Stainless chair weld straightening: distortion, jigging, correction limits and the tolerances a frame has to meet
A stainless frame that leaves the jig 3 mm out of square will rock, twist a stack and make a polished finish look wrong from three metres away. Distortion is not a defect to be corrected afterwards; it is a process number to be set beforehand. These are the inputs, the measurements and the correction rules we hold on every stainless run.
Published by JPChairs · Chair manufacturer in Foshan, China · 2026-11-02
Why austenitic stainless moves more than mild steel
Two material numbers explain most of it. 304 conducts heat at about 16 W/m K against roughly 50 for mild steel, so the heat from a TIG arc stays near the weld instead of spreading down the tube. It also expands more, 17.3 micrometres per metre per degree against about 12. On a 1.5 mm wall tube that combination gives 1.5 to 3 mm of leg splay per 400 mm of length on an unjigged frame, roughly double what the same joint gives in mild steel.
Heat input drives the rest. We weld 1.2 to 1.5 mm wall tube at 0.30 to 0.45 kJ/mm, pulsed where the joint allows. Past 0.6 kJ/mm on a thin tube the distortion stops being predictable, and unpredictable distortion is what turns a cheap correction into a re-weld.
Jig, tack sequence and 1.5 degrees of pre-spring
- Fixture: four clamps on a steel jig, contact on the tube not the joint, so the frame can pull and still be held square.
- Tacking: four tacks at 90 degrees, 5 to 8 mm long, laid at the same heat as the final pass. Cold tacks crack and pull the joint open.
- Sequence: skip or back-step, alternating across the frame rather than working clockwise around it. Around the frame is what builds twist.
- Pre-spring: legs set 1.5 degrees inboard in the fixture. The weld pulls them back to square.
With those four in place a 450 mm seat frame comes out within 1 mm of nominal instead of 3, and the correction bench stays empty.
The five numbers measured on every frame
- Seat height: plus or minus 2 mm at the front edge.
- Leg levelness: rock of 1.0 mm or less on a granite surface plate.
- Squareness: front to back within 2 mm over 450 mm.
- Diagonal difference: 2 mm over a 500 mm frame, the one reading that catches a twist before the chair is finished.
- Backrest tilt: plus or minus 1 degree, which is 7 mm over a 400 mm post.
The frame goes on the plate before it goes to finishing, not after. A polished frame that rocks cannot be corrected without cutting the finish back and re-passivating the joint.
Correction also costs time, which is the real argument for jigging properly. A cold set takes 6 to 10 minutes including the re-measure, a re-jig and re-weld takes 25 to 40 minutes, and a second pass through finishing adds another 15. On a 300-frame week, moving from 3 mm of distortion to under 1 mm is worth about 20 hours of bench time.
Correction: cold setting, two attempts, then re-jig
Up to 3 mm over 400 mm we correct cold in a three-point press. Austenitic stainless springs back 20 to 30 per cent, so the ram travels about 1.5 times the measured error and the frame is re-measured after it releases.
Two corrections per joint is the limit. A third work-hardens the heat-affected zone, the tube starts to crack at the toe and the frame is cheaper to re-jig and re-weld than to keep bending.
Heat straightening is not used on 304. Between 500 and 800 C chromium carbides precipitate at the grain boundaries and the joint loses corrosion resistance exactly where a chair is wiped down every day. Any frame that has been heated past 300 C goes back through passivation before finishing; below that threshold it does not need to.
Tolerance stack, and what stacking does to it
Four legs each within 1 mm of level give 2 mm of worst-case rock at the frame, which adjustable glides take up within their 5 mm of travel. Add a nylon glide that compresses 0.3 mm under 100 kg and the floor evens out the rest.
Carton pressure adds one more term. A 5-ply carton holding four frames compresses 4 to 6 mm over a 30-day voyage, and when the frames are packed face to face with a single divider sheet the outer two carry the load and arrive 1 to 2 mm out. Two dividers and a 10 mm foam pad between the pairs keep the readings we took at the bench.
Stacking multiplies it. Ten frames each 2 mm out of square put up to 20 mm of lean at the top of a stack, which is why 10 high is the practical limit for a stainless side chair and why the stack bumpers are set to contact 15 mm in from the tube centreline rather than on it.
What to write into the order
- Wall thickness and heat input range: 1.2 to 1.5 mm tube at 0.30 to 0.45 kJ/mm.
- Jigged weld, four tacks minimum, skip sequence, 1.5 degrees of pre-spring.
- Tolerances: seat height plus or minus 2 mm, leg levelness 1.0 mm, diagonals within 2 mm over 500 mm, backrest plus or minus 1 degree.
- Correction policy: cold setting only, two attempts per joint, re-jig beyond that, full re-passivation if any frame is heated past 300 C.
- Measurement sheet: five frames per 200 on a surface plate, readings packed with the order.
The tolerances above are the ones we hold on our stainless steel chair frame tolerances, and the same readings are what a buyer re-checks in our stainless weld inspection criteria before shipment.
