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© 2012 Delmar, Cengage Learning Hardware, Fasteners, Drills, and Thread Repair Chapter 7.

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Presentation on theme: "© 2012 Delmar, Cengage Learning Hardware, Fasteners, Drills, and Thread Repair Chapter 7."— Presentation transcript:

1 © 2012 Delmar, Cengage Learning Hardware, Fasteners, Drills, and Thread Repair Chapter 7

2 © 2012 Delmar, Cengage Learning Objectives List fastener grades and describe clamping force Describe the various methods of thread repair Describe characteristics of hand drills

3 © 2012 Delmar, Cengage Learning Introduction Good automotive technicians –Know many things about fasteners (e.g., nuts and bolts) Different methods of repairing them How not to break them

4 © 2012 Delmar, Cengage Learning Characteristics of Fasteners Key concepts –Bolt Externally threaded fastener used with a nut When used without a nut it is a capscrew –Fastener size Determined by screw thread –Tempering Toughens a metal by heating and quenching –Annealing Softens a metal by heating and slowly cooling

5 © 2012 Delmar, Cengage Learning Bolt Stretch and Fastener Grades Bolt stretch –Typical fastener is stretched to 70% of elastic limit Fastener grades –SAE bolt grades measure tensile strength Higher number means greater strength –ISO bolt grades First number is tensile strength Second number is yield strength –Strongest bolts are not always the best choice

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8 Thread Lubricants Antiseize compounds –Used where a bolt might become difficult to remove over time Example: exhaust manifold bolts Lubricants may cause hydrostatic lock –Oil trapped in a blind hole –Bolt cannot compress the oil Cannot be properly tightened Part may crack

9 © 2012 Delmar, Cengage Learning Nuts, Washers, and Removing a Stud Nut grade must match bolt grade Nut must be slightly softer than bolt –Distortion is permanent Nuts lose effectiveness after repeated use –Each thread carries progressively less load Washers are necessary for correct load on a bolt Special stud puller may be used to remove studs –If not available, two nuts with a lock washer between them may be used

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11 Fastener Failures Fatigue breaks account for 75% of fastener problems –Bolts become fatigued from working back and forth when too loose Shear or torsion breaks –Result from: Poor grade fastener Too much friction Improper thread fit Bolts can be broken when bottomed out

12 © 2012 Delmar, Cengage Learning Drill Bits Used to make or enlarge holes in metal and remove broken fasteners Tip must be sharp –Flatter for hard metals and steeper for softer metals Hand sharpening requires practice Prevent drill bit wear by keeping to recommended speed Drill size is printed on the shank of the drill bit –A micrometer can determine size of a drill bit

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14 Taps and Threads Tap: used to cut internal threads in a previously drilled hole and cleans existing threads –Tapered, plug, and bottom tap Pipe threads –Have tapered threads designed to wedge against each other Tap drill –Usually provides about 75% of a full thread

15 © 2012 Delmar, Cengage Learning Taps and Threads (cont'd.) Tapping a hole –Advance the tap clockwise –Back off ¼ turn and break off any metal chips –Thread lubricants required when tapping steel and nonferrous metals Cast iron can be tapped dry Removing a broken tap –Drive a broken tap counterclockwise with a centerpunch –EDM erodes the fastener, leaving the thread

16 © 2012 Delmar, Cengage Learning Dies Make external threads on a round rod –Advance the die and then turn backward as with a tap Useful for chasing burred threads

17 © 2012 Delmar, Cengage Learning Repairing Broken Fasteners Broken bolts may be removable with a chisel, pliers, or stud extractor –Left-hand drill bit with reversible drill motor may remove broken bolt that is not bottomed out Screw extractors –Easy out screw extractor –Extractor with flutes Nut may be welded to the fastener and a wrench unscrews it

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19 Repairing Broken Fasteners (cont'd.) Broken fasteners can be removed by drilling and tapping the hole Stripped screw threads can be repaired with thread inserts –Locking inserts Solid-Threaded Inserts –Spark plug inserts

20 © 2012 Delmar, Cengage Learning Repairing Broken Fasteners (cont'd.) Pop rivets –Fasten sheet metal parts to engine parts Inexpensive and easy to use Retaining rings –Prevent a pin from sliding out of position

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