Complete guide
Machined Weldments
A machined weldment is a structure welded from plate, tube or section, then machined where a tolerance has to hold. The welding makes the shape. The machining cuts the bores, faces and rails the rest of the machine is located from.
Almost everything that goes wrong with one happens in the gap between those two operations, which is why this guide spends most of its time there.
We are a matching service, not a fabrication shop. We own no machines and hold no welding certificates.
The eight steps you are buying
- Cut and preparePlate and tube cut to size, bevels prepared, holes left undersize where they will be bored later.
- Fit and tackParts set in a fixture and tacked. Fit-up decides the distortion you fight for the rest of the job.
- WeldMIG, flux cored, TIG or submerged arc, run to a written procedure with a defined sequence.
- Inspect the weldsVisual as standard, then dye penetrant, magnetic particle, ultrasonic or radiographic where the drawing calls for it.
- Stress relieveThermal or vibratory, before machining, so the frame moves in the oven instead of on the machine.
- MachineBores, mounting faces, rails and threads cut on a bed mill, horizontal boring mill or gantry, from datums that exist on the drawing.
- Finish and checkBlast, prime or paint with machined faces masked, then a dimensional report against the drawing revision.
When a weldment is the right answer
The choice is usually settled by size and quantity before anyone looks at a machine rate.
Machined weldment
Plate and tube welded into shape, then machined where it matters.
- Best for
- Large, stiff, low to medium volume parts
- Tooling
- Fixtures only, no pattern and no die
- First part
- Weeks
- Watch for
- Distortion, residual stress, machining stock
Casting
Poured near net shape, then machined on the critical faces.
- Best for
- Repeating volume, complex organic shapes
- Tooling
- Pattern or die, paid before the first part
- First part
- Months
- Watch for
- Porosity found only after machining opens it
Machined from solid
Cut out of one block or one thick plate.
- Best for
- Small parts, very tight tolerance, no weld allowed
- Tooling
- None beyond workholding
- First part
- Days
- Watch for
- Material cost and cycle time on anything large
The full argument, including the machine base case where damping matters as much as stiffness, is in weldment, casting or machined from solid.
The one thing that catches everybody
Welding shrinks metal. Every joint pulls as it cools, and a frame that was square on the fit-up table is not square when it comes off. That part is normal and shops design around it.
What catches people is the second movement. The stress locked into the structure stays hidden until a cutter removes the material holding it in balance. The frame measured correct, then a face was opened up and it moved.
That is why a machined weldment is not a welded part that then gets machined. It is a sequence: weld, inspect, stress relieve, machine, coat. Move stress relief after machining and you get a part that measured right at the shop and measures wrong on your floor.
- Leave machining stockA common starting point is 3 to 6 mm on faces and about 3 mm on a bore radius, more on long or flexible sections. Work out a starting range.
- Split the tolerancesAn ISO 13920 class on the welded structure, a normal drawing tolerance on the machined features. One general tolerance across both gets ignored.
- Put datums on machined facesA datum on an as-welded surface moves with the distortion, and every feature referenced to it moves with it.
- Say when each number appliesFlat within 0.1 mm after stress relief and finish machining. Otherwise nobody knows when it is checked.
Where to go deeper
Twenty guides, grouped by the decision you are making.
What it costs
Eight lines decide the price, and two shops pricing their own bottleneck can land 40 per cent apart on the same drawing.
- Why Two Shops Quote the Same Weldment 40% ApartThe eight lines that decide what a machined weldment costs: plate grade, weld length, quality level, inspection, stress relief, setups, geometric tolerance and freight.
- A36, S355 or A514: What the Grade Does to Your Weldment PriceHow structural steel grade changes welding, preheat, distortion and machining on a weldment, and when the higher strength plate is worth paying for.
- Why One Setup Is Cheaper Than Three on a Two Tonne FrameHow machine envelope, crane capacity and the number of setups drive the machining cost and the tolerance risk on a large weldment, and how to design for fewer setups.
