Clean square HMI cutouts: cutting methods compared

Short answer: on a new door, order the HMI opening machined before the steel is coated, so no cutting tool ever touches the finish. On a door already painted, cut from the inside with a jigsaw, over tape covering everywhere the saw base will travel, and expect fine scratches rather than burns. Plasma stays acceptable only after scrap practice, with paint burn-back held to about 1/8 in. Waterjet at a fabrication shop is the cleanest retrofit of all. The opening itself always comes from the display's installation sheet — one compact terminal calls for 190 x 135 mm at +1/-0 mm — and this comparison gives you each method's numbers and the decision for each job.

What does each cutting method do to the paint?

Pre-machining and waterjet leave the coating untouched, a taped jigsaw leaves fine scratches you can live with, and plasma run well burns about 1/8 in of paint back from the cut line; plasma run badly burns far more.

Read the damage around a finished opening and it tells you the mechanism. Fine scratches mean chips rolled under the saw base. A brown band means heat reached the coating. Paint chipped along the outside edge means the blade cut on its upstroke from the outside. A corner cut past the line means the layout had no relief holes and the cut had no stopping point.

Which cutting method gives the cleanest square opening?

Machine the opening before the door is coated: the cutter meets bare steel, the paint goes on afterward, and that wins for every new build; for a door already painted, waterjet is the cleanest retrofit because the cut carries no heat.

That is the route we build. Custom stainless cutouts leave the shop with every opening machined before finishing, and the cutout designer places the display opening and every device hole on any face before your quote request.

Waterjet has moved from the machine-shop tier to the local fabrication shop. Small tables are common now, and most shops cut from a CAD layout of the opening plus every pushbutton and pilot-light hole. You unbolt the door, they cut it cold, and the edge needs one pass of deburring. Laser sits one step further back: it belongs at the enclosure builder, on a body being made to order, and it leaves a faint heat tint on stainless.

A metal-cutting circular saw leaves an edge that looks machined and beats a jigsaw for speed on straight runs, with two limits. A round blade cannot cut a square corner, so you still drill corner relief holes and saw the straight runs between them. And the saw can kick back if it is pulled backward, so clamp the door and feed it forward only.

How do you keep a jigsaw from scratching a painted door?

Cut from the inside face, tape every bit of the saw base's travel, and tilt the door so you saw downhill: one reported field comparison put duct tape at about three quarters of the chip marking prevented, and sweeping the chips does the rest.

Take the door off its hinges or lock out the supply first, and tape over open wiring ducts so chips stay out of terminals. Then work the sequence:

1. Support the door flat with the inside face up, and brace the middle so the sheet cannot flex mid-cut.
2. Lay out the opening from fixed references — door edge and hinge line — at the installation sheet's A x B, and mark the waste side.
3. Drill a relief hole just inside each corner, sized about to the blade's width, plus a start hole inside the waste.
4. Cover the saw base's whole travel area with duct tape; fix the template with blue painter's tape, which is translucent enough to read the line through.
5. Fit a fine-tooth metal blade — the same field comparison puts its kerf at about 1 mm — and keep that kerf entirely in the waste.
6. Tilt the door and cut downhill, so the saw's vibration walks the chips down and away from the base.
7. Start in the waste hole, reach full blade speed, then feed steadily; when chips gather, lift the saw clear and sweep instead of dragging the base across them.
8. Turn corners through the relief holes or by re-entering from the adjoining side, rather than by forcing the blade.
9. Break the edge with a mill file so the bezel gasket finds a flat land all the way round.

On a stainless door the method is identical and the stakes shift: there is no paint to burn, and the tape now protects the brushed finish. The field cutting guide covers tooling for round holes in stainless, with the same inside-first discipline.

How much paint does plasma burn off?

With the balance between heat and travel speed practiced on scrap, and an aluminum template clamped over the layout line, plasma holds burn-back to about 1/8 in each side of the cut; without that practice the burn runs wider and slag sticks to the sheet.

Run the numbers before you pick up the torch. A compact terminal's 190 x 135 mm opening has a perimeter of 2 x (190 + 135) = 650 mm. Burn 3 mm of coating back from that edge and you are refinishing 650 x 3 = 1950 mm², about 20 cm² on one door, before you count the slag on the inside face. The burn is not cosmetic either: the ring lands exactly where the bezel gasket sits, so the coating under the gasket has to end up smooth and flake-free or the front seal leaks.

