Why Two Shops Quote the Same Weldment 40% Apart
A machined weldment is priced in two halves that rarely sit in the same part of the shop. The welding half is labour and consumables. The machining half is machine hours and workholding. Shops are strong in one or the other, so the same drawing comes back with quotes that look like they were for different parts.
Here is what actually sits inside those numbers.
Plate grade and thickness
Material is the easiest line to read and the one buyers overweight. What matters more is what the grade does to everything after it. High strength steel such as A514 or S690QL needs controlled preheat and interpass temperature. Abrasion resistant plate has heat input limits from the plate maker that, if ignored, remove the hardness you paid for at the joint. Stainless moves roughly one and a half times as much as carbon steel for the same weld, so the fixture has to be better.
Thickness drives weld volume, and weld volume drives arc time, consumables, distortion and the stress relief decision. A 10 mm increase in plate thickness on a heavily welded frame is not a 10% change.
Weld length and joint preparation
Add up the weld length on the drawing, then look at how many of those joints are prepared full penetration welds rather than fillets. A full penetration joint means a bevel cut, a root pass, fill passes and almost always an inspection method. One line on a drawing changing a fillet to a full penetration weld can add hours.
Weld quality level and inspection
Naming ISO 5817 level B on every joint is the most common way to make a weldment expensive by accident. Level B is a stringent acceptance level. Applied to the whole part, it slows welding, adds grinding and creates rework on joints where nobody cared about the result.
The same is true of inspection. Visual inspection of every weld is normal and cheap. Dye penetrant and magnetic particle are modest. Ultrasonic and radiographic testing bring a report, a technician and often a subcontractor, and they push the schedule. Mark which welds get which method and at what percentage, rather than writing “NDT per drawing” on the title block.
Stress relief
Thermal stress relief costs oven time, two crane moves and, if the shop has no oven that fits the part, a round trip on a truck. For carbon steel it typically runs at 580 to 650 degrees C with a soak of about an hour per 25 mm of thickness plus controlled heating and cooling.
It is still cheap compared with finish machining a frame that then relaxes and takes a 0.3 mm bow into the bore alignment. If the part is heavily welded and carries machined features that must hold position, budget for it.
Machining envelope and number of setups
This is where the quotes separate. Ask one question: does the part fit one machine in one setup?
Each time a 2 tonne weldment is re-fixtured, you pay for crane time, re-alignment, a fresh datum pickup and the risk of stacking tolerance across the two positions. A part that fits a horizontal boring mill table in one go can be several times cheaper to machine than the same part that has to be flipped and re-indicated twice, even though the cutting time is identical.
Flatness, parallelism and bore position
Geometric tolerance across a long part costs more than the same tolerance on a small one, because the error budget is fixed while the distance grows. Holding 0.05 mm flatness across 2.4 m means slower finishing cuts, measurement between passes and sometimes a second pass after the part has been allowed to relax.
Put the tight numbers only where the machine design needs them.
Coating and masking
Blast, prime, paint or galvanise, then mask every machined face and every thread. The masking is manual and it is charged as such. Hot dip galvanising deserves its own conversation, because the part goes into a zinc bath at around 450 degrees C and comes out having moved. If a galvanised weldment also has machined features, the sequence has to be agreed at quote time, not discovered later.
Weight and freight
Weldments are heavy and bulky and they do not stack. On a large frame the transport line can rival the machining line, which is why supplier region is a real technical question rather than a preference. A shop 200 km away with slightly higher machine rates often lands cheaper on the dock than a shop on the other side of an ocean.
What to do with this
Before you send a drawing out, check three things: that the weld quality level is applied selectively rather than globally, that the inspection method is marked per joint, and that the machined features share datums a fixture can reach. Those three edits move a weldment quote more than negotiating on the shop rate.
Frequently asked questions
Why do weldment quotes vary so much between shops?
Because each shop prices its own bottleneck. A shop with a large horizontal boring mill and a small welding bay prices the welding high. A shop with twenty welders and one old bed mill does the opposite. The same drawing can honestly come back 40% apart from two competent suppliers.
Is it cheaper to weld a frame or machine it from solid?
On anything larger than roughly a 300 mm cube, welding is almost always cheaper, because you are buying plate rather than paying to cut most of a block into chips. Below that, and wherever no weld is allowed, machining from solid usually wins.
Does asking for ISO 5817 level B cost much more than level C?
Yes, and often more than people expect. Level B tightens what is accepted on undercut, porosity and profile, which means slower welding, more grinding and more rework on the joints that fail. Specify level B where the joint is loaded and level C or D elsewhere.
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