Welding two steel tubes together at a right angle is a fundamental fabrication skill used in everything from garden gates to trailer frames. With the right preparation, a decent welder, and some patience, a capable DIYer can achieve a strong, clean joint — but good fit-up and safety practice are absolutely essential before you strike an arc.
Understanding the Joint Type
When joining two tubes at 90°, you're typically creating either a T-joint (one tube meeting the side of another) or an L-joint (two tube ends meeting at a corner). The most common in DIY projects is the T-joint, where the end of one tube (the branch) sits against the outside wall of another (the run).
For square or rectangular hollow section (RHS/SHS) tube, the branch end can be cut straight and simply butted against the run — this is the easiest approach. For circular hollow section (CHS/round tube), the branch end needs to be cope-cut (also called a saddle cut or fish-mouth cut) to match the curved profile of the run tube. Without this, you'll have large gaps that are very difficult to weld properly.
Tools and Materials You'll Need
- MIG welder (most practical for DIY tube work) or stick welder
- Angle grinder with cutting and grinding discs
- Steel square and tape measure
- Clamps, magnets, or a welding jig
- Marker or soapstone
- Wire brush and chipping hammer
- Appropriate welding wire or electrodes for mild steel
- PPE: auto-darkening welding helmet, leather gloves, welding jacket, steel-capped boots
For mild steel tube (the most common type in NZ hardware stores), ER70S-6 MIG wire or E6013 stick electrodes are standard choices. Always check the manufacturer's recommendations for your specific material thickness.
Step 1 — Mark and Cut Accurately
Accuracy at this stage saves a lot of grief later. Use a steel square to mark your cut lines clearly. For RHS/SHS, a straight 90° cut on the branch is all you need. For round tube, you'll need to mark and cut a saddle profile — this can be done freehand with an angle grinder, or you can use a cope-cutting template (printable online for common tube sizes) taped around the tube as a guide.
Cut carefully and take your time. A poor cut means poor fit-up, and poor fit-up is the number-one cause of weak welds.
Step 2 — Clean the Steel
Clean steel is critical. Grind or file the cut ends to remove burrs, mill scale, rust, paint, and any galvanising (see safety note below). Wipe down with acetone or a clean rag to remove oil or grease. Welding through contamination causes porosity and weak joints.
⚠️ Safety — Never weld galvanised steel without proper precautions. The zinc coating produces toxic fumes that can cause serious illness. If your tube is galvanised, grind the coating back at least 50 mm from the weld area and work in a very well-ventilated space. Refer to WorkSafe NZ for guidance on welding fume hazards.
Step 3 — Set Up and Tack Weld
Position the branch tube against the run tube and confirm the angle with a steel square or combination square. Use magnetic welding clamps or a purpose-built jig to hold everything firmly in place — even a small shift during tacking will throw your angle out.
Once you're happy with the position, place tack welds at opposing points around the joint (e.g., front, back, left, right for a round tube; each corner for square tube). Tacks should be small but solid — just enough to hold the joint. Check the angle again after each tack, as heat can cause slight movement.
Step 4 — Complete the Weld
With the joint tacked and confirmed square, weld the full joint in sections, alternating sides to minimise heat distortion. For tube walls up to about 3 mm, a single-pass fillet weld is usually sufficient. Thicker walls may need a root pass followed by a cap pass.
MIG Welding Tips for Tube
- Use a push or drag technique depending on your welder and position
- Keep a consistent travel speed — too slow and you'll burn through thin tube; too fast and you'll get poor fusion
- For the inside corners on RHS, slow down slightly to ensure good penetration
- Let each section cool slightly before moving to the next to reduce warping
Step 5 — Inspect and Finish
Once cool, chip any slag (if stick welding), then wire-brush the weld. Inspect for:
- Porosity (small holes — indicates contamination or shielding gas issues)
- Undercut (groove along the weld toe — indicates too much heat or speed)
- Incomplete fusion — the weld should blend smoothly into both tubes
Grind the weld smooth if appearance matters, then prime and paint to prevent rust. Resene and Dulux both offer suitable metal primers for NZ conditions.
When to Call a Professional
Some welding work in NZ is subject to specific standards and may require inspection or certification — particularly for structural, load-bearing, or safety-critical applications such as trailer frames, tow bars, vehicle roll cages, or anything covered by the NZ Building Code. If your project is structural, consult a certified welder or fabricator who can work to the relevant standards.
For trailer and tow bar work especially, incorrect welds can be a serious road safety issue. You can find qualified local welders and fabricators on tradedirectory.co.nz.
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