Shrinking Sheet Metal: Procedure

 

Now let's learn the basic method for shrinking sheet metal...

 

This operation is used to remove small dents in which the direct damage is not too severely stretched or torn, and which the work-hardness of the buckles is not too severe. Hail damage and other similar small dents are examples.This operation is used:

  • when it is determined that the use of heat is necessary.

  • when dents are located behind inner construction that hinders regular bumping.

  • when it is necessary to reduce the size of the dent.

  • when it is necessary to avoid removal of certain parts.

 

View the interactive lesson below for a deeper look at this type of repair.

 

 

Hail Damage Procedure

  1. Use a relatively high heat by bringing the neutral flame of a torch about 5mm (3/16") from the surface. The spot will heat up and expand rapidly. “CAUTION!!” Keep the flame pointed straight down on the spot. NOT at an angle, so the spot will lift in the center.

  2. As the expansion raises the metal of the low spot above the surface, and before the metal turns red, quickly remove the flame from the surface.

  3. Allow the spot to air cool for a few seconds.

  4. As the contraction brings the surface down to the level, and while the metal is still hot, file the heated spot with a body file.

  5. Allow to cool and operation is complete.

 


Click on the icon below to access a "Hail Damage Repair" video from the I-CAR website.

Link to: Hail Damage Repair Video


 

Shrink Preparation

  • If you are using an oxy/acetylene outfit the size of tip used is the same as the one you would use to weld the panel (neutral flame).

  • Some MIG welders come with carbon heat tips to heat the metal for shrinking. They are set to a shrink mode on the selector dial and adjust automatically.

  • Spoons or dollies selected should have a crown or shape slightly less than that of the panel being shrunk.Note:If the spoon or dolly is greater than the crown or shape you will stretch the metal.

  • Hammer face should be flat and have the edges rounded so that crescent shaped marks are not left by the hammer face edge (see image at right).

Note:Be sure both hammer and dolly surfaces are clean.

 

General Shrinking Procedure

 

Arrange your tools - Arrange tools so time is not wasted leaning or bending down to grab them after heat has been applied.

 

 

Concentrate stretched metal - Hammer stretched section up and outward, into one or more concentrated areas. The number of areas will be determined by the amount of shrinking required.

 

 

Locate the highest spot - locate the highest point as this will be the location of the first shrink.

 

 

Select the appropriate torch tip - Select a torch tip as you would to weld the metal (as per metal thickness).

 

 

Setup a neutral flame - Light the torch and adjust to a neutral frame

 

 

Apply heat - Hold the torch perpendicular to the panel approximately 3 mm (1/8") from the surface until the desired temperature is reached.

Holding the torch perpendicular to the panel will ensure the spot is heated evenly. This will give you greater control of the restricted expansion and contraction. The size and colour of the heated spot will be governed by the shape of the panel and how much stretching has occurred.

Heating the metal softens a small spot in the centre of a larger stretched area. This allows the technician to compress the metal towards the centre. As the heated spot cools, it contracts and draws the stretched metal with it, firming up the panel. Heating can be accomplished using an oxy-acetylene torch setup or electrically with panel repair equipment. Most GMA (MIG) welders can be setup for shrinking by installing a carbon shrinking tip.

For better control on a flat or low-crowned panel it may be beneficial to perform 2 or 3 smaller blue shrinks than one large cherry red shrink. Overshrinking causes the metal to be held under tension and may be mistaken for stretched metal.

 

 

Level the damage - Choose a dolly with a slightly lessor crown than the panel being shrunk. Hammer blows should be directed around the outside of the heated spot pulling the metal towards the centre. Hold the dolly loosely against the backside of the panel and level it with a body hammer

 

 

Quench the area- The shrink may now be quenched (if desired) with a wet rag or cooled with air. Additional shrinks must not be attempted until previous shrinks have cooled. Wait times can be shortened by quenching. Quenching increases the amount of contraction and allows you to continue working without having to wait for the metal to cool.

 

 

Video unavailable at this time...

Shrink

 

 

 

 

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