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Drill Design. Many holes can be drilled with more than one type of drill, but discriminating selection may allow closer tolerances. For example, if a screw-machine drill can be used, somewhat closer tolerances can be met than with a longer drill, because of the decrease in drill overhang. In some applications, a screw-machine drill is used instead of a standard- length drill, thus eliminating the need for bushings. Modification of the drill point often results in greater accuracy. Predrilling with a center drill often improves accuracy because the final drill has no chance to walk at the start. Walking causes out-of-line drilling, which results in holes that are less accurate in size and roundness. Drilling in steps can improve accuracy, although with some types of equipment it can also reduce productivity. For example, to drill a 75 mm (3 in.) deep, 19 mm ( in.) diam hole, it may be advantageous first to drill a 25 mm (1 in.) deep hole with an 18.6 mm ( in.) diam twist drill and then switch to an 18.2 mm ( in.) diam twist drill to drill a 50 mm (2 in.) deep hole. This is followed by a 17.9 mm ( in.) diam twist drill to reach the final 75 mm (3 in.) depth desired. Finally, a 19 mm ( in.) diam four-flute core drill is used as a roughing reamer to obtain an accurate hole. Drill bushings in fixtures ensure uniformly centered starts, which result in better alignment, and thus improve hole roundness, straightness, and accuracy of location. Rotating the workpiece (provided its size and shape permit) will invariably result in more accurate drilling than rotating the drill while the work remains stationary. Machines in which the work is rotated include lathes, turret lathes, and horizontal multiple-station drilling machines. A turret lathe also makes it possible to drill a hole in steps, completing it with a core drill, and then to ream it for still greater accuracy. Speed and Feed Optimum speed and feed for drilling depend on workpiece material, tool material, depth of hole, design of drill, rigidity of setup, tolerance, and cutting fluid. Consequently, it is impossible to recommend speeds and feeds that are applicable under all conditions. The nominal speeds and feeds given in Tables 6(a) and 6(b) are useful as starting points in selecting an optimum combination for a specific job. Table 6(a) Recommended operating parameters for producing holes in a variety of materials with twist drills Hardness Peripheral speed, Material drilled HB HR Cutting tool material (a) m/min (sfm) Feed rate (b) Helix angle, degrees Point angle, degrees Point style Aluminum and aluminum alloys 45-105 to 62 HRB HSS 107 (350) Z 32-42 90-118 . . . Asbestos . . . . . . WC 17 (55) Y 17-20 90 . . . Bakelite . . . . . . WC 24 (80) Y 30-40 90-118 . . . Carbon . . . . . . HSS 18-21 (60- 70) W 25-35 90-118 . . . Copper and copper alloys . . . High machinability to 124 10-70 HRB HSS 61 (200) Z 15-40 118 . . . Low machinability to 124 10-70 HRB HSS 21 (70) Z 0-25 118 . . . Fiberglass-epoxy . . . . . . WC 198 (650) 0.063-0.127 mm (0.0025- 0.0050 in.) 35-40 118-130 Four- facet Glass . . . . . . WC 4.6- 7.6 (15- 25) Light hand 0 . . . Spear High-temperature alloys Cobalt-base 180-230 89-99 HRB HSS-Co 6.1 (20) W 28-35 118-135 Split Iron-base 180-230 89-99 HSS-Co 7.6 (25) X 28-35 118-135 Split HRB Nickel-base 150-300 to 32 HRC HSS-Co 6.1 (20) W 28-35 118-135 Split Iron 120- 150 to 80 HRB HSS 43-46 (140-150) Z 20-30 90-118 . . . Cast (soft) WC 27-50 (90- 165) Y 14-25 90-118 . . . 