Welded assembly tolerances that hold in production

A 2,000 mm × 600 mm elevator frame leaves the weld shop with a 9 mm diagonal difference. Every individual part is inside its drawing tolerance. The welds are sound and pass visual inspection. The frame still will not close onto the guide rails without a fitter pulling one corner with a sling, and on site it is reported as a supplier quality failure.

Welded assembly tolerances fail in that gap between part tolerance and assembly tolerance. Each component can be correct while the sum of part error, fixture location and weld shrinkage exceeds the callout. The fix is not a stricter part tolerance; it is knowing which of the three dominates and controlling that one.

The bending and welded assemblies hub collects the distortion, fixture and inspection guides that sit around this topic.

What a welded assembly tolerance is made of

An assembly dimension is the vector sum of four contributions. The parts carry their own tolerance from cutting, forming and any machining. The fixture locates those parts with its own clearance and wear. The weld adds or removes metal as it shrinks, and the sequence decides where the movement lands. Coating adds a film on every face.

Those contributions add as a stack, and the stack is what the drawing should be written against. If the parts are held to ±0.5 mm, the fixture to ±0.3 mm and the weld shrink to ±0.8 mm, the root-sum-square is 0.99 mm and the worst case is 1.6 mm. A ±1.0 mm callout is therefore achievable with discipline and marginal without it, while ±0.5 mm is not achievable at all by welding alone.

That arithmetic is the decision rule. When the callout is looser than the RSS of the three controlled errors, weld it. When it is tighter, weld it oversize and machine the interface afterwards, or move the tolerance to a feature that is set after welding, such as a drilled or reamed hole.

The values a steel frame can hold

The diagonal is the single most useful check on a rectangular frame, because it catches both squareness and length error in one measurement. Four millimetres over 2,000 mm corresponds to about 0.11° of skew, which most bolted assemblies absorb. Eight millimetres is 0.23° and usually does not, because the hole clearance in a 10 mm bolt is around 1 mm and the skew consumes it in one corner.

Flatness is the second. A welded panel that measures 3 mm per metre is within a normal enclosure callout, but a panel that has been stitch-welded to a frame will take the frame's shape, and a frame with a 4 mm bow will show as a 4 mm bow in the panel. Check the frame's straightness before the panel goes on, not after.

Where ISO 13920 fits, and what it does not cover

ISO 13920 sets general tolerances for welded constructions. It covers dimensions for lengths and angles, and shape and position, in four classes from A to D, with class A the tightest. It exists so that a drawing does not need a tolerance on every welded dimension.

The scope matters. ISO 13920 applies to welded fabrications and to the welds in them; it does not cover machined features, and it does not overrule a specific tolerance on the drawing. Where a frame is welded and then machined, the machined features follow ISO 2768-1 or an explicit callout, and the welded features follow ISO 13920. Mixing the two on one drawing without saying which applies where is the most common source of a first-article argument.

The classes are a starting point, not a guarantee. A class A callout on a 3,000 mm frame asks for accuracy that depends on the fixture and the sequence, so the drawing should also state the sequence assumption or accept that the supplier will propose one. Weld symbols follow ISO 2553, and the symbol should carry the size and the finish, not only the type.

The shrink arithmetic, worked

Take a frame 2,000 mm × 600 mm in 3 mm CRCA with four corner fillet welds and eight stiffener welds. Each fillet pulls about 0.6 mm across the joint as it cools. If all twelve ran in the same direction the frame would lose roughly 7 mm on one side and bow by a comparable amount.

Two controls change the outcome. First, balance the sequence: weld opposite corners in the same pass, then the next pair, so the pulls cancel in pairs. That halves the effective movement to about 3.5 mm in total. Second, pre-set the fixture and cut the closing parts oversize by 1.5 mm per side, so the frame closes to nominal instead of shrinking away from it.

The third control is heat input. A 3 mm fillet at 0.8 kJ/mm puts less heat into the joint than a 5 mm fillet at 1.4 kJ/mm, and the smaller weld shrinks less because there is less molten metal. Specify the throat or leg you need for the load, not a larger weld for comfort. The frame distortion guide sets out the sequence patterns in detail.

Datum strategy: locate on the interface

A frame has three candidate datums: the outer profile, the mounting face, and the hole pattern. Locating on the outer profile is the easiest and the least useful, because the profile is formed from a flat blank whose edge tolerance is the loosest on the part. Locating on the mounting face and the hole pattern costs more fixture work and matches what the customer measures.

The rule is to fixture on the surface that touches the next assembly. On an elevator counterweight frame that is the guide shoe mounting face; on a genset base frame it is the engine foot pads; on a rack it is the 19-inch mounting rails. Everything else is reference geometry.

