Machining a welded assembly is one of the least forgiving jobs in contract manufacturing, because the blank that reaches the milling machine is not a piece of solid stock. It is a structure with internal stresses, a heat-affected zone around every weld, and dimensions that have already moved once during cooling. Whether a welded frame, bracket, or base plate holds a tight machined tolerance depends less on the cutting tools than on decisions made before and during welding, and on how clearly those decisions are written into the RFQ.
This article explains the risks of finish machining weldments and the information a buyer should supply: the weld sequence, the expected distortion, the stress relief decision, the machining datums, and the inspection condition in which measurements are taken.
Why a weldment is a different starting point
During welding, metal near the joint is heated quickly, melts with the filler, and cools inside a constrained geometry. Residual stresses form because regions expand and contract by different amounts while the surrounding structure resists that movement. Heat from welding causes localised expansion taken up by the molten metal or by part movement, and as the weldment cools, regions contract at different rates and leave stress behind.
Two consequences follow. The weldment is not stress-free when clamped onto the machine, and the stress level depends on process, heat input, joint design, and weld order. A machinist who removes a face can release stored stress, and the part can bow after the clamp is released.
Material choice amplifies the effect. Austenitic stainless steels are noted as especially susceptible to distortion because of high thermal expansion, while aluminium alloys bring hot-cracking risk that calls for controlled heat input.
Weld sequence: the variable most RFQs leave out
The weld sequence is the order, direction, and layering of individual passes. Each pass adds heat to a structure that is already partly restrained. A sequence that concentrates heat on one side, or completes one end of a frame before the other, tends to produce bow or twist; a balanced sequence that alternates sides distributes heat more evenly. Backstep welding, laying short sections against the overall direction of progression, is used for the same reason.
For finish machining the practical questions are whether the assembly is welded in a fixture or free, how much restraint is applied, whether the sequence is numbered on the drawing or left to the fabricator, and whether temporary braces stay on during machining. A drawing that shows weld symbols but no sequence note delegates these decisions to whoever builds the part — acceptable for a simple bracket, risky for an assembly with machined faces on several sides.
Distortion: what to expect and how to plan for it
Distortion is the change of shape produced by uneven heating and cooling and by stress release. In weldments it appears as angular distortion at fillet joints, bowing along long members, twist across a frame, and shrinkage that closes a gap dimensioned at design stage. It is also a recognised weld defect category alongside cracking, porosity, lack of fusion, and undercut, which is why acceptance criteria are usually written against a welding standard.
Three planning consequences follow. Machining stock must exist where distortion has moved a surface. Distortion corrected by clamping can reappear after machining, so a frame pulled flat with clamps and then milled can leave a face that is flat only while the clamps are fitted. And some distortion is better removed before machining, by straightening or a controlled heat operation, than by a heavy cut that simply follows the distorted surface. The RFQ should state the largest deviation the fabricator may deliver to the machining stage, or ask for one to be proposed. General tolerance tables for welded assemblies cover dimensions of the welded structure, not the flatness of a face machined afterwards; machined features need their own callouts.
The stress relief decision
Thermal stress relief heats the part below its lower critical temperature and cools it uniformly. At temperature the yield strength falls, regions carrying stresses above that reduced strength relax plastically, and the part is left with lower residual stress without the microstructure change that annealing or normalising would cause. It is an established industrial step for pressure vessels and air tanks, where welding stresses are known to be significant.
For a machined weldment there are two common sequences, and they are not equivalent. The first is weld, stress relieve, rough machine, stress relieve again if required, then finish machine; it removes most welding stress before precision cuts and suits thick, restrained assemblies with tight faces. The second is weld, rough machine, stress relieve, finish machine; it also releases roughing stress, but the rough cut is taken on a distorted blank, so allowance must cover it.
The decision is not automatic. Temperature must stay below the level at which the material loses required properties; for low-alloy steels it must not exceed the point associated with the required hardness or strength condition. For precipitation-hardening aluminium alloys a thermal cycle can affect temper and must be agreed with the material supplier. Distortion during stress relief is also possible for long, slender assemblies not properly supported in the furnace. An RFQ that says only “stress relieve after welding” leaves temperature, holding time, atmosphere, support method, and records undefined.