160-220 80-97 HRB HSS 24-34 (80- 110) Y 20-30 90-118 . . . Cast (medium hard) WC 27-50 (90- 165) X 14-25 90-118 . . . Hard chilled 400 41 HRC WC 9 (30) X 0-25 130-140 Notched 112-126 to 71 HRB HSS 27-37 (90- 120) Y 20-30 90-118 . . . Malleable WC 30-46 (100- 150) X 14-25 118 . . . 190-225 to 98 HRB HSS 18 (60) Y 20-30 118 . . . Ductile WC 24-30 (80- 100) X 14-25 118 . . . Magnesium and magnesium alloys 54-90 to 52 HRB HSS 46- 122 (150- 400) Z 25-35 118 . . . Marble . . . . . . WC 4.6- 7.6 (15- 25) Light hand 0-10 90-130 . . . . . . . . . HSS 30 (100) Y 15-25 118 . . . Plastics WC 30-61 (100- 200) X 15-25 118 . . . . . . . . . HSS 30-91 (100- 300) X 10-20 90-118 . . . Rubber (hard) WC 61-91 (200- 300) W 15-25 118 . . . Steel Plain carbon to 0.25 C 125-175 71-88 HRB HSS 24 (80) Y 25-35 118 . . . to 0.50 C 175-225 88-98 HRB HSS 20 (65) Y 25-35 118 . . . to 0.90 C 175-225 88-98 HRB HSS 17 (55) Y 25-35 118 . . . Alloy steel Low carbon (0.12- 0.25) 175-225 88-98 HRB HSS 21 (70) Z 25-35 118 . . . 175-225 88-98 HRB HSS 15-18 (50- 60) X 25-35 118 . . . Medium carbon (0.30-0.65) 488+ 50+ HRC WC 23-30 (75- 100) 0.013-0.038 mm (0.0005- 0.0015 in.) 25-35 135 Notched Maraging 275-325 28-35 HRC HSS 17 (55) Y 25-32 118-135 Split Stainless steel Austenitic 135-185 75-90 HRB HSS-Co 17 (55) X 25-35 118-135 Split Ferritic 135-185 75-90 HRB HSS 20 (65) X 25-35 118-135 Split Martensitic 135-175 75-88 HRB HSS-Co 20 (65) Z 25-35 118-135 Split Precipitation- hardened 150-200 82-94 HRB HSS-Co 15 (50) X 25-35 118-135 Split 196 94 HRB HSS 18 (60) Y 25-35 118 . . . Tool 241 24 HRC HSS 15 (50) Y 25-35 118 . . . Titanium Pure 110-200 to 94 HRB HSS 30 (100) X 30-38 135 Split and - 300-360 31-39 HRC HSS-Co 12 (40) Y 30-38 135 Split 275-350 29-38 HRC HSS-Co 7.6 (25) W 30-38 135 Split Zinc alloys 80-100 41-62 HRB HSS 76 (250) Z 32-42 118 . . . Source: Ref 2 (a) HSS, high-speed tool steel; HSS-Co, high-speed tool steel with cobalt; WC, tungsten carbide. (b) See Table 6(b). Table 6(b) Feed rates for materials listed in Table 6(a) Feed rate, mm/rev (in./rev), at a drill diameter of: Code 3.2 mm ( in.) 6.4 mm ( in.) 12.7 mm ( in.) 19.1 mm ( in.) 25.4 mm (1 in.) W 0.038 (0.0015) 0.08 (0.003) 0.089 (0.0035) 0.114 (0.0045) 0.13 (0.005) X 0.05 (0.002) 0.089 (0.0035) 0.15 (0.006) 0.216 (0.0085) 0.267 (0.0105) Y 0.08 (0.003) 0.13 (0.005) 0.20 (0.008) 0.267 (0.0105) 0.317 (0.0125) Z 0.08 (0.003) 0.15 (0.006) 0.25 (0.010) 0.394 (0.0155) 0.483 (0.0190) When unfavorable conditions prevail, such as less-than-normal rigidity or restrictions on the use of cutting fluid, slower speeds and lower feed rates than those given in Tables 6(a) and 6(b) may be required. Oversize drilling (Fig. 28 and 29), increases as speed increases. Therefore, drill grinding practice, drilling rigidity, and drill design are closely related to the accuracy that can be obtained at a given speed. Feed rate must not exceed that at which chips can be flushed away. Clogging of chips decreases accuracy and eventually leads to drill breakage. Gun drills ordinarily have carbide tips, which can withstand greater speeds. The usual practice is to operate them at considerably higher speeds but lower feeds than those for high-speed tool steel