Chapter 10 Metal Straightening Fundamentals Objectives Describe different

Chapter 10 Metal Straightening Fundamentals

Objectives • Describe different types of metals used in vehicle construction • Summarize the deformation effects of impacts on steel • Use a hammer and dolly to straighten • Explain how to straighten with spoons • List the steps for shrinking metal

Objectives (continued) • • • Summarize paintless dent removal Prepare a surface for filler Properly mix filler and hardener Correctly apply and shape filler List common mistakes made when using filler and spot putty

Introduction • An untrained person can spend more time shaping and sculpting body filler than properly reworking the damaged metal • Improperly straightened panel has tension that can cause filler to crack, lose adhesion, or fall off • To do quality sheet metal repairs, you must return the sheet metal to its original shape – Then you can use a thin layer of filler to smooth surface above panel

Sheet Metal • Hot-rolled sheet metal is made by rolling at temperatures exceeding 1, 472 degrees F • Cold-rolled sheet metal is hot-rolled that has been acid rinsed, cold-rolled thin and annealed • Low-carbon or mild steel (MS) is relatively soft, with a yield strength of up to 30, 000 psi – Easily deformed, and relatively heavy • High-strength steel (HSS) is stronger, and has a yield strength of up to 60, 000 psi – More difficult to restore than MSS

Steel Strength • When flat sheet steel is formed into a shape for a panel or part it takes on hardening properties • Shape of panels is changed in a collision, changing the structure of the metal • After a collision, metal is harder and more resistant to corrective forces • Strength is expressed in pounds per square inch (psi), kilograms per centimeter squared (kg/cm 2) • Differences in chemical makeup and crystalline structure of steels affect their strength

Physical Structure of Steel • Steel, like all matter, is composed of atoms, which are arranged to form grains • Grain structure in a piece of steel determines how much it can be bent or shaped • To change the shape of flat steel, you must change the shape and position of the grains • In MSs the individual grains can withstand a considerable amount of change before breaking • Bending steel causes individual grains to move against each other, which causes friction

Effect of Impact Forces • Elastic deformation is the ability of metal to stretch and return to its original shape • Spring-back is the tendency for metal to return to its original shape after deformation • Plastic deformation is the ability of metal to be bent or formed into different shapes • When metal is bent beyond its elastic limit it has a tendency to spring back – Will not spring back to its original shape because grain structure has changed

Work Hardening • Work hardening is the limit of plastic deformation that causes the metal to become hard where it has been bent • If a welding rod is bent back and forth several times, a fold or buckle will appear at bend point – Plastic deformation is so great the metal will be very hard and stiff at bend

Kinds of Damage • A part is kinked when: – It has a sharp bend of a small radius – After straightening there is a visible crack or tear in the metal, or permanent deformation • A part is bent when: – Change in shape of part between damaged and undamaged areas is smooth and continuous – Straightening part by pulling restores its shape without areas of permanent deformation

Figure 10 -2. The radius of the bend determines whether you have a kink or a bend. A kink generally results when metal is folded more than 90 degrees. A bend in sheet metal can be easily repaired but not a sharp kink that has deformed the metal badly.

Using Body Hammers • Body hammer strikes sheet metal and rebounds off surface • Raising means to work a dent outward, lowering means to work a high spot down, into the body • Swing a body hammer in a circular motion at the wrist, not at the shoulder • Hit part squarely and let hammer rebound • Face of hammer must fit contour of panel

Straightening with Dollies • Sometimes you can reach into obstructed areas more easily with a dolly than with a hammer • Contour of a dolly must fit contour of back side of damaged area • If wrong surface hits panel, it will cause further damage to panel • Start with light blows from dolly, hitting exactly where needed • Gradually increase force, using numerous wellplaced blows

Hammer-On-Dolly • Hold dolly against back of damage and hammer right over top of dolly • Repeatedly move point of hammer impact and dolly slightly, each blow overlapping • Start at outside and work toward center • Shapes of dolly and hammer must match desired shape of panel • Start with light hammer blows, and work up to stronger blows

Figure 10 -3. With the hammer-on-dolly method, place the dolly right behind the damage and hit the metal right over the dolly to straighten the metal in the small area between the tools.

Hammer-Off-Dolly • Used to raise low spots and lower high spots simultaneously • Often used to rough out or shape large areas of damage during initial straightening • Hold dolly under lowest area on back of panel, then hit any area next to dolly • Start at outer perimeter and work toward center • Control straightening by altering how hard to hit hammer, how hard to push dolly, how far away dolly is from hammer blows

Figure 10 -7. Note the steps for removing damage from a curved shape using a hammer and dolly. First, start at one end of the damage, on the right. Then work the damage on the other side. Roll out the damage toward the middle. Filler would then be needed to give the area its final contour.

Straightening with Spoons • Spring hammering is a method of bumping out damage with a hammer and dinging spoon • Force of blow on spoon is distributed over a large area of ridge • With a long body spoon, you can reach into restricted places • Spoons can be used to pry up metal or to drive out deep dents • Do not stretch metal by prying out too much

Straightening with Picks • Picking hammers, pry picks, a dolly edge, and a scratch awl can be used to pick or push up metal • After an area has been raised, use a file to identify any remaining low spots • Pick can be inserted in drainage hole in door – No need to drill holes • Paintless dent removal uses picks to remove small dents without having to repaint • Too much pressure can stretch metal

Pulling Damaged Areas • Pulling is often needed because access to the inside of panels is blocked by reinforcements • Suction cup straightens shallow dents • Stud spot welder joins “pull rods” on the surface of a panel so you do not have to drill holes – Pins are welded onto surface of panel – Slide hammer is attached to each of the pins – Pulls out the damage in steps

