brass, copper and galvanized steel stay on the laser, so for most plate parts the choice comes down to the edge, the holes and the price. The laser leaves smaller holes, squarer edges and less cleanup; the mild steel guide covers steel on the laser, and the sheet metal gauge chart lists every stocked thickness. Plasma earns its place on big, simple parts where speed and price count for more than the edge.

Two quick tests help. If a part’s holes are small next to its thickness, or its edges will show or mate, lean toward the laser. If the part is thick, large, or headed for welding and paint where a ground edge is normal, plasma is the usual route — and because both quote instantly, pricing the same file on each settles it. The router vs laser vs plasma vs waterjet comparison sets all four side by side.

Hole size versus plate thickness

A plasma arc needs room to start and turn, so the smallest clean hole is tied to the plate’s thickness: holes much smaller than the plate is thick come out tapered, out of round or not at all. The plasma page gives the usual ratios. The quote flags holes that may be too small for the plate, and what suits your plate is confirmed with the quote before anything is cut.

For bolt holes that must fit, the usual approach is to cut the hole undersize, or only mark its center, and drill or ream it to size afterward — so mark those holes on the drawing with their finished size. Threads and counterbores come after cutting, and so do countersinks as a rule; only some bevel-cutting heads can cut one. And a hole crowded against an edge or its neighbor leaves a thin web that overheats and distorts, so keep at least the plate’s thickness of material between them.

Bevel, taper and edge finish

A plasma-cut edge isn’t quite square. The arc swirls as it cuts, so one side of the kerf comes out squarer than the other, and programs run outside profiles one way and holes the other so the better side lands on the part. Expect a slight bevel and taper that grows with thickness, so the top and bottom of an edge measure a little differently.

Two things follow for design. First, if one face shows or mates with another part, say so in the notes for the shop, and the parts can be cut with that face up. Second, a beveled edge changes a fit-up — a tab in a slot touches at its top or bottom corner first — so allow clearance for it. If an edge needs a true weld-prep bevel, note it when you approve the proof; plasma parts ship as cut, so a bevel is normally ground afterward in your own shop, and anything else is confirmed with the quote before cutting.

The cut edge also carries a heat-affected zone. On carbon steel it can be harder than the plate around it, which shows when a hole tapped right at the edge, or a tight bend across a cut edge, fights back. If parts go straight to welding, ask how they’ll be cut, since edges cut with some plasma gases should be ground first.

Dross and cleanup

Dross is the resolidified metal that clings to the bottom of a plasma cut. Some is normal. Dross from cutting too slowly forms beads that chip off easily; dross from cutting too fast is a thin, tight ridge that has to be ground, and stainless tends to hold on to it harder than mild steel.

Plasma parts from The Job Shop ship as cut — the dross and edge condition are as the cut leaves them. Plan the cleanup: many buyers grind every edge before welding or paint anyway; if edges must be ready to hand to a coater, budget the grinding and blasting in your own shop or your fabricator's. Parts headed for hot-dip galvanizing also need vent and drain holes in any enclosed space, such as a plate welded over the end of a tube, placed where the galvanizer asks.

Small features, text and thin webs

• Slots, fingers and letter strokes narrower than the plate is thick burn away or round over. Draw them no narrower than the plate’s thickness, and wider on decorative work meant to read from a distance.
• Inside corners come out slightly rounded; if a square part must seat in a cutout, relieve the corners or round the mating part.
• Sharp outside points — star tips, arrowheads, serifs — melt back, so blunt them a little in the drawing.
• Text should be bold, simple and converted to outlines, with counters in letters like A, O and R large enough to stay open; a stencil font keeps those islands attached.
• Dense screens and grilles put a lot of heat into the plate, and thinner plate can warp. For fine patterns, the laser is usually the better fit.

Lead-ins, pierces and nesting

Every cut begins with a pierce — the arc punching through the plate — and a pierce leaves a rough crater, so each cut starts a short way off the part and runs a lead-in onto the line: in the surrounding scrap for an outside profile, in the slug for a hole. Where the lead-in joins the edge there’s usually a small mark, and on a visible edge you can ask for it to go where it won’t show.

That explains two design rules. A hole needs room inside for a pierce and a lead-in, one more reason small holes in thick plate don’t cut well. And every hole, slot and cutout is its own pierce, so forty small holes take longer to cut than four larger openings of the same total length — and the quote prices each pierce.

Nesting is how parts are arranged on a plate. Parts in the same grade and thickness share plates, so putting all of a job’s parts in one thickness into the same quote lets them nest together. If a part will be bent and its bend lines should run across the plate’s rolling direction, say so, since that limits how it can turn in the nest.

Heavy plate parts: base plates, gussets and brackets

• Base and embed plates: take the grade, thickness and hole pattern from the engineer’s drawing. Anchor-bolt holes are often oversized on purpose so the plate can be placed in the field, so draw the hole the drawing calls for rather than the bolt diameter.
• Gussets: clip the inside corner where a gusset meets the two parts it stiffens, so it clears the weld already in that corner. Draw the clip in the file — the cut follows the outline.
• Brackets that get bent: draw the flat pattern with the holes kept clear of each bend, and keep a drawing of the bent part for whoever bends it. Bending heavy plate is a separate press-brake step, and plasma parts ship flat, so plan it with your fabricator.
• Skid plates and wear parts: abrasion-resistant plate cuts, drills and bends very differently from mild steel, and it isn’t in the online catalog, so ask the shop before designing around it.
• Ballast plates for exhibits and signs: work out the thickness from the weight you need, then price a couple of thicknesses in the quote to compare.

Uploading parts to the plasma quote

The core is a DXF, SVG or STEP file at full scale with closed outlines, one part per file or clearly separated; the laser file prep guide covers clean outlines and units, and the same habits apply. In the quote, choose the grade, thickness and quantity for each part and check what it read from your file. Then add what the file can’t say in the notes for the shop: fit-critical holes with their finished sizes, which face shows, and any edge headed for a weld-prep bevel. For a critical fit, send the part to an engineer from the quote before you pay.

Approve the proof, and choose pickup or local delivery in the Las Vegas valley. While a plasma price is shown as an estimate, the shop checks it and replies by email before cutting, and payment waits until you agree. Plate parts that need finer detail can go to the laser quote instead, where you pay online once the shop confirms the price. Holes, threads and faces that need machining after cutting are covered in how to send a drawing for a machining quote, tube members for the same frame in the tube bending and tube laser design guide, and welding or a whole build in the custom fabrication inquiry.