twist drills. This allows gun drills to form thin chips that are more readily flushed away by cutting fluid under pressure. Nominal speeds and feeds for the gun drilling of ferrous materials with carbide-tip drills are given in Table 7. Table 7 Recommended starting conditions for gun drilling Gun drill diameter Coolant pressure Low- and medium-carbon steels to 200 HB Alloy steel to 240 HB Tool steel to 200 HB mm in. kPa psi rev/ min mm/ min in./ min m m in. (a) rev/ min mm/ min in./ min m m in. (a) rev/ min mm/ min in./ min m m in. (a) 1.9 8 0.07 81 10,3 42 15 00 10,00 0 36 1.4 11 4 4.5 10,00 0 41 1.6 11 4 4.5 8560 33 1.3 13 2 5.2 2.3 8 0.09 38 10,3 42 15 00 10,00 0 46 1.8 12 7 5.0 10,00 0 51 2.0 12 7 5.0 7135 36 1.4 15 0 5.9 3.1 8 0.12 50 10,3 42 15 00 10,00 0 61 2.4 14 7 5.8 9,165 64 2.5 15 5 6.1 5345 38 1.5 20 3 8.0 3.9 7 0.15 62 8,96 3 13 00 10,00 0 76 3.0 16 5 6.5 7,335 64 2.5 19 3 7.6 4280 38 1.5 25 4 10. 0 4.7 6 0.18 75 7,92 9 11 50 9,170 81 3.2 19 0 7.5 6,110 64 2.5 23 4 9.2 3565 36 1.4 30 5 12. 0 5.5 5 0.21 88 7,24 0 10 50 7,860 79 3.1 21 8 8.6 5,240 61 2.4 26 9 10. 6 3055 36 1.4 35 3 13. 9 6.3 5 0.25 00 6,37 8 92 5 6,875 79 3.1 25 4 10. 0 4,585 61 2.4 31 2 12. 3 2675 36 1.4 40 9 16. 1 7.1 4 0.28 12 5,86 1 85 0 6,110 76 3.0 28 7 11. 3 4,075 58 2.3 35 1 13. 8 2375 33 1.3 46 0 18. 1 7.9 4 0.31 25 5,34 4 77 5 5,500 74 2.9 32 0 12. 6 3,665 56 2.2 39 1 15. 4 2140 33 1.3 51 1 20. 1 8.7 3 0.34 38 4,99 9 72 5 5,000 71 2.8 35 1 13. 8 3,335 56 2.2 42 9 16. 9 1945 33 1.3 56 4 22. 2 9.5 3 0.37 50 4,65 4 67 5 4,585 71 2.8 38 4 15. 1 3,055 53 2.1 47 0 18. 5 1780 30 1.2 61 5 24. 2 10. 32 0.40 62 4,30 9 62 5 4,230 69 2.7 41 7 16. 4 2,820 53 2.1 50 8 20. 0 1645 30 1.2 66 5 26. 2 11. 11 0.43 75 4,13 7 60 0 3,930 66 2.6 44 7 17. 6 2,620 51 2.0 54 9 21. 6 1530 30 1.2 71 6 28. 2 11. 91 0.46 88 3,79 2 55 0 3,670 66 2.6 47 2 18. 6 2,445 51 2.0 58 7 23. 1 1425 30 1.2 77 0 30. 3 12. 70 0.50 00 3,62 0 52 5 3,440 64 2.5 51 1 20. 1 2,290 48 1.9 62 7 24. 7 1335 28 1.1 82 0 32. 3 13. 49 0.53 12 3,44 7 50 0 3,235 64 2.5 54 4 21. 4 2,155 48 1.9 66 5 26. 2 1260 28 1.1 87 1 34. 3 14. 29 0.56 25 3,44 7 50 0 3,055 61 2.4 57 7 22. 7 2,035 48 1.9 70 6 27. 8 1190 28 1.1 92 2 36. 3 15. 08 0.59 38 3,27 5 47 5 2,895 61 2.4 61 2 24. 1 1,930 46 1.8 74 9 29. 5 1125 28 1.1 98 0 38. 6 15. 88 0.62 50 3,10 3 45 0 2,750 58 2.3 64 3 25. 3 1,835 46 1.8 78 7 31. 0 1070 25 1.0 10 31 40. 6 16. 67 0.65 62 2,93 0 45 0 2,620 58 2.3 67 6 26. 6 1,745 46 1.8 82 8 32. 6 1020 25 1.0 10 82 42. 6 17. 46 0.68 75 2,93 0 42 5 2,500 56 2.2 70 9 27. 9 1,665 43 1.7 86 6 34. 1 970 25 1.0 11 35 44. 7 18. 25 0.71 88 2,75 8 40 0 2,390 56 2.2 73 4 28. 9 1,595 43 1.7 89 9 35. 4 930 25 1.0 11 79 46. 4 19. 05 0.75 00 2,75 8 40 0 2,290 56 2.2 77 2 30. 4 1,530 43 1.7 94 5 37. 2 890 25 1.0 12 40 48. 8 Gun drill diameter Coolant pressure Type 300 stainless steels to 200 HB Type 400 stainless steel to 240 HB Bronze m m in. kPa psi rev/ min mm/ min in./ min m m in. (a) rev/ min mm/ min in./ min m m in. (a) rev/ min mm/ min in./ min m m in. (a) 1.9 8 0.07 81 10,3 42 15 00 9,780 30 1.2 11 7 4.6 10,00 0 36 1.4 11 4 4.5 10,00 0 41 1.6 11 4 4.5 2.3 8 0.09 38 10,3 42 15 00 8,150 33 1.3 14 0 5.5 10,00 0 46 1.8 12 7 5.0 10,00 0 51 2.0 12 7 5.0 3.1 8 0.12 50 10,3 42 15 00 6,110 33 1.3 18 8 7.4 7,640 46 1.8 17 0 6.7 10,00 0 71 2.8 14 7 5.8 3.9 7 0.15 62 8,96 3 13 00 4,890 33 1.3 23 6 9.3 6,115 46 1.8 21 1 8.3 10,00 0 86 3.4 16 5 6.5 4.7 6 0.18 75 7,92 9 11 50 4,075 33 1.3 28 4 11. 