Both spot welding and projection welding are the forms of resistance welding, but they differ significantly in their heat generation, welding position control, and typical applications.

Spot Welding                

- Principle: Two flat sheets are clamped between copper electrodes. Current flows through the contact area to form a weld nugget.

- Features: The welding position is determined by electrode alignment. Spot welding produces joints one at a time. It is best suited for flat, thin sheet metal. 

- Applications: Automotive body panels, home appliance casings, and thin structural parts.

- Pros & Cons: The equipment and jigs are simple and low-cost with high flexibility. However, accuracy depends entirely on alignment, electrode wear is higher, and there is limited control when attempting multiple points.

Projection Welding

- Principle: A projection is pre-formed on one workpiece. Current concentrates through this point to generate heat and form the weld.

- Features: The location is defined by the projection design, allowing for multiple points to be welded simultaneously.

- Applications: Nut and bolt welding, cross-wire grids, and joining thin-to-thick workpieces.

- Pros & Cons: This method offers high accuracy, repeatable strength, and longer electrode life, making it ideal for mass production. The trade-off is a higher tooling cost and less flexibility for design changes due to the need for pre-formed projections.