That choice also sets the inspection plan. Measure the assembly against the same locating surfaces the customer uses, record the diagonal and the hole pattern, and state whether coating goes on before or after the final fit check. Measuring from a convenient edge produces a passing report and a failing assembly. The fixture control guide covers pin placement and clamp direction.

The bolt-clearance budget is the quickest way to decide the callout. An M8 bolt in a 9 mm clearance hole leaves 1 mm of total float, or 0.5 mm per side. Two parts each carrying ±0.5 mm of hole position can therefore just bolt together, and any additional error pushes the joint into force-fit. The same M8 in a 10 mm hole leaves 2 mm, which absorbs ±1 mm per part and is why rack and canopy frames are usually drawn with the larger clearance. Set the tolerance from the joint, not from habit.

Coating has to be inside that budget too. Powder at 70 µm adds 0.07 mm per coated face, so a 9 mm hole becomes 8.86 mm and a 0.1 mm designed clearance becomes an interference. Either mask the hole, specify a clearance allowance of 0.15 mm per side, or ream after coating. The dimension report will pass in every case, which is why coating clearance is the error most often found by the fitter rather than by quality.

Welded assembly tolerances at the frame, not the part

Three measurements do most of the work on a welded frame: diagonal difference, flatness against a surface plate or straight edge, and hole position from the datum. A tape measure and a digital protractor are enough for the first two to a working accuracy of about 0.5 mm; a CMM or a portable arm is needed for the third when the callout is ±0.5 mm.

Record the measurements against the assembly serial and the welding cell or station. That linkage is what turns a site complaint into a process change, because it shows whether the failures cluster on one fixture, one shift or one coil. The welding inspection guide covers the record set for a repeat programme.

Destructive or semi-destructive checks belong on the weld, not the geometry. A fillet weld that is undersized will distort less and pass the diagonal check, so geometry inspection alone can reward the wrong outcome. Weld size, throat and any required penetration have to be checked separately.

How Ganesh holds welded assembly tolerance

Repeat welded assemblies run on three CLOOS robotic MIG cells, with fixtures designed and built in-house. The robot matters for tolerance because it repeats the weld sequence and the heat input on every part; on a manual station the same frame is welded by an operator whose travel speed varies through a shift, and the shrink varies with it. More than 30 manual MIG/MAG, spot and special-purpose stations handle the larger frames and the low-volume work where a fixture would not pay back.

Fixtures are located on the assembly interface and the hole pattern, with replaceable pins and pads, and the fixture is checked at the start of a production run. Frames then go through seven-tank pretreatment and the 18 ft × 14 ft powder booth, with masking on any face that has to stay bare for an earth connection or a sliding fit.

Customer work in this shape includes elevator frames for TKE and Eros Elevators and base frames and canopy structures for Mahindra Powerol. The welded assemblies capability sets out the cell and station range.

Ganesh has manufactured sheet metal since 1992 across five units in Mumbai (Vasai) and Nagpur, with ISO 9001:2015 and ISO 14001:2015 certification. PPAP and first article inspection are available, and a first article for a welded frame reports the diagonal, flatness, squareness and hole position against the stated datums.

What to put on the drawing

• General tolerance class to ISO 13920 and, separately, the class or standard for any machined feature.
• The datum faces, marked, and which one is primary.
• Diagonal or squareness callout with the measuring method, not only the value.
• Hole positions referenced to the datum after welding, with the drill-after-weld note where it applies.
• Weld symbols to ISO 2553 with size, length and any intermittent pattern.
• The sequence assumption, or a note that the supplier proposes one for approval.
• Coating system, DFT and the faces that must be masked.
• The assembly the frame fits into, so the interface can be checked in context.

The RFQ checklist lists the fields a welded frame quotation needs before the tolerance stack can be assessed.

When the callout should change instead

Some tolerances should not be met by welding at all. A ±0.3 mm position on a bearing bore, a 0.1 mm flatness on a machined pad, or a parallel face that carries a gearbox all belong to a machining operation after welding, and the drawing should say so.

The other case is a callout tighter than the assembly needs. A ±1 mm frame tolerance that is copied from a machined part drawing adds fixture cost and inspection time for no benefit. If the mating part has 2 mm of bolt clearance, ±2 mm is the honest number, and paying for ±1 mm is paying for something the assembly cannot use.

The third case is a frame that is too flexible to hold any tolerance. A 2,000 mm frame in 1.5 mm steel will move when it is lifted, and the tolerance is lost in handling rather than in welding. Thicker material, a gusset or a temporary brace is cheaper than a fixture that fights the part.

Send the frame drawing with the weld symbols, the datums and the dimension that fails at assembly. The engineering team will return the fixture datum plan, the weld sequence and the tolerance stack, with the achievable value stated against each callout.