2 5,090 46 1.8 25 7 10. 1 9,170 94 3.7 19 0 7.5 5.5 5 0.21 88 7,24 0 10 50 3,490 30 1.2 33 0 13. 0 4,360 43 1.7 29 5 11. 6 7,860 91 3.6 21 8 8.6 6.3 5 0.25 00 6,37 8 92 5 3,055 30 1.2 38 1 15. 0 3,820 43 1.7 34 0 13. 4 6,875 89 3.5 25 4 10. 0 7.1 4 0.28 12 5,86 1 85 0 2,715 28 1.1 42 9 16. 9 3,395 43 1.7 38 4 15. 1 6,110 89 3.5 28 7 11. 3 7.9 4 0.31 25 5,34 4 77 5 2,445 28 1.1 47 2 18. 8 3,055 41 1.6 42 7 16. 8 5,500 86 3.4 32 0 12. 6 8.7 3 0.34 38 4,99 9 72 5 2,220 28 1.1 52 6 20. 7 2,780 41 1.6 47 0 18. 5 5,000 84 3.3 35 1 13. 8 9.5 3 0.37 50 4,65 4 67 5 2,040 28 1.1 57 4 22. 6 2,545 38 1.5 51 3 20. 2 4,585 81 3.2 38 4 15. 1 10. 32 0.40 62 4,30 9 62 5 1,880 25 1.0 62 2 24. 5 2,350 38 1.5 55 6 21. 9 4,230 81 3.2 41 7 16. 4 11. 11 0.43 75 4,13 7 60 0 1,745 25 1.0 67 1 26. 4 2,180 38 1.5 60 2 23. 7 3,930 76 3.0 44 7 17. 6 11. 91 0.46 88 3,79 2 55 0 1,630 25 1.0 71 9 28. 3 2,035 36 1.4 64 3 25. 3 3,670 76 3.0 48 0 18. 9 12. 70 0.50 00 3,62 0 52 5 1,530 25 1.0 76 7 30. 2 1,910 36 1.4 68 6 27. 0 3,440 74 2.9 51 1 20. 1 13. 49 0.53 12 3,44 7 50 0 1,440 23 0.9 81 5 32. 1 1,800 36 1.4 72 9 28. 7 3,235 74 2.9 54 4 21. 4 14. 29 0.56 25 3,44 7 50 0 1,360 23 0.9 86 4 34. 0 1,700 33 1.3 77 2 30. 4 3,055 71 2.8 57 7 22. 7 15. 08 0.59 38 3,27 5 47 5 1,290 23 0.9 91 4 36. 0 1,610 33 1.3 82 0 32. 3 2,895 71 2.8 61 2 24. 1 15. 88 0.62 50 3,10 3 45 0 1,220 23 0.9 96 5 38. 0 1,530 33 1.3 86 1 33. 9 2,750 69 2.7 64 3 25. 3 16. 67 0.65 62 2,93 0 45 0 1,165 23 0.9 10 13 39. 9 1,455 33 1.3 90 7 35. 7 2,620 66 2.6 67 6 26. 6 17. 46 0.68 75 2,93 0 42 5 1,110 23 0.9 10 62 41. 8 1,390 33 1.3 95 0 37. 4 2,500 66 2.6 70 9 27. 9 18. 25 0.71 88 2,75 8 40 0 1,060 20 0.8 11 02 43. 4 1,330 30 1.2 98 6 38. 8 2,390 66 2.6 73 4 28. 9 19. 05 0.75 00 2,75 8 40 0 1,020 20 0.8 11 56 45. 5 1,275 30 1.2 10 34 40. 7 2,290 66 2.6 77 2 30. 4 Gun drill diameter Coolant pressure Aluminum Gray cast iron to 180 HB Ductile cast iron to 200 HB mm in. kPa psi rev/ min mm/ min in./ min m m in. (a) rev/ min mm/ min in./ min m m in. (a) rev/ min mm/ min in./ min m m in. (a) 1.9 8 0.07 81 10,3 42 15 00 10,00 0 64 2.5 11 4 4.5 10,00 0 91 3.6 11 4 4.5 10,00 0 71 2.8 11 4 4.5 2.3 8 0.09 38 10,3 42 15 00 10,00 0 84 3.3 12 7 5.0 10,00 0 124 4.9 12 7 5.0 10,00 0 97 3.8 12 7 5.0 3.1 8 0.12 50 10,3 42 15 00 10,00 0 122 4.8 14 7 5.8 10,00 0 180 7.1 14 7 5.8 9,165 127 5.0 15 5 6.1 3.9 7 0.15 62 8,96 3 13 00 10,00 0 152 6.0 16 5 6.5 9,780 226 8.9 16 8 6.6 7,335 130 5.1 19 3 7.6 4.7 6 0.18 75 7,92 9 11 50 10,00 0 180 7.1 18 3 7.2 8,150 226 8.9 20 1 7.9 6,110 127 5.0 23 4 9.2 5.5 5 0.21 88 7,24 0 10 50 10,00 0 208 8.2 19 6 7.7 6,985 224 8.8 23 4 9.2 5,240 124 4.9 26 9 10. 6 6.3 5 0.25 00 6,37 8 92 5 10,00 0 231 9.1 21 1 8.3 6,110 218 8.6 26 9 10. 6 4,585 122 4.8 31 2 12. 3 7.1 4 0.28 12 5,86 1 85 0 10,00 0 257 10.1 22 4 8.8 5,435 213 8.4 30 5 12. 0 4,075 119 4.7 35 1 13. 8 7.9 4 0.31 25 5,34 4 77 5 9,780 272 10.7 23 9 9.4 4,890 211 8.3 33 8 13. 3 3,665 117 4.6 39 1 15. 4 8.7 3 0.34 37 4,99 9 72 5 8,890 264 10.4 26 4 10. 4 4,445 206 8.1 37 3 14. 7 3,335 114 4.5 42 9 16. 9 9.5 3 0.37 50 4,65 4 67 5 8,150 259 10.2 28 7 11. 3 4,075 201 7.9 40 6 16. 0 3,055 112 4.4 47 0 18. 5 10. 32 0.40 62 4,30 9 62 5 7,520 254 10.0 31 2 12. 3 3,760 198 7.8 43 9 17. 3 2,820 109 4.3 50 8 20. 0 11. 11 0.43 75 4,13 7 60 0 6,895 249 9.8 33 5 13. 