Shrinking Metal • Small spot in center of warped area is heated to a dull red, causing it to swell • Surrounding area is cool so panel can’t expand • If heating continues, stretching of the metal is centered in red-hot portion, pressing it out – Causes it to thicken, relieving compression load • If red-hot area is suddenly cooled, steel contracts and surface area shrinks

Torch Shrinking • Torch shrinking uses an oxyacetylene torch to release tension in the panel • Average-sized area to heat is about size of a quarter, but never larger than size of hammer face being used • When area has been heated, tap around it to drive molecules of metal closer together • Once redness has disappeared and area has been smoothed, cool area with a wet rag to cause metal to contract

Shrinking a Gouge • Gouge is caused by a focused impact that forces a sharp dent or crease into a panel • Gouge causes metal to be stretched and must be shrunk to original size to repair damage • Simply picking up low area distorts panel • Filling gouge without restoring panel’s original contour leaves tension in panel that could cause filler to crack or fall off

Identifying Stretched Metal • Stretched metal has been forced thinner in thickness and larger in surface area by impact • Metal is often stretched in badly buckled places • These same areas are sometimes stretched during straightening • When an area is stretched, grains of metal are moved farther away from each other • Before shrinking, dolly the damaged area back as close to its original shape as possible

Filing the Repair Area • When area has been straightened as smooth as possible, file across damaged area to undamaged metal on opposite side • Keep filing action on correct plane with good part of panel • Push file forward by its handle for cutting stroke, using as long a stroke as possible • Scratch pattern created by file identifies high and low spots

Figure 10 -18. Filing will help you locate high and low spots that need further work. Low spots will NOT have file marks.

Working Aluminum • Aluminum is much softer than steel yet is more difficult to shape once it is work-hardened • Aluminum does not readily bend back to its original shape after being buckled by impact • Spring hammering with a hammer and spoon is an excellent way to unlock stress in aluminum • Sand carefully on aluminum • Heat shrink stretched aluminum slowly to avoid distorting the panel – Aluminum does not turn red as it is heated

Using Body Filler • Body fillers cover up minor surface irregularities • Prepare area by washing with soap and water, use a wax and grease remover • Mask trim, parts or adjacent panels that could be damaged by grinding, sanding or filling • Grind area to remove paint 3 -4 inches around area to be filled • Blow away sanding dust with compressed air, and wipe with a tack rag to remove dust

Preparing Surface for Filler • Many problems are linked to improper surface preparation • Mix can of filler to a uniform and smooth consistency, free of lumps and not wet on top • Mix hardener by squeezing tube back and forth with your fingers, to a paste-like consistency • Add hardener according to proportion indicated on product label

Preparing Surface for Filler (continued) • Filler over-catalyzation results from using too much hardener • Filler under-catalyzation is caused by not using enough hardener • Mix filler and hardener thoroughly to achieve a uniform color • Do not redip spreader or mixed filler into can, or whole can will harden in time • Use different spreaders to mix and apply filler

Applying the Filler • Apply filler as soon as you have finished mixing, applying a thin coat to the repair area • Press firmly to force filler into sand scratches and holes to strengthen bond • Work filler patch in two directions – Left to right, then top to bottom – Build up layers not more than 1/8 inch thick • Spread filler about 3" beyond the repair area

Applying the Filler (continued) • When one layer cures and has been sanded apply more coats to build up repair area – Allow each layer to cure before sanding • Build up final layer of filler slightly above panel surface • Sand filler down smoothly on an equal plane with existing panel • If needed, use fingers to bend spreader to match shape of contour • Avoid using filler in cold temperatures

Shaping Filler • Allow filler to cure to a semi-hard consistency • Use a coarse body file to rough shape filler • Many vehicles have sharp body lines in doors, quarter panels, hoods, etc. • Apply masking tape along one edge then apply filler to adjacent surface

Shaping Filler (continued) • Before filler sets, pull tape off to remove excess filler from body line • After first application is dry and sanded, tape opposite edge and coat adjacent surface with filler • Do not cover a seam sealer with body filler

Sanding the Filler • After filing, sand out file marks and shape filler accurately • Do not try to sand all imperfections in first coat of filler • Smooth final coat of filler with No. 180 grit sandpaper – Do not oversand

Sanding the Filler (continued) • After sanding, blow with air gun and wipe with tack cloth • Featheredging involves sanding the repair area until the filler and old paint blend smoothly – Commonly done with a DA sander

Figure 10 -36. When featheredging, hold the sander flat on the surface. Sand the body filler until it is flush with the surrounding undamaged area. A common mistake is oversanding, trying to remove small surface imperfections. If you find a low spot, apply more filler to the low area. Then resand.

Priming • After using filler, primer-surfacer is often sprayed on the repair area, to help fill small sand scratches in the filler and paint • Apply primer-surfacer in 2 – 3 coats with 5 – 15 minute flash time between coats • Wait an hour or more before sanding primersurfacer

Finishing Fillers (Putties) • Once primer is dry, small pinholes and scratches can be filled with spot or glazing putty • Apply a thin coat over the primer, usingle strokes and fast scraping motion • A guide coat is a thin layer of a different color primer or special powder applied to the repair – Low spot: guide coat does not sand off – High spot: sands off too quickly

Summary • Two types of sheet metal used in auto body work are hot-rolled and cold-rolled • Tools used to straighten metals include body hammers, spoons, dollies, suction cups, slide hammers, and spot-welded studs • Shrinking metal removes strain or tension on a damaged, stretched area • Aluminum is much softer than steel yet it is more difficult to shape once it is work-hardened

Summary (continued) • Body fillers fill minor surface irregularities that remain after straightening • Featheredging involves sanding repair area until filler and finish blend smoothly into each other • Priming is done after sanding body fillers to prepare surface for refinishing