Bodywork Basics
How to perfectly replicate factory welds using DIY equipment
How to spot-weld your classic.
The majority of post-war classics are held together solely by resistance spot welds. It’s probably not the way you’d choose to stick a car together if you were building something in your shed or preparing a shell for rallying. If you’re massproducing cars, however, it’s an elegant method that’s strong, fast and lends itself perfectly to automation.
Resistance in an electrical circuit causes heat, which is why crusty wiring looms catch fire. But that resistance can be used to accurately form a molten nugget between sheets of metal, strong enough to build a motor industry. The spot welding process has dictated the way cars are put together. Panels are formed with flanges that can be accessed on both sides by the spot welder’s electrodes, rather than being butt or seam welded. The prominent external flanges of the Mini illustrate this.
You can closely replicate resistance spot welds using MIG or TIG, as demonstrated in previous features. However, for about £420 for a Telwin spot welding set like the one shown here, you can do the real thing in your shed. A secondhand set may also be a good option. Bear in mind, though, that the windings eventually deteriorate with use, so beware of very high-mileage sets.
Resistance spot welding requires no real skill – just the right kit, some attention to detail and careful appraisal of your test welds. With a little knowledge of the process, you’ll be able to adjust your set to ensure spot-welding perfection. Once you’ve used a resistance spot welder, you’ll wonder how you ever did without one.
1 How it works
The welding set is a low-voltage high-current transformer. Its low-resistance copper arms and electrodes focus high resistance between the metal sheets, creating a molten nugget. Weld time, current and electrode pressure are the key parameters.
2 Protective equipment
Resistance spot welding is the most benign of welding processes. That said, it’s possible for molten metal to be ejected. Fumes will be created, too, especially if weld-through primer is used. Wear flame retardant overalls, gloves and eye protection.
3 Spot welder anatomy
When the handles are squeezed, the electrodes close together against adjustable spring pressure. A micro-switch will energise the windings and current will flow via the interchangeable arms to the joint. Choose a welding set that has a timer.
4 Joint types
On your car you’ll typically find a combination of lap-type joints where panels overlap [A] and flange-type joints where panels meet at rightangles or where sills are joined underneath [B]. You can joddle lap-type joints to create a flat surface [C].
5 Adjust the set
If the clamp force is too high, the resistance between the metals will be lower and the weld will be too cold and weak. Too little force and it’ll be too hot – and the molten nugget can be ejected. Fine-tune force and time on test piece. Start at mid-range.
6 Make a test weld
Bring the electrodes squarely into contact with the joint and squeeze the handles together. The micro-switch will start the weld when a certain pressure has been reached and it’ll be ended by the timer. Release the handles and inspect the weld.
7 Test the spot
If molten metal was ejected, reduce the weld time and/or increase electrode pressure. The weld should look uniform. Peel apart the weld. The metal itself should tear before the nugget comes apart. Increase time and/or decrease pressure if not.
8 Size and frequency
For 0.7mm steel, the nugget diameter should be 4mm and spot spacing should be 15mm for two sheets (22mm for three); 0.9mm steel should have 4.3mm spots and 18mm (27mm) spacing; 1.2mm should have 5mm spots and 24mm (30mm) spacing.
9 Other materials
More exotic materials, such as aluminium, require much higher output sets. However, it’s possible to make strong spot welds in aluminium if you sandwich it between sheets of mild steel. Stainless steel can be spot welded directly.