2 3,490 193 7.6 47 5 18. 7 2,620 107 4.2 54 9 21. 6 11. 91 0.46 88 3,79 2 55 0 6,520 244 9.6 36 1 14. 2 3,260 190 7.5 50 8 20. 0 2,445 107 4.2 58 7 23. 1 12. 70 0.50 00 3,62 0 52 5 6,110 239 9.4 38 4 15. 1 3,055 185 7.3 54 4 21. 4 2,290 104 4.1 62 7 24. 7 13. 49 0.53 12 3,44 7 50 0 5,750 234 9.2 40 9 16. 1 2,875 183 7.2 57 7 22. 7 2,155 102 4.0 65 5 26. 2 14. 29 0.56 25 3,44 7 50 0 5,430 229 9.0 43 2 17. 0 2,715 180 7.1 61 2 24. 1 2,035 99 3.9 70 6 27. 8 15. 08 0.59 38 3,27 5 47 5 5,145 226 8.9 45 7 18. 0 2,575 175 6.9 64 8 25. 5 1,930 99 3.9 74 9 29. 5 15. 88 0.62 50 3,10 3 45 0 4,890 221 8.7 48 3 19. 0 2,445 173 6.8 68 3 26. 9 1,835 97 3.8 78 7 31. 0 16. 67 0.65 62 2,93 0 45 0 4,655 218 8.6 50 5 19. 9 2,330 170 6.7 71 6 28. 2 1,745 94 3.7 82 8 32. 6 17. 46 0.68 75 2,93 0 42 5 4,445 216 8.5 53 1 20. 9 2,220 168 6.6 75 2 29. 6 1,665 94 3.7 86 6 34. 1 18. 25 0.71 88 2,75 8 40 0 4,250 211 8.3 51 21. 7 2,125 165 6.5 78 0 30. 7 1,595 91 3.6 89 9 35. 4 19. 05 0.75 00 2,75 8 40 0 4,075 208 8.2 57 9 22. 8 2,040 163 6.4 81 8 32. 2 1,530 91 3.6 94 5 37. 2 Source: Eldorado Tool & Manufacturing Corporation (a) Maximum allowable unsupported gun drill shank length. Length in excess of these values will cause shank to whip (balloon). Cutting Fluids Cutting fluids serve the same purposes in drilling as in other metal cutting operations: to cool workpieces and tools, to flush away chips, and to minimize adherence of tool and work metal. It is suggested that lubrication problems be referred to a reputable manufacturer of cutting oils. The following list of lubricants should be used as suggestions only: • Aluminum and aluminum alloys: Kerosene, kerosene and lard oil, and soluble oil • Brass and bronze: Dry. Deep holes: kerosene and mineral oil, lard oil, and soluble oil • Magnesium and magnesium alloys: Mineral lard oil, kerosene, or dry • Copper: Mineral lard oil and kerosene, soluble oil, or dry • Monel metal: Mineral lard oil • Low-carbon steels: Mineral lard oil • Tough alloy steels: Sulfurized oil • Steel forgings: Sulfurized oil • Cast steel: Soluble oil • Wrought iron: Soluble oil • High-tensile steels: Soluble oil • Manganese steel: Dry • Cast iron: Mineral oil • Malleable iron: Soluble oil or dry • Stainless steel: Soluble oil • Titanium alloys: Soluble oil • Tool steel: Mineral lard oil • Abrasives, plastics: Dry • Fiber, asbestos, wood: Dry • Hard rubber: Dry Detailed information on types of cutting fluids and principles of selection and use are described in the article "Metal Cutting and Grinding Fluids" in this Volume. Cutting fluids are used in most drilling applications, except on such materials as cast iron (for which an air jet may be used instead) or when the use of fluids is incompatible with subsequent operations or the end use of the part. The use of sulfurized cutting oils is almost mandatory for gun drilling operations because of the more accurate dimensions and smoother finishes that are usually required. Variables Affecting Drill Life Apart from drill design and material, and rigidity of the setup, the variables that most affect the service lives of drills are speed and feed and the hardness and composition of the work metal. Speed and Feed. Figure 30 relates speed and feed to drill life. The curves shown in Fig. 30(a) for drilling 4130 and 4340 steels at 341 HB (37 HRC) indicate that: • Drill life decreased rapidly as cutting speed was increased • At a given cutting speed, drill life was shortened when the feed rate was increased from 0. 