Nissan Murano: Sheet Metal Work / Shrinking the Sheet Metal

Shrinking The Sheet Metal

Plastic deformation may cause reduced panel thickness and the panel may stretch. Even when it is repaired using a hammer and dolly, the panel tends to bulge, losing its original shape.

In such cases, it must be shrunk to its original shape. This is called shrinking the sheet metal.

(1) ELECTRIC WELDING MACHINE

The body panel is connected to the negative power supply terminal and the tip is connected to the positive terminal. Then, an electric current is supplied to heat the panel. The shrinking principle is the same as that of gas welding. This method features no hammering and greater workability than gas welding and is suitable for repairing local panel deformations.

(2) CARBON ROD

The panel is connected to the negative power supply terminal and a carbon rod is connected to the positive terminal. The panel is heated so that heat is conducted from the outside to the center of the dent. Wet rags are then applied to cool it quickly, thus shrinking the panel.

This method is suitable for repairing wide, shallow panel deformations.

(3) GAS WELDING (OXY-ACETYLENE TORCH)

(a) Heating and expansion

As the sheet metal is heated with a gas welding torch, it stretches.

However, stretching is restricted in the unheated surrounding portion. As a result, the heated portion bulges.

(b) Sudden cooling

When the bulge is cooled suddenly, it shrinks.

This shrinking is accelerated by tapping with a wooden hammer.

(4) HAMMER AND DOLLY

A shrinking hammer and shrinking dolly are used by the hammer-on-dolly method. Many tiny dents are formed on the panel surface. This method is suitable for shrinking comparatively small areas of panel deformation.

• Use a carburizing flame when shrinking sheet metal with a gas welding torch.

  

• Hold the torch at a right angle to the center of the sheet metal.

• Maintain a distance of 3 mm - 5 mm (0.12 in - 0.20 in) between the inner core and sheet metal.

• Heat the sheet metal to approximately 800°C (1,472°F) (until the heated portion turns red). Increase the temperature if sheet metal stretching is insufficient.

• Locate the stretched portion of the panel.

  

  Press the surface being repaired in several places.

  The point where the largest elastic dent is formed is the center, where the stretch is the maximum. The highest portion of the panel being repaired can also be considered the most stretched portion.

• The area heated with a welding torch must be approximately 3 mm - 5 mm (0.12 in - 0.20 in) in diameter if the panel shape is complex, and approximately 6 mm - 15 mm (0.24 in - 0.59 in) in diameter if it is flat.

  

• Small stretch

• Apply a dolly to the back of the heated panel. Tap the panel with a wooden hammer using the hammer-ondolly method in the sequence shown in the figure.

  (A) and (B) → 1 - 8

  (C) → 9

• Excessive stretch

  

  Shrink the panel, starting with the most stretched portion, and proceed toward the edge of the dent so that the dent surface is lower than the original surface.

  Using a gas welding torch makes the panel concave.

  To correct this, strike the concave portion using the hammer-on-dolly method to stretch the panel bit by bit until the original surface is restored.

• Apply wet cloths to the shrunken portion of the panel to cool it quickly.