The aluminum template earns its keep twice — it guides the torch and it sinks heat out of the edge. If the burn is already there, touch-up paint on the burn ring covers the band. On stainless there is no paint to burn, and the same energy shows as a blue-brown heat tint along the kerf about as wide as that burn ring.

So plasma earns its spot on repeat installs: same display, same template, same practiced line speed. For a single opening in a finished painted door, it is the fastest route to a repaint.

What tolerance does an HMI cutout actually need?

Take the opening straight from the display's installation sheet and cut to the band it prints: one compact terminal specifies 190 x 135 mm with a +1/-0 mm tolerance in panels 1.6 to 5 mm thick, so your entire oversize budget is 1.0 mm.

Both ends of that band bite. Cut undersize and the terminal's rear housing and connectors will not clear the opening — enlarging an HMI cutout after the fact is a swarf-control job you can avoid. Cut more than 1 mm oversize and the bezel gasket loses land, and the seal weakens first at the corners.

Kerf arithmetic decides which tools can hold that band. A jigsaw blade's 1 mm kerf equals the whole budget, so the blade runs in the waste and the finished edge lands on your line. A nibbler's 4-5 mm swath is four to five times the budget, which is why nibbler openings come out oversize with big corner radii unless you nibble up to the line from the waste side. Check the door gauge against the sheet too: a 14 ga [1.9 mm] steel door sits inside the 1.6-5 mm band, while 16 ga [1.5 mm] falls just under the 1.6 mm minimum, and a thin door needs the display's clamp reach read off its mounting page. The LCD keypad cutout checklist lists the same dimension-first checks for keypad terminals.

Does cutting the door change the enclosure's rating?

The finished front takes the display's rating wherever it is mounted: a display with an IP65 front seal — under IEC 60529, dust-tight and jetted water from a 6.3 mm nozzle at 12.5 L/min — sets the front, and the door's own rating covers the rest.

The door keeps its rating through the cut when the opening is flat, deburred and inside the sheet's tolerance, because the bezel gasket seals on the door's face and the surface under it is part of the seal. Our stainless bodies are rated NEMA 4X / IP66 by the manufacturer, not UL listed; mount a display with an IP65-type front and the assembly's front is IP65-class, since the weakest part sets the rating. What rating a panel keeps after you cut it and mount devices works that rule through device by device, and the cutouts, glands and HMI windows guide covers how each opening is specified to hold its seal.

This is where machining before coating pays twice: the edge comes out flat, and the coating under the gasket is intact factory finish rather than a filed repair.

Which cutting method should you choose for your job?

Order the opening machined before coating on every new door; jigsaw from the inside over tape for a one-off retrofit; waterjet when you can take the door to a shop; plasma only on repeat installs with a template and a practiced 1/8 in burn-back.

• New door, any quantity — machined before coating. One drawing carries the display opening and every device hole, and nothing touches the finish afterward.
• One painted door, hand tools — inside jigsaw with taped travel, relieved corners, filed edge. Damage stays inside the taped band.
• Painted door, shop access — waterjet from your layout. Cleanest retrofit edge, cut cold, corner radii to your drawing.
• Repeat installs, same display — plasma with an aluminum template after scrap practice, budgeting the burn ring and touch-up.
• Stainless door in the field — jigsaw from the inside: no paint to burn, tape protecting the finish, no heat tint.

What should you specify so the opening arrives already cut?

Send the opening's A x B with its tolerance (+1/-0 mm on most terminal sheets), the corner radii the datasheet allows, every pushbutton and pilot-light hole center, and the door's gauge: that one drawing is everything a fabricator needs to cut before coating.

On our side the route is one page. The cutout designer lays display openings, glands and operators on any face of a catalog or custom stainless body, checks spacing and sends the layout with your quote request. The finished enclosure is delivered in 10 business days, counted from your approval of the drawing.

HMI enclosures take this to the end of the line. A 10.1 in or 15 in panel PC housing arrives in 304 stainless already cut for the display, with sizes running from 7 in to 23.8 in and an IP65-type front seal with the display installed, manufacturer rated. If your door is painted steel bought elsewhere, the same drawing goes to whoever coats it — cut first, paint after, and the burn-back question never comes up.

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