Weld symbols on machine frame drawings, and what they cost you when they are wrong

“Weld all round” written across an assembly view is the single most expensive phrase on a machine frame drawing. It is not a specification — it is a delegation, and every shop that reads it will resolve the ambiguity in whatever direction suits its quote. The cheapest bid is the shop that read it most loosely.

Weld symbols exist so that the joint is unambiguous. They are a small notation, they take an afternoon to learn properly, and on a frame with sixty joints they are worth more than any other drafting skill.

The anatomy of the symbol

Every weld symbol is built on a horizontal reference line with an arrow pointing at the joint. The convention, set out in ISO 2553 and in AWS A2.4, is consistent once you know which side you are reading:

• Below the reference line means the weld is on the arrow side — the side of the joint the arrow touches.
• Above the reference line means the other side.
• Both means welds on both sides.

That sounds trivial and it is the most common reading error in the industry, because the ISO and AWS conventions differ in where the “other side” symbol sits relative to a dashed identification line. State which standard the drawing follows in the title block. A drawing that does not say is a drawing that will be misread eventually.

The symbol shape tells you the weld type: a right triangle for a fillet, a V for a single-V groove, a U, a J, a bevel, a square for a square-groove butt. A circle at the elbow means all round. A flag means field weld.

The numbers, and where they go

For a fillet — which is most of what a machine frame consists of — the size goes to the left of the symbol, the length to the right, and for intermittent welds the pitch follows the length after a dash.

So 6 ▷ 50-150 on a fillet symbol means: 6 mm leg, 50 mm long segments, on 150 mm centres. That is a complete, unambiguous, priceable instruction. Every shop quoting it will price the same amount of deposited metal, and the quotes will cluster.

Compare that with “weld all round”. On a 600 mm rib the difference between a continuous 6 mm fillet on both sides and a staggered intermittent pattern is roughly a threefold difference in deposited metal, and therefore in welding hours, heat input, distortion and cost. That entire range is inside the ambiguity of the phrase.

Fillet size: the calculation people skip

Deposited volume scales with the square of leg length. A 10 mm fillet contains about 2.8 times the metal of a 6 mm fillet. It takes more than 2.8 times as long to lay, because it usually needs multiple passes where the smaller one is a single pass.

Most machine frame fillets are specified larger than the load requires, for the same reason most safety factors get rounded up — it feels prudent and the cost is invisible on the drawing. It is not invisible on the invoice, and it is not invisible in the distortion, as controlling distortion on large weldments sets out.

A working sequence: size the fillet from the load it carries, check it against the minimum fillet size for the thicker plate in the joint (a code requirement that exists to avoid excessive cooling rates), and take the larger. That usually lands well below what gets specified by instinct.

Intermittent welds, and when not to use them

Intermittent fillets are excellent on rib-to-plate joints that carry distributed load and need stiffness rather than sealing. They cut heat input substantially and they are structurally adequate far more often than people assume.

They are wrong in three situations, and the drawing should say so explicitly where they apply:

• Sealed sections. A closed box that will be filled, or that must exclude coolant and swarf, needs continuous welds. Water sitting inside a frame section is a slow, expensive failure.
• Fatigue-loaded joints. Each intermittent weld end is a stress concentration and a crack initiation site. Where the joint sees reversing load, continuous is correct.
• Joints in a corrosive environment. The unwelded gap is a crevice corrosion site.

Where you want continuous, draw continuous. Where intermittent is acceptable, draw intermittent with a pitch. Do not leave it to the shop, because the shop’s interest and yours point in opposite directions here.

The notes that belong alongside the symbols

Symbols specify geometry. Three things they do not specify belong in the drawing notes:

The governing standard and the qualification requirement. Something like: All welding to AWS D1.1. Welders and procedures qualified to the standard; WPS and PQR available on request. This is the clause that separates export-capable shops from the rest. Which standard fits which market is covered in welding standards for machine frames.

The critical weld list. A short table — three to six joints — naming the welds that carry the important loads, with the NDT method and acceptance class for each. This is worth far more than a blanket “NDT all welds” note and costs a small fraction of it. The reasoning is in NDT for machine frame welds.

Weld preparation and access. If a joint needs a specific groove preparation, draw it. If a weld is only accessible before an adjacent part is fitted, say so — assembly sequence problems discovered on the shop floor turn into change requests.

Reading a drawing back before you release it

A quick self-audit that catches most of it:

1. Does every joint on the assembly have a symbol, or is any of it covered by a general note?
2. Does the title block state ISO 2553 or AWS A2.4?
3. Does every fillet have a size, and does every intermittent weld have a length and pitch?
4. Are the sizes calculated, or inherited from the previous drawing?
5. Are the continuous welds — sealed sections, fatigue joints — drawn continuous deliberately?
6. Is there a critical weld table with NDT called out per joint?
7. Are the field welds flagged, if any?

A drawing that passes that audit produces quotes that land within a few percent of each other. A drawing that fails it produces a 40% spread, and the spread is not competitive pricing — it is five shops guessing at five different scopes of work.

If you want a frame drawing reviewed for weld callouts, sizing and access before it goes out to a shortlist, send it through — the manufacturability review comes back within 24 business hours, and it is separate from quoting. The broader drawing audit is set out in a drawing checklist for welded machine bases.