Quoting Welded Assemblies from a BOM: Cost Rollup Guide

A welded assembly RFQ never arrives as one drawing. It arrives as a package: a general-arrangement drawing, a BOM with twenty-odd lines, a handful of child part drawings, and a note about paint. The tempting shortcut is to cost the lines you recognise — some laser parts, some tube, a bag of hardware — add "a few hours of welding", and send it. That is how welded assemblies become the most reliably underquoted product a job shop sells, because quoting a welded assembly from a BOM is not the sum of its part quotes. The money hides between the lines: in the joints, the fit-up, the handling, the distortion, and the machining that has to happen after the heat has moved everything.

This guide builds an assembly quote the way it should be built: explode the BOM into what you make, what you buy, and what you outsource; roll up the component costs without double-counting; price the weld joints instead of guessing at the weldment; then add the blocks the BOM never mentions — fixturing, straightening, post-weld machining, inspection — and close with a worked frame example in euros.

Why welded assemblies get underquoted

Two mechanisms do the damage, and neither shows up on a spreadsheet until the job has lost money. The first is the interface problem: a BOM gives every component a line, so every component gets a cost — but no line exists for the act of joining them. The joints, the tacking sequence, the fit-up of parts that were cut to ±0.2 mm and must meet at a corner, the grinding of a weld that lands where a later part must sit: none of it belongs to any single component, so in a component-by-component costing it belongs to nobody and is priced at zero.

The second is handling. A 30-part weldment gets touched far more often than 30 single parts: components are kitted, carried to the fixture, clamped, tacked, re-clamped, the assembly is turned — with a crane, once it weighs enough — moved to the weld-out station, turned again, moved to straightening, to machining, to paint. Each move is minutes, and on a heavy frame each move is minutes of two people and a crane. Shops that track actuals on weldments consistently find handling and fit-up at 30–50% of total assembly labour, which means a quote that only counts arc time can be short by a third before anything else goes wrong.

Underquoting an assembly is also structurally worse than underquoting a part. The absolute money is bigger, the lead time is longer, and the error repeats across the whole batch before the first actuals come back.

Explode the BOM before you price anything

The first pass through the BOM is classification, not costing. Every line lands in one of four buckets, and each bucket is priced with a different method:

• Parts you make — laser or plasma cut plates, sawn and mitred tube, press-brake parts, machined details. Each needs its own small quote, exactly as if it had arrived alone: a cut part follows the laser cutting cost method, a machined foot follows a machining estimate.
• Purchased hardware — levelling feet, weld nuts, threaded inserts, fasteners, hinges. These carry supplier price plus procurement handling and a kitting cost, and they deserve more respect than they get; the full treatment is in purchased parts and hardware cost quoting.
• Outsourced processes — the note that says "hot-dip galvanized" or "RAL 7016" is a real supplier line with a real minimum charge, priced by area like any powder coating estimate, plus two transport legs and days of lead time.
• What the BOM forgets — weld wire and gas, consumable grinding discs, and every block of labour this article is about. The BOM is a list of things, and most of an assembly's cost is not a thing.

While classifying, fix the quantities: BOM quantities are per assembly, so everything gets multiplied by the order quantity, and cut parts get a scrap allowance — on a batch of five frames, one ruined gusset means a new nest or a remnant hunt, and a 3–5% allowance on make-parts is honest calibration.

Roll up component costs without double-counting

The rollup itself is arithmetic with two traps. Cost every make-part at the total produced quantity — four gussets per assembly on a batch of five is a run of twenty, and that is the quantity the laser setup and the press-brake setup amortize over, not four. Nesting all the flat parts of the assembly together on shared sheets is where material yield is won, and it only happens if the parts are costed as one cutting campaign instead of as strangers.

The traps: first, margin belongs at the top. If every child part is priced with its own margin and the assembly then takes margin again, the quote compounds itself out of the market; roll up child parts at cost, and apply the target margin once, on the assembly. Second, surcharges belong in one place. A material surcharge applied inside each child part and then again as an assembly-level contingency is the same money counted twice — pick the level, apply it once.

For Italian-speaking readers, the general method for pricing fabrication work — rates, hours, and the structure of a carpenteria quote — is covered in the preventivo carpenteria metallica guide; this article layers the assembly-specific blocks on top of it.

Weld time: cost the joints, not the weldment

"A few hours of welding" is not an estimate; it is a hope with a unit attached. The estimate comes from the general-arrangement drawing: count the joints, read their type and size, sum the lengths, and convert metres of weld into minutes with a calibration factor for your process and positions. A continuous a4 fillet laid down in position with GMAW runs at one speed; the same throat welded out-of-position on an assembly that cannot be turned runs at a fraction of it. The full method — deposition rates, arc-on factors, joint prep — is in the welding and fabrication cost estimation guide; the assembly-level point is the ratio behind it: on real weldments, arc-on time is typically only 20–35% of weld-bay time. The rest is fit-up, tacking, measuring, clamping, turning, and grinding — which is exactly why a quote built on arc time alone is short by design.

As 2026 calibration for a job shop welding structural assemblies: an all-in weld-bay rate of €50–65 per hour, and 3–6 minutes per metre of a4 fillet all-in (arc plus the handling that surrounds it) in position, doubling or worse out-of-position. Calibrate against your own actuals; the ratio between arc time and bay time is the number most worth knowing about your own shop.

