Design for Fabrication: 12 Drawing Changes That Cut Heavy Frame Cost

The cheapest hour in a heavy fabrication project is the one spent on the drawing before it is released. Almost every meaningful cost decision on a multi-tonne weldment is made in CAD, and most of the savings below cost nothing in function.

These are the twelve we raise most often when a drawing lands in front of us.

1. Reduce fillet size to what the calculation says

Weld metal volume rises with the square of leg length. A 12 mm fillet holds roughly 2.25 times the metal of an 8 mm one, and more than that in arc time, because it needs multiple passes.

Blanket fillet call-outs (“all welds 10 mm FW unless noted”) are convenient and expensive. Size the structural welds and let the rest be minimum-size.

2. Replace full-penetration welds where a fillet will do

Full-pen joints need edge preparation, root access, often backing, more passes, and usually volumetric NDT. They belong where the joint carries tension or fatigue. They do not belong on brackets, covers and non-structural attachments.

Go through the drawing and ask, joint by joint, what is being carried.

3. Bevel heavy plate instead of leaving square grooves

Above roughly 25 mm, a single or double bevel removes a large amount of weld metal compared with a square-groove joint, which cuts both cost and distortion. Double bevel also balances shrinkage about the neutral axis, see welding distortion control on heavy plate.

4. Give the torch somewhere to go

A weld reachable in one clean pass is cheap. The same weld inside a closed box, reached through a hand hole, can take four times as long, cannot be inspected properly, and may need a different process.

If a joint cannot be seen, it probably cannot be welded well. Design access holes, or split the assembly and weld it in stages.

5. Bring machined features onto one plane

Machining cost scales with setups, not features. Four pads on one face costs little more than one pad on that face. Four pads on four faces costs four times as much, plus re-datuming between each.

Rotating a bracket 90° in the model to bring its machined face into an existing plane is free. It is the highest-value change on this list.

6. Specify coplanarity, not individual flatness

If what you need is four feet landing on one plane, say coplanarity. Four individually flat pads can sit on four different planes and still pass inspection. Detail in flatness tolerance on large machined weldments.

7. Design in a shim pack

Three millimetres of shims under each foot costs nothing and gives the millwright somewhere to go. Without it, every dimensional deviation becomes a machining problem.

8. Round dimensions to the plate

Standard plate comes in standard sizes. A part that needs 2,450 mm and is drawn at 2,520 mm may force a second plate for the sake of 70 mm. Check your major dimensions against stock sizes before release, the shop will nest better and the material line will drop.

9. Standardise plate thicknesses across the assembly

A frame using 12, 16, 20 and 25 mm plate needs four material calls, four nests and four offcut streams. The same frame using 16 and 25 mm is materially lighter to buy and faster to cut, even if a couple of members end up slightly over-strength.

Aim for two or three thicknesses on a medium assembly.

10. Repeat parts instead of mirroring them

A left-hand and a right-hand bracket are two parts, two nests, two fit-up jigs and two chances to fit the wrong one. A symmetrical bracket used both sides is one part with twice the quantity.

Look for mirrored details in the model and ask whether symmetry costs anything real.

11. Specify NDT by joint, not by blanket statement

“100% UT on all welds” is an expensive sentence. The volumetric NDT that matters is on full-penetration joints carrying tension or fatigue. Everything else is visual, with MT or PT on selected joints where surface cracking is the concern.

Mark the joints on the drawing. Reasoning in NDT for heavy weldments.

12. Check the shipping envelope before you freeze the geometry

A frame that fits a 40 ft flat rack ships at a predictable rate. The same frame 20 cm taller becomes out-of-gauge cargo and can cost several times as much on the ocean leg alone.

Draw the container envelope in the model as a reference body. If the frame breaks it, consider split-and-bolt design before you consider paying for breakbulk, see out-of-gauge and breakbulk shipping.

Three more that are worth a sentence each

• Do not mix grades without reason. Two grades in one weldment means two material streams, two mill certs and a preheat question at every dissimilar joint. See steel grades for heavy weldments.
• Avoid closed sections if the finish is galvanising. Every closed volume needs vent and drain holes, which have to be located and sized properly, and they will show.
• Put the centre of gravity on the drawing. It costs a calculation and it prevents a rigging incident.

How to actually use this list

Run it as a review before you release, with the drawing on a screen and someone who has welded asking the questions. Twenty minutes of that catches most of the twelve.

Better still, run it with the fabricator. The shop knows its own crane, oven, machine table and nesting software, and will tell you which of these matter for your specific piece. That conversation is free and it happens before the quote, not after.

That is what we do with a drawing. Send it over, even unfinished, and we will come back within 24 business hours with the changes that carry real money and the ones that do not.