Groove preparation to ISO 9692-1
Thickness, process, and whether you can reach the far side — those three settle the preparation type, the included angle, the root gap and the root face. The calculator gives the usual values and the deposit area with them.
Where the values come from
A preparation reconciles two things that pull against each other. The arc has to reach the root — hence the opening and the gap. At the same time every extra millimetre of opening is more deposit, more heat and more shrinkage — hence the wish to keep the angle small. ISO 9692-1 collects the compromises that have proved themselves.
| Thickness, mm | Usual preparation | Angle α | Gap b | Root face c |
|---|---|---|---|---|
| up to 3–4 | Square edge (I) | — | 0–3 mm | — |
| 4–12 | Single V | 50–60° | 1–4 mm | 1–2 mm |
| 12–40, both sides reachable | Double V (X) | 50–60° | 1–3 mm | 1–3 mm |
| over 30, one side only | Single U | 8–12° | 1–3 mm | 1–3 mm |
| over 40, both sides | Double U | 8–12° | 1–3 mm | 1–3 mm |
The angle depends on the process too: MAG allows a narrower opening because the arc is more concentrated and digs deeper, while TIG takes a wider one so a hand-fed rod fits.
What the angle costs
The area grows with the square of the thickness and with the tangent of half the angle. The practical upshot: the difference between 60° and 50° on 20 mm plate is about a fifth of the deposit, and moving from a V to an X at the same thickness saves roughly half. Run the same joint through the deposited metal calculator — the numbers argue better than words.
Welding from one side there are three ways out: penetrate the root fully (needs a steady hand and an accurate gap), back-weld after gouging, or use backing — ceramic, copper or permanent. The choice goes into the WPS along with the geometry, because it changes the range of qualification.
Frequently asked questions
When a V and when an X?
Access and thickness decide. Up to 12 mm a V usually does. Above that, if the work can be turned and reached from behind, a double V wins twice over: roughly half the deposit, and the angular shrinkage is symmetrical so the part does not pull to one side. With no access to the back a V is what is left — and the root has to be got right first time.
Why leave a root face at all?
Without one the thin edge simply runs away. A 1–2 mm root face gives the root bead something to stop against and lets it be run at a steady current. Too large a root face does the opposite: the arc cannot melt through it and you are left with lack of root penetration.
Can these numbers go straight into a WPS?
As a starting point yes, as a final entry no. ISO 9692-1 gives shapes and ranges; the values for a particular joint are set by the engineer, allowing for the steel grade, the position, the requirements on the root and whether the procedure was qualified.
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