Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. PowerPoint to accompany Krar Gill Smid Technology of Machine Tools 6 th Edition Thread-Cutting Tools and Procedures Unit 24

24-2 Objectives Calculate the tap drill size for inch and metric taps Cut internal threads using a variety of taps Know the methods used to remove broken taps from a hole Cut external threads using a variety of dies

24-3 Hand Taps Cutting tools used to cut internal threads Made from high quality tool steel, hardened and ground Two, three, or four flutes cut lengthwise across threads to form cutting edges –Provide room for chips –Admit cutting fluid to lubricate tap End of shank square so tap wrench can be used to turn tap into hole

24-4 Hand Taps Inch tap markings –Major diameter –Number of threads per inch –Type of thread ½ in.—13 UNC ½ in. = major diameter of tap 13 = number of threads per inch UNC = Unified National Coarse (type of thread)

24-5 Hand Taps: Sets of Three Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Taper –Tapered from end six threads –Used to start thread easily –Used for tapping hole that goes through work as well as blind hole Plug –Tapered for three threads –Tread hole through workpiece Bottoming tap –Not tapered but chamfered at end –Used for threading to bottom of blind hole

24-6 Another Method of Identifying Taps Use of annular ring(s) cut around top of shank Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. One – taper tap Two – plug tap Three – bottoming tap

24-7 Tap Drill Size Hole drilled to correct tap drill size –Leave proper amount of material for tap to cut thread (75%) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. A = body size B = tap drill size C = minor diameter TDS = tap drill size D = major diameter of tap N = number of threads per inch Find tap drill size for a 7/8 in. –9NC tap

24-8 Metric Taps Several thread forms and standards in metric thread system International Standards Organization (ISO) has adopted standard metric thread –Will be used in US, Canada and other countries –Have only 25 thread sizes (1.6 to 100 mm) Available in sets of three: taper, plug and bottoming taps –Identified by letter M followed by nominal diameter of thread in mm times pitch in mm

24-9 Tap Drill Sizes for Metric Taps Tap drill size for metric taps calculated in same manner as for U.S. Standard threads TDS = major diameter (mm) – pitch (mm) Example: Find the tap drill size for a 22 – 2.5 mm thread TDS = 22 – 2.5 = 19.5 mm

24-10 Tapping a Hole Operation of cutting internal thread using tap and tap wrench Taps hard and brittle Easily broken –Extreme care used to prevent breakage –Broken tap in hole difficult to remove and often results in scrapping work

24-11 To Tap Hole By Hand 1.Select correct taps and tap wrench for job 2.Apply suitable cutting fluid to the tap No fluid required for tapping brass or cast iron 3.Place tap in hole as vertically as possible; press downward on wrench, applying equal pressure on both handles; turn clockwise (for right-hand thread) for two turns

Remove tap wrench and check tap for squareness Check two positions at 90º to each other 5.If tap not entered squarely, remove from hole and restart it by applying pressure in direction from which tap leans 6.When tap properly started, feed it into hole by turning tap wrench 7.Turn tap clockwise one-quarter turn, and turn it backward about ½ turn to break the chip (must turn with steady motion)

24-13 Removing Broken Taps Several methods may be used to remove broken tap (some successful, others not!) –Tap Extractor Tool with four fingers that slip into flutes of broken tap; wrench fitter to extractor and turned to extract –Drilling Drill through broken tap –Acid Method Acid acts on steel and loosens tap for extraction –Tap Disintegrators Uses electrical discharge principle to cut through tap

24-14 To Remove Broken Tap Using a Tap Extractor 1.Select proper size extractor for tap 2.Slide collar, to which fingers attached, down body so fingers project well below end of body 3.Slide fingers into flutes of broken tap, making sure they go down into hole as far as possible

Slide body down until it rests on top of broken tap Give maximum support to fingers 5.Slide collar down until it rests on top of work Also provides support for fingers 6.Apply wrench to square end of body top 7.Turn wrench gently in counterclockwise direction Note: Do not force extractor – will damage fingers

24-16 To Remove Broken Tap (Carbon Steel) By Drilling 1.Heat broken tap to bright red color and allow it to cool slowly 2.Center-punch tap close to center 3.Using drill smaller than distance between opposite flutes, carefully drill holl through broken tap 4.Enlarge hole to remove as much of metal between flutes as possible 5.Collapse remaining part with punch and remove pieces

24-17 To Remove Broken Tap (High- speed Steel) Using the Acid Method 1.Dilute one part nitric acid with five parts water 2.Inject mixture into hole; acid will act on steel and loosen tap 3.Remove tap with extractor or pliers 4.Wash remaining acid from thread with water to neutralize acid

24-18 Threading Dies Used to cut external threads on round work Most common threading dies –Adjustable split die –Adjustable screw plate die –Solid die Used for chasing or recutting damaged threads May be driven by suitable wrench Not adjustable

24-19 Threading Dies Adjustable split die –Has adjusting screw that permits adjustment over or under standard depth of thread –Fits into die stock Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Adjustable screw plate die –Two die halves held in collet by threaded plate –Adjusting screws against each die half –Greater adjustment than split die

24-20 To Thread With a Hand Die 1.Chamfer end of workpiece with file or on grinder 2.Fasten work securely in vise 3.Select proper die and die stock 4.Lubricate tapered end of die with suitable cutting lubricant 5.Place tapered end of die squarely on workpiece

Press down on die stock handles and turn clockwise several turns 7.Check die to see that it has started squarely with work 8.If not square, remove die and restart 9.Turn die forward one turn and reverse it approximately one-half turn to break chip 10.During threading process, apply cutting fluid frequently If thread must be cut to shoulder, remove die and restart it with tapered side of die facing up (complete the thread)