Inconsistent Spot Weld Quality

Definition

Spot weld quality varies with some welds failing to meet applicable quality standards.

Description

Quality may vary as a result of a trend (Fig. 1) or in a more random pattern (Fig. 2).

• A gradual decrease in spot weld quality (Fig. 1) can occur because of an increase in the face diameter of the tips. Random variation about this trend may eventually fall below the minimum standards. Countermeasures to address the trend may include current steppers, tip-dressing etc.
• More random variation (Fig. 2) may occur due to intermittent factors such as voltage sag and spike, workpiece surface variation, etc. Further system analysis may be required to determine the root cause of the variation.

Detection

Inconsistent weld quality can be determined through properly scheduled quality checks.

Significance

Quality, Workplace Issues, Cost, Downtime, Maintenance, Throughput (cycle time; PPH), are all potentially affected by this condition.

Possible Causes

Strong Possibilities

• Dirty Material
• Electrode Skidding/Sliding
• Excessive Sealer
• Poor Electrical Connections
• Poor Electrode Follow-up
• Poor Mechanical Connection
• Poor or Varying Part Fit-up
• Shunting of Guns or Parts
• Weld Wrongly Located
• Wrong Cables/Shunts

Weak Possibilities

• Damaged Part
• Defective Air or Hydraulic System
• Incorrect Cylinder
• Incorrect Electrode Dressing
• Incorrect Material/Coating
• Incorrect Test Procedure
• Incorrect Workpiece Selected
• Insufficient Cooling
• Wrong Transformer

Nonmatrix Possibilities

• Interference due to other operations, electro-magnetic interference from nearby sources
• Power system surges or drops