Fixture-controlled welding for dimensional accuracy

A welded frame is a part that was accurate for a few seconds and then cooled. Between those two states the fixture decides how much of the accuracy survives. A fixture that locates well and clamps hard can still deliver a frame that pulls 3 mm on the diagonal, because clamping force and weld heat are not the same problem wearing the same name.

The useful way to think about fixture-controlled welding is in two layers. The fixture fixes the geometry before the arc starts, and it fixes nothing once the arc has started. Everything after the first tack is a question of heat, restraint and sequence. A supplier who treats both layers as one operation produces parts that pass in the fixture and fail at the customer.

The bending and welded assemblies hub collects the forming and tolerance material that decides what arrives at the fixture in the first place.

Six locators, six degrees of freedom, and no more

A rigid body has six degrees of freedom: translation in three axes and rotation about three axes. The 3-2-1 principle removes them with six contacts — three on the primary plane, two on the secondary, one on the tertiary. That is the whole theory, and it is enough to diagnose most fixture problems.

The failure mode is the seventh locator. A fourth pad on the primary plane, a clamp that pushes a panel down against a surface it does not naturally touch, or a pin that pulls a hole to nominal all add constraint the part cannot satisfy. On a 3 mm panel that is 0.8 mm out of flat, forcing it onto four pads stores elastic energy; when the clamps open, the panel returns part of that 0.8 mm and takes the welded assembly with it.

The same arithmetic applies to a hole. A frame hole held to ±0.2 mm on the drawing, located by a pin with 0.1 mm clearance and a pin position held to ±0.05 mm, gives a pre-weld worst case of 0.35 mm. Tighten the pin clearance to zero to "improve accuracy" and the part must be sprung onto the pin, which distorts the frame around the hole and moves the feature the pin was meant to protect.

What the fixture controls, and what it does not

Heat input is the number the fixture cannot hold

Weld shrinkage is not a mystery once the heat input is expressed in kJ per millimetre of weld:

HI = 60 × V × I / (1000 × v)

with V in volts, I in amps and v the travel speed in mm per minute. A MIG run at 24 V, 180 A and 400 mm/min gives 60 × 24 × 180 / (1000 × 400) = 0.65 kJ/mm.

That figure is the energy per unit length entering the part, and it decides how much of the surrounding steel is heated above the temperature at which it will contract. A robotic cell repeats 0.65 kJ/mm on every part of a batch. A manual operator whose travel speed drifts from 480 mm/min at the start of a shift to 320 mm/min at the end, with the same wire feed and voltage, moves the heat input from 0.54 to 0.81 kJ/mm — a change of half again, on the same drawing and the same fixture.

The consequence for a frame is proportional. A 1,000 mm steel section heated through 200 °C would expand 1000 × 12 × 10⁻⁶ × 200 = 2.4 mm if it were free. It is not free: the fixture and the rest of the frame restrain it, so the expansion becomes plastic compression near the weld, and on cooling that region pulls. The rest of the frame has to absorb the pull, and it absorbs it as bow, twist and a shorter diagonal.

Sequence and symmetry beat brute force

Three sequence rules do most of the work on a fabricated frame.

Weld from the centre outward. A run started at one end drags the whole length of contraction towards that end. Starting in the middle and working outwards lets the two halves pull against each other, so the net movement along the length cancels rather than accumulates.

Balance across the neutral axis. A stiffener welded on one face of a panel pulls that face and bows the panel towards the weld. At 3 mm thickness, one degree of angular distortion across a weld line 500 mm from the edge produces 500 × tan(1°) = 8.7 mm of bow. Put an equivalent weld on the opposite face at the same distance and the two angular distortions oppose each other; the panel stays flat and the cost is one more bead.

Backstep long seams. Instead of a single run, weld short lengths alternately in the reverse direction. Each segment starts in cool metal, so the peak temperature and the restrained length are both smaller. The seam takes longer, is no stronger, and moves less.

For a product where the paint finish is visible, the sequence also decides where the heat marks fall. A balanced sequence keeps the panel flat enough that the coating does not highlight the weld line, which is a quality issue long before it is a dimensional one.

The weld distortion article covers frame-specific countermeasures, and the fixture design guide covers the geometry of the fixture itself.

Release: the only measurement that describes the shipped part

A part measured in the fixture describes the process, not the product. The product is the frame that comes out when the clamps open, sits on a bench overnight and goes into a crate.

Release inspection therefore has three requirements. The part must be free, with no clamps and no pins. It must be supported at defined points, because a 2,000 mm frame on two end supports sags under its own weight and reads a bow that the steel does not have. And it must be measured against the datum the customer uses — usually a mounting face and a hole pattern, not the outer envelope.

