The Brunsfield Center/Manufacturing Technologies/Welding/Spot Welding

About

A spot welder is a type of resistance welding machine used to join two or more metal surfaces at small points by applying pressure and passing a strong electrical current through the metal. The heat generated by the electrical resistance at the interface of the workpieces causes them to melt and fuse. Spot welding is commonly used in the automotive industry, metal fabrication, and manufacturing of appliances.

Safety Considerations

• Risk of burns from hot metal and electrodes.

• Electrical hazards due to high current.

• Eye protection needed for sparks.

• Proper ventilation required to avoid inhalation of fumes.

Principle of Operation

Spot welding operates on the principle of Resistive Heating. Two copper alloy electrodes are used to clamp the workpieces together. A high-current, low-voltage electric pulse is then passed through the metals, typically for a few milliseconds. Because the current is concentrated at the point of contact and the resistance is highest there, the material heats and melts at that spot, forming a weld nugget.

Components

A typical spot welder consists of:

• Control System: Regulates weld time, pressure, and current.

• Transformer: Steps down voltage and increases current.

• Electrodes: Copper alloy tips that conduct current and apply pressure.

• Tongs: Provide leverage and spacing for the workpieces.

• Cooling System: Often water-cooled to prevent overheating of electrodes.

Applications

Spot welders are widely used in:

• Automotive Manufacturing: For joining body panels and frame components.

• Battery Packs: To weld tabs on cylindrical and pouch-type battery cells.

• Sheet Metal Fabrication: In appliances, cabinets, and enclosures.

• Aerospace and Electronics: For precise, localized joining of components.

Advantages

• Fast and efficient for mass production.

• No need for filler material.

• Minimal heat-affected zone (HAZ).

• Consistent and repeatable weld quality with proper control.

Limitations

• Limited to thin sheet metals (typically less than 3 mm or 1/8” thick).

• Not suitable for non-conductive materials or thick components.

• Weld strength may vary with contamination or improper setup.

• Electrode wear requires regular maintenance.

• The MTC spot welder cannot weld aluminum since it requires higher current than the machine is rated for

Training and Operation

Spot welders are often rated as a Class 2 or 3 operation in machine shop environments like Brunsfield Center, meaning users require a brief training and oversight to safely perform welds. Training focuses on:

• PPE use (e.g., safety glasses, gloves)

• Setting weld time and current

• Electrode alignment

• Handling hot workpieces safely

To operate the spot welder, particular procedures must be followed to ensure safe and effective operation. Before use, make sure to have MIG welding gloves or pliers immediately available to handle the workpiece after welding and avoid burns.

• Turn the machine on, set the timer to the correct length of time
  • For mild/galvanized steel, set the timer between 0.75 and 1.00 seconds
  • For stainless steel, set the timer between 0.25 and 0.50 seconds
  • Setting the timer too short will result in a cold joint and lack of fusion. Setting the timer too long will deform the material and cause the weld cross section to be smaller. Both result in a weak weld
  • While some spot welders can weld aluminum, the MTC spot welder cannot. It does not have AC capability which aluminum requires to weld.

• Position the pieces to be welded between the tongs
  • Make sure the pieces are aligned correctly relative to each other
  • Make sure no part of the piece is touching any part of the tong other than the contact tip. This will split the current, causing the weld to not be as hot, which can cause lack of fusion
  • For pieces more than 4” across, use a free hand to support the piece and prevent tipping
  • Use a MIG glove to support the piece to avoid burns

• Hold the trigger for the full duration of the timer
  • Failing to do so can result in a cold weld and lack of fusion
  • The timer shuts the welder off automatically after it runs out; don’t worry about over-doing it

• Once the timer runs out, release the clamp and remove the workpiece
  • DO NOT TOUCH with bare hands
  • The piece will be hot, use pliers or gloves to handle until it cools
  • Running the piece under the sink will cool it quickly, but the rapid change in temperature may cause cracks in the weld. For any joint that will be under load, allow to cool slowly