Getting the tolerances right
The part you machine is not the part you drew. Allowance, datums and two separate tolerance systems on one drawing.
- How Much Stock to Leave on a Weldment Before You Machine ItHow much machining allowance to leave on a welded frame, how to split weldment and machining tolerances on the drawing and where to put the datums.
- Putting Datums and Geometric Tolerances on a Weldment That MovesHow to apply datums, flatness, position and parallelism to a machined weldment, why welded and machined features need separate tolerance systems and where ISO 13920 fits.
- Which Weld Quality Level to Put on Your DrawingWhat ISO 5817 quality levels B, C and D actually accept, how the level changes welding cost and rework, and how to apply different levels to different joints.
How it is actually made
Distortion, fixturing, stress relief, inspection and the material specific behaviour that changes all of them.
- Why Your Frame Came Out Bowed, and What a Shop Does About ItThe six causes of weld distortion in large weldments and the controls fabricators use: joint design, weld sequence, heat input, fixturing, prebending and balanced welding.
- When a Welded Frame Needs Stress Relief, and When It Does NotThermal and vibratory stress relief for weldments: when residual stress will move your machined faces, what a PWHT cycle involves and where it belongs in the sequence.
- Which Weld Inspection Method to Specify, and on Which WeldsVisual, dye penetrant, magnetic particle, ultrasonic and radiographic testing on weldments: what each method finds, what it misses and what it adds to the quote.
Choosing who makes it
Machine travels, crane capacity and whether welding and machining happen in the same building.
- The Questions That Separate a Weldment Shop From a Welding ShopA practical assessment for machined weldment suppliers: machine travels, crane capacity, stress relief, inspection, qualifications and the questions that reveal real capability.
- AWS D1.1, ISO 3834 and EN 1090 Explained for People Who Buy WeldmentsWhat AWS D1.1, ISO 3834 and EN 1090 each cover, which one applies to your part and market, and what to ask a supplier to show you for each.
- What a WPS Is, and Why You Should Ask to See OneThe difference between a WPS, a PQR and a welder qualification record, what a qualification range covers and the three questions to ask a weldment supplier.
Questions buyers ask
What is a machined weldment?
A structure built from plate, tube or section, welded together, then machined where a tolerance has to hold. The welding makes the shape. The machining cuts the bores, mounting faces and rails that the rest of the machine is located from. Between the two there is usually a stress relief cycle, because welding leaves residual stress that moves the part as soon as a cutter opens it up.
Why not just machine the part from solid?
Above roughly a 300 mm cube it stops making sense. You buy a block, pay to turn most of it into chips and pay again for the machine hours doing the removing. A weldment buys plate at plate prices and only cuts where a cutter is needed.
What tolerance can a machined weldment hold?
The welded structure and the machined features hold different tolerances and the drawing has to say which applies where. The structure is normally held to a general fabrication class such as ISO 13920. The machined features carry a normal drawing tolerance. Tight geometry on machined faces is achievable, and it holds only if the part was stress relieved before finish machining.
How long does a machined weldment take?
For a mid sized frame, plan in weeks rather than days. Material procurement, cutting, fit-up, welding, inspection, stress relief, machining, coating and final inspection are mostly sequential, and two or three of those steps often happen in somebody else's building. Ask any supplier for the schedule broken down by stage rather than a single number.
Do you manufacture machined weldments?
No. We are a matching service, not a fabrication shop and not a machine shop. We own no welding bay, no machines and no oven, and we hold no welding certification. We read your drawing and connect you with up to 3 shops that weld, stress relieve and machine parts like it themselves. They quote you directly.
What do I need to send to get a quote?
A STEP file plus a PDF drawing with weld symbols, the material grade, the weld quality level, any inspection requirement, which features are machined and to what tolerance, the finish and the quantity. A weldment quoted from a model with no weld symbols gets re-quoted later, usually higher.
Have a drawing?
We read it, tell you what the part actually needs, and match it with up to 3 shops that weld and machine it in house. Free for buyers, reply within 24 business hours.