Spot Welding

Spot welding joins metals through electrical resistance heating:

Process Principles:
1. Squeeze: Electrodes clamp sheets together with force
2. Weld: High current flows through joint, resistance generates heat
3. Hold: Current stops, pressure maintained while nugget solidifies
4. Release: Electrodes separate, part moves to next weld location

Key Variables:
- Current (Amps): Determines heat generation (heat = I²Rt)
- Time (Cycles): Duration of current flow (1 cycle = 1/60 second at 60Hz)
- Force (Pounds): Electrode pressure affects resistance and nugget formation
- Electrodes: Shape, material, and condition affect weld quality

Heat Generation:
Heat concentrates at the interface between sheets (highest resistance point), forming a molten nugget that solidifies into the weld:

```
Electrode (Low resistance)
↓
Sheet 1 (Medium resistance)
↓
Interface (High resistance) → Heat concentrates here
↓
Sheet 2 (Medium resistance)
↓
Electrode (Low resistance)
```

Electrode Materials:
- Class 2 (RWMA): Copper alloy, most common
- Class 3: Higher hardness for coated materials
- Dispersion Strengthened: Longer life on coated steels

Electrode Geometries:
- Truncated Cone: Most common, general purpose
- Dome (Radius): Larger contact, lower current density
- Flat: Maximum contact for projection welding

Spot welding equipment ranges from portable guns to automated systems:

Machine Types:

Rocker Arm:
- Simple mechanical design
- Fixed lower electrode, moving upper
- Limited throat depth
- Entry-level equipment

Press Type:
- Vertical motion, consistent pressure
- Larger throat depths available
- Better for heavier materials
- Programmable controls

Portable Gun:
- Transformer in hanging gun
- Flexible positioning
- Used in assembly and repair
- Limited to thinner materials

Robotic Systems:
- Servo guns with precise control
- Integrated with welding controllers
- Thousands of welds per hour
- Dominant in automotive production

Setup Parameters:

Material Thickness Guidelines:
| Total Thickness | Current (kA) | Time (Cycles) | Force (lbs) |
|-----------------|--------------|---------------|-------------|
| 0.030" (2x 0.015") | 6-8 | 6-10 | 300-500 |
| 0.060" (2x 0.030") | 8-10 | 10-15 | 500-800 |
| 0.125" (2x 0.062") | 10-14 | 15-25 | 800-1200 |

Electrode Maintenance:
- Tip dress regularly (restore geometry)
- Replace when worn beyond specification
- Monitor electrode life tracking
- Water cooling required for production

Ensuring spot weld quality requires systematic inspection:

Visual Inspection:
- Surface appearance (indentation, expulsion, cracks)
- Electrode alignment marks
- Sheet metal distortion
- Surface discoloration

Destructive Testing:

Peel Test:
Pry apart welded samples to examine nugget:
- Good weld: Nugget pulls from one sheet (button)
- Poor weld: Interface separation (interfacial failure)
- Measures nugget diameter against specification

Chisel Test:
Drive chisel between sheets:
- Production verification method
- Nugget should remain intact
- Similar failure criteria to peel test

Cross-Section Examination:
Cut through weld and polish:
- Measure nugget diameter and penetration
- Check for porosity, cracks, voids
- Laboratory quality control method

Ultrasonic Testing:
Non-destructive nugget evaluation:
- Measures nugget size and quality
- 100% inspection possible
- Requires trained technicians and calibration

Common Defects:

Undersized Nugget:
- Insufficient current or time
- Worn electrodes
- Poor fit-up

Expulsion:
- Excessive current
- Insufficient force
- Contamination

Surface Cracks:
- Too rapid cooling
- Excessive force
- Material issues

Porosity:
- Contamination (oil, coatings)
- Trapped gas
- Parameter issues

Spot welding is fundamental to several industries:

Automotive Manufacturing:
- Vehicle bodies: 3,000-5,000 spot welds per car
- Robotic welding cells
- High-speed production requirements
- Mix of steel and aluminum assemblies

Appliance Industry:
- Refrigerator cabinets
- Washing machine drums
- HVAC components
- High-volume production

Electronics Enclosures:
- Computer cases
- Server racks
- Electrical cabinets
- Sheet metal chassis

Career Paths:

Spot Welding Operator:
- Set up and operate equipment
- Monitor quality
- Maintain electrodes
- $35,000-$50,000

Resistance Welding Technician:
- Program and troubleshoot
- Develop weld schedules
- Implement new applications
- $50,000-$70,000

Welding Engineer:
- Design welding processes
- Specify equipment
- Solve production problems
- $70,000-$100,000

Robotics/Automation:
- Program robotic spot welding
- Integrate cells into production
- Optimize cycle times
- $65,000-$95,000

Skills for Advancement:
- Understanding resistance welding theory
- Metallurgy of sheet steels
- Quality system knowledge
- Robotic programming (for automation roles)
- Data analysis for process optimization

Certifications:
- AWS D8.1M (Automotive spot welding)
- Certified Resistance Welding Technician (CRWT)
- Manufacturer-specific robot certifications