Tack welding is the process of placing small, temporary welds at intervals along a joint to hold your workpieces in the correct position before you run the final, full weld. Done properly, tack welds prevent warping, keep your alignment true, and make the finished job far cleaner and stronger. Here's how to do it right.
What You'll Need
Before you start, gather your equipment:
- MIG, TIG, or stick welder suited to your material and thickness
- Welding helmet (auto-darkening is ideal)
- Welding gloves, leather apron or jacket, and closed-toe leather boots
- Wire brush and angle grinder for surface prep
- Clamps, magnets, or a welding jig to hold pieces in rough position
- Steel square or tape measure for checking alignment
- Chipping hammer and wire brush (for stick/MMA welding)
Safety First
Welding produces intense UV light, heat, sparks, and fumes. Always work in a well-ventilated area — outdoors or with extraction — to avoid inhaling metal fumes, which can be a serious health hazard. Check WorkSafe NZ for guidance on welding fume exposure limits and safe work practices. Never weld near flammable materials, LPG cylinders, or fuel lines. Keep a fire extinguisher within reach.
Step 1 — Prepare Your Metal
Clean metal is essential for a sound weld. Use an angle grinder or wire brush to remove mill scale, rust, paint, galvanising, or oil from the joint area. Contaminants cause porosity and weak welds. If you're working with galvanised steel, be aware that zinc fumes are toxic — grind the coating back well beyond the weld zone and ensure excellent ventilation.
Step 2 — Set Up and Align Your Pieces
Position your workpieces as accurately as possible before you strike an arc. Use:
- Clamps to hold pieces firmly together
- Magnetic welding squares to maintain 90° angles
- A welding jig or fixture for repeat work or complex assemblies
Double-check your measurements with a steel square and tape measure. Small errors at this stage become big problems after the full weld.
Step 3 — Set Your Welder Correctly
For tack welds, use the same settings as your full weld (or very close to them). A common beginner mistake is turning the heat down too low, which produces cold, brittle tacks that crack when stressed. As a general guide:
| Material Thickness | Process | Approximate Amperage |
|---|---|---|
| 1–2 mm sheet | MIG / TIG | 40–80 A |
| 3–5 mm flat bar | MIG / Stick | 80–130 A |
| 6–10 mm plate | MIG / Stick | 130–180 A |
Always check your welder's manual and run a test bead on scrap first.
Step 4 — Place Your Tack Welds
This is the core skill. Follow these principles:
- Start at the ends of the joint, then work towards the middle. This locks the alignment before the centre is fixed.
- Space tacks evenly — as a rough guide, place a tack every 50–150 mm depending on material thickness and joint length. Thinner material needs more frequent tacks to control warping.
- Keep tacks small — aim for 10–20 mm long. They need to be strong enough to hold, but small enough that the full weld can fuse through or over them cleanly.
- Alternate sides on long joints (e.g., left end, right end, middle-left, middle-right) to distribute heat and minimise distortion.
- Hold the torch or electrode at the correct angle and pause briefly — don't just flick the arc on and off. A proper tack weld should fuse fully into both pieces.
Checking Your Tacks
After each tack, inspect it. A good tack weld is smooth, slightly convex, and fully fused at the edges with no undercut or porosity. Tap it lightly with a chipping hammer — it should ring solid, not crack or pop off. If a tack fails, grind it out and redo it. Never leave a cracked or poorly fused tack in place.
Step 5 — Check Alignment Again Before the Full Weld
Once all tacks are in place, re-measure and re-square your assembly. Heat from tacking can shift pieces slightly. Make any adjustments now — it's far easier than correcting a fully welded joint. If a piece has moved, carefully grind out the relevant tack, reposition, and re-tack.
Controlling Distortion
Heat causes metal to expand and contract, which pulls joints out of alignment. To minimise distortion:
- Use backstep welding (welding short runs in the opposite direction to travel) during the full weld
- Clamp the assembly to a flat surface or strongback where possible
- Allow tacks to cool before placing adjacent ones
- On symmetrical assemblies, weld opposing sides alternately
When to Call a Professional
For structural welding — including anything that forms part of a building's structure, load-bearing frames, trailers, pressure vessels, or safety-critical components — you should engage a qualified, certified welder. In New Zealand, structural welding is governed by standards such as NZS 3404 (steel structures) and work on buildings may require a building consent and inspection. Check with your local council and refer to MBIE Building Performance for guidance on what requires consent. If you're unsure whether your project is structural or safety-critical, err on the side of caution. You can find qualified local welders on tradedirectory.co.nz.
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