Direct answer: Cutting an opening in a dished head is not flat plate work. The path is a closed curve lying on a curved surface, and the torch attitude has to be planned together with it rather than corrected afterwards. Once the opening is out, the thing to assess is the cut section and edge condition, because that decides whether the next operation can weld directly.
Who this is for: Written for fabricators cutting openings and bevels in dished heads and shells for vessels, tanks and similar assemblies.
Scope: This EVST guide covers why a curved surface changes the planning problem, what has to close, and what to read on the finished edge. It does not select a thermal cutting process, quote edge tolerances, or cover weld procedure qualification.
Why curvature changes robotic head cutting
On flat plate a round opening is a circle in one plane. On a dished head the same opening is a closed curve in three dimensions, and every point on it sits at a different surface angle.
In the reference footage a robot runs a full circuit across the curved wall of a head standing on a low stand and takes the opening out. The motion looks like the flat-plate case; the underlying path does not.
This is why programs written for plate rarely transfer. Robotic head cutting has to have its geometry generated against the actual head form. According to ISO 12100:2010, the limits of the machinery are determined first, and on a dished head those limits are geometric before they are anything else.
Closing the path
The first thing to confirm is that the path closes: the trajectory returns to its start without the robot running out of joint travel or hitting a singular pose part way round.
Reachability on a curved surface is not uniform. A pose that is comfortable at the crown may be awkward near the knuckle, and that is usually where a path fails to close.
Checking closure before committing to a fixture position is far cheaper than discovering it with the head clamped. According to ISO 10218-2:2025, the application rather than the robot is assessed, and in robotic head cutting the fixture is part of that application.
Attitude travels with the path
The angle between the torch and the local surface governs the cut. Where the surface angle changes continuously, the attitude has to change with it or the kerf and any bevel angle drift around the circuit.
Treating attitude as a correction applied after the path is planned tends to produce exactly that drift. The two are one specification.
Where a bevel is required, the bevel angle is defined relative to the local surface, which makes attitude planning a requirement rather than a refinement.
What to read on the finished edge
The footage ends on the finished opening and its cut section. That view is worth more than the cutting itself.
According to ISO 9013:2017, thermal cuts are classified by criteria including perpendicularity and angularity tolerance and mean height of the profile, which gives a vocabulary for stating what an edge from robotic head cutting has to meet.
What matters commercially is whether the edge as cut is acceptable for the next operation, or whether it needs grinding or machining first. That decision belongs in the specification, not in the shop.
What the footage proves and what it does not
It proves that a robot carrying a thermal cutting torch completed a closed circuit on the curved wall of a dished head, and that the resulting opening and its cut section are visible afterwards.
It does not prove cutting speed, kerf width, bevel angle, edge roughness or heat-affected zone. It is also not sufficient to identify which thermal cutting process was used, so this article does not name one.
Where a process name matters commercially, it should come from the process specification rather than from a picture.
A short decision table
Opening in flat plate, no bevel: a plate program is usually adequate.
Opening in a dished head, no bevel: path closure and attitude planning against the real form are required.
Opening with a bevel in a dished head: attitude is a specification input, and the edge criteria should be stated before programming rather than after the first cut.
| Case | Path planning | Attitude planning |
|---|---|---|
| Opening in flat plate, no bevel | Circle in one plane | Constant |
| Opening in a dished head, no bevel | Closed curve on a curved surface | Follows the surface angle |
| Opening with a bevel in a head | Closed curve, checked for reach | A specification input, set before programming |
What to prepare before asking for a quotation
Head specification: form, diameter, wall thickness and material. Opening layout: position, quantity, diameter, and whether a bevel is required.
Edge requirement: whether the next operation welds directly on the cut edge or expects preparation.
Fixturing and handling: how the head is supported and whether it is rotated during the cut.
Frequently asked questions
Can a flat plate cutting program be reused on a head?
Not usually. The path becomes a closed curve in three dimensions and the surface angle changes continuously, so both trajectory and attitude have to be generated against the head form.
What decides whether the edge can be welded directly?
The edge condition against the requirement for the next operation. ISO 9013:2017 gives categories for stating that requirement.
Is the bevel angle set by the robot or by the geometry?
By the geometry, expressed relative to the local surface. The robot has to hold that relationship as the surface angle changes.
Does the head have to be rotated during cutting?
That is a fixturing decision. Rotating can improve reachability, and it changes what has to be checked for path closure.
Project inputs for an application review
If you are evaluating robotic opening and bevel cutting on heads, the geometry and the edge requirement are what make a quotation meaningful. EVST checks path closure against the actual head form before quoting.
- head form, diameter, thickness and material
- opening layout, diameter and bevel requirement
- edge condition expected by the next operation
- fixturing and whether the head is rotated
Send those and EVST will check path closure and attitude against your actual head. Related reading: cylinder path boundaries, cutting automation acceptance.