05 to 0.13 mm/rev (0.002 to 0.005 in./rev) Fig. 30 Effect of speed and feed on drill life. Holes for both series of tests were drilled through 13 mm ( in.) thick specimens 75 to 102 mm (3 to 4 in.) in diameter, with 6.4 mm ( in.) diam drills (118° included point angle, 29° helix angle, 7° clearance), using a 1:1 mixture of thread- cutting oil and light machine oil as cutting fluid. Drills for data in (a) were 102 mm (4 in.) long and made of M2 high- speed tool steel. Drills for data in (b), (c), and (d) were 70 mm (2 in.) long, of T15 high-speed tool s teel, and ground with crankshaft points. The end point of drill life was the breakdown of the drill or a 0.38 mm (0.015 in.) wear land on the drill margin, whichever occurred first. Tests on 4340 steel at 514 HB (52 HRC), as shown in the bar graphs of Fig. 30(b), 30(c), and 30(d), indicate trends somewhat different from those observed for the softer steels. Maximum drill life was found at the intermediate feed rate of 0.025 mm/rev (0.001 in./rev), with shorter drill life observed at either a higher or a lower feed rate. Only at 0.025 mm/rev (0.001 in./rev) feed did drill life decrease progressively as speed was increased; at both the higher and the lower feed rates, maximum drill life was obtained at a cutting speed of 9.1 m/min (30 sfm). The use of a light feed reduces cutting temperature and cutting force. However, if the feed rate is reduced by half, the area of chip passing over the cutting edge is doubled. As a result, tool wear is likely to increase. Therefore, whether or not reducing the feed rate is advantageous depends on whether the lower cutting temperature and lighter cutting force offset the increased area of chip passing over the cutting edge. At high cutting speed, for which cutting temperature is a critical factor in drill life, reducing the feed is advantageous. At low cutting speed, for which cutting temperature is less critical, the longer chip and the increased rubbing over the cutting edge are likely to offset the advantages of lower feed, and drill life is likely to decrease. Hardness and Composition of Work Metal. The effect of workpiece material on tool life in drilling carbon and low-alloy steels similar in composition and microstructure can be interpreted in terms of hardness. The results of one comparison of this type are given in Fig. 31. Although these results were affected to some degree by the use of a different tool material for drilling the softest steel, drill life (the amount of metal removed before the development of a predetermined wear land on the drill) was progressively shorter for increasing hardness of the steel being drilled. Fig. 31 Effect of workpiece hardness on drill life for 6.4 mm ( in.) (top row) and 13 mm ( in.) (bottom row) diam holes. Factors considered were cutting speed (a), feed rate (b), removal rate (c), and drill life (d). Comparative drilling tests were conducted on three similar steels, each at a different hardness. On each steel, the tools and machining conditions used had been previously determined to provide maximum economy in drilling that material. Drill life was based on the development of a predetermined amount of wear on the edge of the drill. The composition of carbon