Fixturing, fit-up, and tacking

An assembly is only as good as its fit-up, and fit-up costs time somewhere: either in a fixture that positions the parts, or in a welder with a tape measure, a square, and patience. For one-offs, modular clamping on a fixture table is the honest answer and its time goes straight into the quote. For recurring assemblies, a dedicated jig converts fit-up minutes into clamping seconds on every future frame — and its cost needs an explicit home in the quote, either as a tooling line or amortized over a committed volume with a clawback, exactly as covered in tooling and fixture cost amortization.

The estimating consequence: fit-up and tacking is its own labour block, estimated from the number of components and the datum scheme, not a rounding error on weld time. A 14-part frame located from two datum faces tacks together in under an hour in a jig; the same frame freehand on a table with the drawing balanced on a toolbox is two to three hours, and the difference belongs in the price of whichever scenario you actually quoted.

Distortion, straightening, and the machining that follows the heat

Every weld shrinks as it cools, and the assembly moves. On light frames the movement hides inside loose tolerances; on anything with a flatness or parallelism callout, it does not. Three blocks follow, and hand-built assembly quotes miss all three with impressive regularity.

Straightening — flame or press straightening to bring the welded frame back within tolerance is skilled, unglamorous time. On welded machine bases and frames, 5–15% of weld-bay time is a defensible allowance, more if the design concentrates weld on one side of the neutral axis.

Post-weld machining and the datum shift — when the drawing calls flat, machined pads on a welded frame, the machining must happen after welding, because welding after machining un-machines the pads. That sequencing has consequences: the whole assembly goes on a machine big enough to hold it, the setup is awkward, and the datums the child parts were made to no longer quite exist — the machinist is re-establishing a datum scheme on a distorted weldment, which is slower than machining any of the child parts ever was. Price it as its own operation with its own setup.

Inspection — a welded assembly is inspected as an assembly: overall dimensions, weld sizes and visual criteria, and any callout the customer will measure on receipt. If the spec says welds per ISO 5817 level C, someone has to look, and the time is real.

Worked example: a machine-base frame, batch of five

A welded machine base, 1200 × 800 × 600 mm, in 80 × 80 × 4 tube with laser-cut gussets and four machined foot pads, powder coated, batch of five. The rollup per frame, at the calibration rates above:

Cost block	Basis	Per frame
Cut tube parts	14 pcs, material + saw/tube-laser, nested as one campaign	€182
Laser-cut plates and gussets	4 pcs, 8 mm S355, incl. material	€58
Purchased hardware	4 levelling feet, 8 weld nuts, kitting	€43
Fit-up and tacking	0.9 h in modular fixture @ €55/h	€50
Weld-out	~5.8 m a4 fillet, 1.1 h all-in @ €55/h	€61
Straightening + check	0.4 h @ €55/h	€22
Post-weld machining	Foot pads milled flat, 0.8 h @ €75/h incl. setup share	€60
Inspection	0.3 h @ €50/h	€15
Powder coating	~2.6 m² external area, coater line + transport share	€62
Fixture amortization	€900 modular jig plates ÷ 30 committed frames	€30
Assembly cost		€583

At a 25% target margin, price = €583 ÷ 0.75 ≈ €777 per frame. Now look at where the money sits: the make-parts and hardware are €283 — less than half. The blocks that a parts-only rollup would have missed — fit-up, weld-out, straightening, post-weld machining, inspection, coating logistics, fixture — are €300 per frame, €1,500 across the batch. That is the underquote hiding in "cost the BOM and add some welding", and these figures are calibration points to check your own actuals against, not numbers to copy.

FAQ: quoting welded assemblies

How do you quote a welded assembly? Explode the BOM into make-parts, purchased hardware, and outsourced processes; cost each bucket with its own method at the total batch quantity; then add the assembly-level blocks the BOM does not show — fit-up and tacking, weld time from counted joints, straightening, post-weld machining, inspection — and apply margin once at the top. The joints and the handling are where hand quotes go short.

What is a BOM cost rollup? The sum of every BOM line's cost at assembly level: manufactured parts costed as their own mini-quotes, purchased parts at supplier price plus procurement handling, outsourced coatings as supplier lines. Done properly it amortizes setups over the total produced quantity, nests flat parts as one campaign, and applies margin and surcharges exactly once.

Why do welded assemblies get underquoted? Because BOMs list components and quotes follow BOMs — so the work between components goes unpriced. Joints, fit-up, clamping, turning heavy assemblies, grinding, straightening and re-establishing datums after distortion belong to no single part line. On tracked jobs, that in-between work is commonly 30–50% of assembly labour.

Should an assembly carry a different margin than a part? The margin target can be the same, but it must be applied once, at assembly level, on the full rolled-up cost. Compounding child-part margins under an assembly margin prices you out; skipping assembly margin because "the parts already carry it" leaves the interface work — often a third of the job — earning nothing.

From drawing package to itemized assembly quote

The reason assembly quotes get shortcut is not that estimators do not know the method — it is that the method is long. Reading a GA drawing, classifying twenty BOM lines, taking off cut lengths and weld metres, and pricing four child parts properly is an afternoon, and the RFQ wants an answer this week alongside every other RFQ in the inbox.

QuoteBuddy compresses the reading. It interprets the technical drawings in the package — title blocks, materials, geometry, the operations each part implies — and builds the itemized quote deterministically from your own rates: each make-part costed the same way every time, hardware and outsourced lines kept explicit, the assembly-level labour blocks visible as lines instead of vanishing into a lump. The estimator's judgement goes where it pays: the joint count, the distortion risk, the fixture decision — not into re-typing dimensions.

Start a 30-day trial and run a real assembly package through it, BOM and all. Compare the itemized rollup against the last weldment you quoted by feel — and see which blocks the feel had quietly set to zero.