Metal Straightening, Shrinking, and Stretching
Metal Straightening, Shrinking, and Stretching tests how steel and aluminum move under force and heat, and how to restore shape without destroying strength.
Behavior of metal
- Stretching thins and lengthens metal; shrinking gathers it. A dent often has stretched crowns and compressed edges.
- Work hardening increases with bending; excessive hammering makes steel brittle.
- Heat changes strength. Many high-strength and UHSS grades prohibit or limit heat; follow OEM limits.
- Aluminum work-hardens quickly, cracks with repeated bending, and needs dedicated tools.
Tools and technique
- Hammer and dolly: hammer off-dolly to stretch, on-dolly to planish and smooth.
- A slapping file or pick file shows high and low spots.
- Shrink with a torch and cold quench or a shrinking disc, controlling heat and using a wet rag.
- Relief cuts can relieve a buckle, but they add a weld and corrosion risk.
Worked example
A fender has a shallow dent with a stretched crown. Reasoning: the metal is longer than the original surface, so hammering it down further will oil-can. Planish the crown with an on-dolly hammer, then shrink the stretched area with controlled heat and quench, cooling between passes. Check with a straightedge and avoid over-shrinking.
Traps
- Using a torch on UHSS because it worked on mild steel.
- Hammering off-dolly on already stretched metal.
- Over-shrinking and creating a low spot.
- Skipping the pick file and guessing at the contour.