Building Prototypes Dan Gelbart part 5 of 18 Spot Welding

Spot Welding for Prototyping

Dan Gelbart demonstrates how to use a spot welder on sheet metal, rods, and captive hardware. He also explains how to choose settings and make custom electrodes.

Setting up and testing a weld

• Adjust the weld time to suit the material and its thickness. Some machines also allow pressure adjustment; Gelbart recommends leaving pressure at maximum or ignoring that control.
• When working with a new material or thickness, make several test welds at different time settings. For sheet metal, he demonstrates testing 0.7 mm (about 22-gauge) material at two, three, and four cycles.
• Use a destructive test to judge the result: the weld should stay intact while the surrounding metal tears. If the weld opens, increase the setting. A chart of settings for common materials and thicknesses can save time when repeating tests.
• The welder shown works well on mild steel and stainless steel, but not on non-ferrous metals such as copper. Gelbart explains that resistance heating works best on materials that conduct electricity relatively poorly. He recommends keeping separate settings charts for stainless and mild steel.

Choosing current and time

If both current and time are adjustable, set the current high and adjust the time first. Shorter, higher-current welds can reduce heat spreading into adjacent metal.

Lower the current only if the shortest time setting is still too strong or the available time steps are too coarse—for example, when welding very thin material.

Gelbart says the demonstrated 15 kVA machine is near the minimum useful size. Its approximate limit is 3 mm steel to 3 mm steel; a 25 kVA unit can handle somewhat more. Spot welding is intended for relatively thin materials, while TIG welding becomes a practical alternative for thicker stock.

Temporary tacks, permanent welds, and removal

Lower settings can make temporary welds that hold parts in position but can later be twisted apart. Once the alignment is right, increase the setting to make permanent welds.

A single permanent weld may sometimes be twisted apart, but multiple welds should not be pried apart because the metal is likely to tear. Gelbart recommends drilling out the spot, typically with a 3–4 mm drill. The weld also serves as a center mark.

Other materials and shapes

Spot welders can be especially effective on rods because their small contact area creates high current density and limits heat loss. Test rod welds by twisting the rods: the rod should shear before the weld opens.

The machine can weld rods up to roughly 3/8 inch easily and may handle 1/2-inch rod. Gelbart demonstrates smaller rods and notes that the setting may need to be increased if they can still be twisted apart.

Welding captive hardware and useful features

Gelbart demonstrates spot welding the following hardware and components:

• Studs and screws: Use hollow electrodes that hold the hardware in position. Some holders use a spring-loaded ball to keep a stud from falling out. A small locating hole or mark helps position it. Test the weld by breaking the stud; the weld should remain intact.
• Spot-welding nuts: These have projections for welding and a pilot shoulder that fits a matching hole for alignment. Because they have a larger contact area, they may need a higher setting. Welding can leave metal in the threads, so a tap may be needed to clean them.
• Alternative threaded attachments: Weld a washer or another piece of metal to the back of thin sheet, then tap it; or use a threaded insert installed with a special tool.
• Threaded standoffs, hinges, latches, and closures: These can be welded to sheet metal to simplify assembly. For a hinge, align it so it has room to open and close before welding.
• Balls: Custom electrodes can hold balls for welding them to rods or flat surfaces. Welded balls can serve as pivots, adjustment points, or parts of kinematic mounts. Plan the setup so the part can be removed from the electrode after welding.

Making inconspicuous welds and joining difficult materials

For a weld that should leave a flat exterior surface, Gelbart recommends a special electrode with a shallow dimple at the contact point. The dimple accommodates the small amount the metal pulls inward as it cools, leaving less to sand than a flat electrode.

Some materials that do not spot-weld readily can be trapped between stainless-steel layers and welded through the stainless. Gelbart demonstrates this approach with materials such as Nitinol, phosphor bronze, and beryllium copper.

When trapping a strip or wire, bend its ends with a gradual radius rather than an abrupt transition, which Gelbart says would make the joint weaker.

Making custom electrodes

Electrodes are made from copper bars or rods, which are easy to bend into custom shapes. Copper can be annealed if it becomes difficult to bend.

For longer-lasting tips, use inserts made from copper-chrome or copper-tungsten alloy rather than bare copper, which can wear or deform. Gelbart describes making a tapered hole in the copper body, then pressing in a tapered tip.

He also recommends making specialty electrodes as needed, such as offset or extended electrodes, electrodes for narrow spaces, and fixtures for welding parts at a particular angle.

To avoid marking the part being welded, use a much larger contact area on that side than on the other electrode. The larger area helps prevent that surface from heating significantly.

Safety and additional uses

• Wear goggles or suitable glasses, and do not wear sandals: sparks and molten metal can cause injury.
• Although the demonstrated machine operates at low voltage, Gelbart warns that it carries very high current and can exert strong magnetic forces on nearby metal objects.
• A spot welder can make watertight enclosures by spreading a very thin layer of RTV silicone between the pieces and welding at close intervals. The weld pressure displaces the silicone and seals the joint.
• It can also join scrap material edge-to-edge when preparing stock for a water jet cutter. The weld may not be strong, but it can create a smooth, flush joint suitable for holding the material.

Speakers and sources featured

• Dan Gelbart — presenter and instructor.
• Unidentified audience member or off-camera participant — asks whether to adjust current or time.
• Resources mentioned: McMaster-Carr is named as a supplier of some welding materials and hardware. Textbooks are also referred to generally but are not identified.