The Welded Frame That Wound Up Twisted
We welded a steel conveyor frame from rectangular tubing. After welding, the frame looked straight. When we bolted the conveyor rails on, they didn’t line up — the frame had twisted. The welds pulled the tubing as they cooled. The frame was 6 m long and had about 3 mm of twist across the width. We had to shim the rails on the machine. The mistake was welding the frame and mounting the rails without stress-relieving or machining the mounting surfaces. We should have either stress-relieved (annealed) the weldment, or machined the rail-mounting surfaces flat after welding. The next frame we built, we added machining pads (spacer plates machined flat) for the rails. No more shimming.
Welded frame design and machining is about distortion control. Welds shrink and pull. Unless you plan for it, the frame won’t be straight. This article covers the approach.
Why Welds Distort
When you weld steel, the weld bead (and the heat-affected zone) cools and contracts. The contraction pulls the surrounding material. A weld on one side pulls the frame toward that side. A 6 m long frame, welded along one edge, will bow (curve) as it cools. The distortion is small (a few mm over a few meters), but it’s enough to misalign precision components (linear rails, conveyor rails).
Approach 1: Design for Distortion
If the frame doesn’t carry precision components (a guard frame, a support stand), the distortion is fine. Weld it square, check with a tape measure, and don’t worry about a few mm.
For a frame that carries precision (linear rails, a precision conveyor, a measuring station), you need flat, straight mounting surfaces. You can’t rely on the as-welded tubing.
Approach 2: Stress Relief
Stress relief (annealing) heats the weldment to about 600°C and cools it slowly. This relieves the weld stresses. The frame stops distorting after cooling. But annealing is expensive and adds lead time. For a long conveyor frame, it’s usually overkill.
A cheaper alternative: vibratory stress relief (the frame is vibrated to relieve stresses). Less effective than annealing but faster.
Approach 3: Machine the Mounting Surfaces
The standard approach for automation frames: weld the frame, then machine (mill) the mounting surfaces flat after welding. The rails, actuators, and precision components bolt to machined pads (not directly to the welded tubing).
- Machining pads: Add thicker plates (10–20 mm) where the rails mount. After welding, mill these plates flat (and parallel to each other). The rails bolt to the machined pads.
- One machined reference: Machine one surface as the datum. Everything else is referenced to it.
- Bolt-on sub-frames: For precision axes, bolt a machined aluminum extrusion (or a steel plate) onto the welded frame. The extrusion is straight (extrusions are straight by default). The welded frame only carries load, not precision.
The welded frame rule: Don’t mount precision components directly on welded tubing. Welded frames distort. Add machined pads (or a bolt-on extrusion sub-frame) where the rails go. The frame that twisted had rails bolted directly to welded tubing. Machine the pads after welding, and the rails line up.
Step 1: Welding Sequence (Minimize Distortion)
You can reduce distortion with the welding sequence:
- Skip welds: Don’t weld along one edge continuously. Weld in short, alternating beads on opposite sides. Balanced welding pulls evenly.
- Clamp during welding: Tack the frame on a flat table. Weld while clamped. The table holds it straight as it cools.
- Back-step welding: Weld in short segments, going backward (the beads overlap). Distributes the heat.
These reduce distortion but don’t eliminate it. For precision, still machine the pads.
Step 2: Tube Selection
The tubing itself affects the frame’s stiffness:
- Square/rectangular tube: Stiff in both axes. Standard for frames.
- Thin wall (3 mm): Lighter, but distorts more when welded.
- Thick wall (5–6 mm): Heavier, stiffer, less distortion. For precision frames.
For a precision frame, use thick-walled tube (5 mm). It stays straighter. For a general guard, thin wall (3 mm) is fine.
Step 3: Flatness Tolerance
Specify the flatness tolerance on the machined pads. For linear rails (article 61), the mounting surface must be flat within 0.05 mm over the rail length. For a conveyor frame, 0.5 mm is fine. For a measuring station, 0.02 mm.
Put the tolerance on the drawing. The machine shop machines to the print.
| Frame Type | Treatment | Flatness |
|---|---|---|
| Guard, support stand | Weld, square by eye | ±5 mm |
| Conveyor frame | Weld, clamp, add machined pads | 0.5 mm |
| Linear rail axis | Weld, machine rail pads (or bolt-on extrusion) | 0.05 mm |
| Measuring station | Weld, anneal, machine all surfaces | 0.02 mm |
A Welded Frame Checklist
- What mounts to the frame? (Precision rails? Or just guards?)
- Is the as-welded straightness good enough?
- Are there machined pads for precision components?
- Is a bolt-on extrusion sub-frame used (instead of welding rails on)?
- Welding sequence? (Alternating, clamped?)
- Tube wall thickness? (3 mm general, 5 mm precision?)
- Flatness tolerance specified on the drawing?
- Is stress relief needed? (Annealing or vibratory?)
- Are the mounting holes drilled after welding? (To align?)
- Is the frame checked flat on a surface plate after welding?
The Bottom Line
Welded frame design and machining separates load-bearing from precision. The frame that twisted had rails bolted directly to welded tubing. For precision, machine the mounting pads after welding (or bolt a straight extrusion onto the welded frame). Weld balanced and clamped to reduce distortion. Specify the flatness on the drawing. The frame that goes together without shimming wasn’t the most carefully welded — it had machined reference surfaces.