Machining datums
The machining datums are the features from which machined surfaces are located and measured. On a weldment the natural candidates are often weld-affected faces, mill-scale edges, or surfaces that have already moved. A scheme should locate the part the same way at machining and at inspection, and ideally reflect how the assembly mates in service.
Tolerancing practice treats a datum as a theoretically exact plane, line, point, or axis derived from an identified datum feature; the feature is physical and imperfect, the datum is not. That distinction matters because a welded plate face, before machining, is not a reliable datum feature. Practical options are to machine a datum first, cutting one face and one edge in a single setup and using them for later operations and inspection; to add process pads machined flat as datums; or to use fixture features as temporary datums for roughing only, with finished datums established after stress relief.
Whichever route is chosen, the callouts must be achievable: a datum that exists only after a finishing cut cannot locate that same cut. Inspection alignment should use the same datum reference frame as the machining fixture.
Inspection condition
The inspection condition is the state in which the part is measured: clamped or free, at what temperature, after what delay, and on what supports. For weldments it is often the most disputed line in a quality report. A face measured while the part is still bolted to a fixture describes the setup, not the delivered part; a frame measured on three points describes a different shape from the same frame bolted at its mounting holes.
Measurement practice distinguishes free-state from constrained or functional verification, and the drawing should say which applies. Critical flatness and parallelism are commonly verified free on a surface plate after clamps are released, while mounting-hole position is verified in a simulated assembly condition. Both can be required, but they answer different questions and should be reported separately. Timing also matters, since stress release is not always instantaneous, and coating thickness belongs in the acceptance limits if the part is later plated or painted; our article on plating allowance for CNC parts covers that adjustment.
What belongs in the RFQ
A supplier quoting a finish-machined weldment must estimate operations, fixture cost, furnace cost, and scrap risk, so quotation quality depends on the information provided. The minimum package includes an assembly drawing plus a separate machining drawing with machined surfaces identified, weld symbols and sizes with a sequence note, material specification for every component, the stress relief decision and its timing, datum features, inspection condition and acceptance criteria, and quantity with repeat pattern.
Our guide to CNC machining drawing requirements covers the drawing checklist, and the article on CNC fixture datum strategy explains how fixture location and datum choice interact.
Common failure patterns
The face that was flat on the machine. A frame is clamped flat, the mounting face is milled, and the part springs when released; the report looks good because it was taken in the fixture. The fix is free-state verification plus stress relief or a roughing step before finishing.
The hole that moved after welding. Holes drilled before a plate is welded to a frame are pulled out of position by shrinkage. The fix is to drill after welding, or to produce the holes in the final setup from a machined datum.
The datum that no longer exists. A machined pad used as a datum is cut away later or damaged in handling. The fix is a datum scheme that survives the process and is documented on the drawing. The same principle appears in planning for heat-treatment distortion in CNC machined parts: material moves when constraints are removed, and inspection must reflect the released state.
How to verify a supplier’s approach
Because weldment risk sits in process decisions, the useful selection questions are process questions. Ask for the proposed weld sequence and whether it is written down, whether stress relief is performed in-house or subcontracted and what records are kept, how datums are established, and how the part is supported during inspection.
A first article inspection report is especially informative here, because it shows whether the supplier measured the part as delivered or as clamped. Our first article inspection checklist and the guide to reading a CMM inspection report describe what to look for in alignment and reported uncertainty.
Bottom line
Finish machining weldments is a sequence problem before it is a cutting problem. The weld sequence sets how much stress is locked into the assembly. Distortion sets how much material must be left and whether straightening is needed. The stress relief decision decides whether the machined part stays where it was cut. Machining datums decide whether the part is located the way it functions, and the inspection condition decides whether the delivered part is actually being measured.
None of these can be inferred reliably from a drawing that shows only final geometry. An RFQ that states all five explicitly gives both shops a common basis for quoting and the buyer a basis for acceptance. Where one is genuinely open, the RFQ should say so and ask for a proposal rather than leave the shop to choose silently.
Sources: technical summaries of welding residual stress, weldability, and heat treatment, and ASME Y14.5 datum definitions, as listed with this article. Standard numbers and tolerance tables should be checked against the current edition of the relevant standard before use in a purchase specification.