For a rectangular frame the check that finds the most problems for the least time is the diagonal pair, but it has to be sized to the aspect ratio. On a 1,000 × 600 mm frame, the diagonal changes by only 1000 / 1166 = 0.86 mm for every millimetre the long side moves, against 600 / 1166 = 0.51 mm per millimetre of the short side. A tall, narrow frame is much more sensitive to a bow in the long dimension, and a long, shallow one is relatively deaf to it. Add a squareness check at the corner and the tape stops lying.

Post-weld tolerances belong on the drawing, not in a conversation. ISO 13920 provides general tolerances for welded constructions in four classes covering linear and angular dimensions, straightness, flatness and position, which is exactly the framework for a frame whose drawing would otherwise default to ISO 2768-1 — a standard written for a formed or machined part, not for a weldment that has just been through a thermal cycle.

Fixture wear, records and the checking fixture

A welding fixture is a tool, and tools wear. Locating pins burnish, pads fretting against a part under clamp load lose height, and clamps lose travel. None of this appears in a part inspection until the drift is large enough to fail something.

Three records keep it under control. The fixture drawing states the locator positions, the wear allowance and which items are replaceable. The fixture carries an identification number that appears on the first article report, so a part can be traced to the fixture that made it. And the checking fixture or the CMM program is validated against a measured part, not against the welding fixture — otherwise the acceptance gauge is comparing the process to itself.

Inspection and traceability for welded work runs on the same principle as the welding inspection and traceability route: the weld symbol, the procedure and the record are all identified by the same drawing revision. Weld symbols to ISO 2553 carry the size, type and length on the drawing, which is what makes a sequence instruction checkable rather than advisory.

How Ganesh Multitech runs fixture-controlled welding

Welding runs on three CLOOS robotic MIG cells with fixture-mounted, programmed paths, supported by more than 30 manual MIG/MAG, spot and SPM stations. The fixtures come from the in-house tool room, which also builds press tools, progressive dies and checking fixtures, so the welding fixture and the checking fixture are built from the same drawing and the same datum scheme rather than being bought from two sources.

That matters on the parts this article describes: elevator counterweight frames and car frames for TKE and Eros Elevators, genset base frames for Mahindra Powerol, overhead guards and chassis parts for Godrej & Boyce, and coach brackets and underframe fabrication for Indian Railways. Cutting is on the 12 kW fibre laser with a 40 ft × 16 ft bed, forming on the 200 T, 5-axis brake with a 4,450 mm bed, and finishing through seven-tank pretreatment and the 18 ft × 14 ft powder booth.

Ganesh has manufactured sheet metal since 1992 across five units in Mumbai (Vasai) and Nagpur under ISO 9001:2015 and ISO 14001:2015. First article inspection and PPAP are available, and the first article for a welded frame includes a released-state diagonal and squareness record, not an in-fixture check. The welded assemblies capability page carries the machine and fixture data, and the tooling capability covers how fixtures are designed and maintained.

What to specify with the assembly drawing

• The datum scheme: which face locates, which two holes orient, and which edge stops rotation.
• Weld symbols to ISO 2553 with size, length and any intermittent or staggered pattern.
• The weld sequence, or a note that the supplier is responsible for controlling distortion.
• Post-weld tolerances to ISO 13920, with the class named and the release state stated.
• Which holes are datum holes and must not move, and the post-weld callout on them.
• Whether the part is inspected free or restrained, and the support positions used.
• The finish sequence: blast and coat before or after the final fit check.
• Fixture ownership, if a customer-supplied fixture is to be used.

The repeatability article covers the production data side, and the supplier audit checklist is the place to ask how a fixture is maintained rather than only whether one exists.

When a fixture is the wrong investment

A fixture is the wrong answer when the batch is small and the geometry is simple. A one-off base frame can be tacked on a marked table, checked on a plate, and corrected while it is still hot; building a fixture for ten pieces usually costs more than the scrap it prevents.

It is also the wrong answer when the design is fighting the process. If a frame needs a ±0.5 mm hole position after welding at 3 mm thickness, no fixture will deliver it, because weld pull moves the surrounding steel by more than that. The fix belongs in the design — move the hole to a thicker section, add a machined pad welded before final drilling, or drill after welding.

And it is the wrong answer when the fixture is being used to correct a part that should have been straighter before it arrived. A fixture holds a good part in a good position; it does not make a bowed blank flat.

Send the assembly drawing with the datum scheme, the weld symbols and the release tolerances, or request a quotation for the assembly and the fixture together. The review will come back with the locator plan, the sequence, the heat input assumption and the post-weld tolerances that the thickness and geometry can actually hold.