and low-alloy steels is usually of only secondary importance in its direct effects on drill life. Effects of practical significance include those of free-cutting additives, differences of 0.10% or more in carbon content, and substantial differences in the content of alloying elements. Some of these differences in composition also produce changes in hardness. Composition is of primary importance when it is necessary to improve drill life by heat treatment or cold reduction of the work material, or by selection of a more suitable work material. Effect of Workpiece Hardness on Cost The hardness of the workpiece material exerts a major influence on drilling cost. Meaningful comparisons of the effect of hardness on drilling cost can be made for steels that are similar in composition and microstructure. Drilling Cost. Although drilling cost almost always increases with increasing hardness in the range above 35 HRC (330 HB), the reverse effect is observed for low-carbon steel at lower hardness. For low-carbon steel, there is usually an optimum hardness range for lowest drilling cost; this range varies considerably according to composition. For some low- carbon steels, a heat-treating operation is warranted to increase hardness before drilling. Overall Cost. In considering overall economy of manufacture, cost-per-pound differences among mechanically equivalent steels should be evaluated, along with the differences in cost of machining the steels. Even when machining costs for a grade of steel that is difficult to drill are much greater than for a similar grade that is not, the machining costs may be outweighed by the difference in cost per pound of the two steels. The heavier the part and the less machining done on it, the more important material cost is as a factor in total cost. Determination of Optimum Speed and Feed A wide range of speeds and feeds can be employed to yield acceptable results in the drilling of most materials, particularly those that present no unusual difficulty in machining. For materials that are unusually hard, soft and gummy, abrasive, or otherwise difficult to machine, the range of economical operating conditions is narrow. For these materials, an optimum speed and feed can be determined by making drilling tests to establish the conditions for minimum cost per cubic inch of metal removed. Optimum drilling conditions are determined by changing one variable (speed or feed) while holding the other constant. The first step is to select a near-optimum feed. Speed is then adjusted in increments to find the most economical speed. Selection of the near-optimum feed is based on previous experience. After this first series of operations (the speed search), speed is held constant, and feed is varied in increments to verify or correct the initially selected feed rate (the feed search). The development of a predetermined wear land on the edge of the drill is the criterion for tool life. Different speeds and feeds cause wear at different rates. Attainment of a predetermined amount of wear, which is ascertained by the use of a hand microscope, marks the point at which each test is stopped. Cost Factors. In such a study, three factors are included in the determination of machining costs: tool-use cost, tool- change cost, and operating cost. Operating cost is based on a standard cost per minute for all operations. Tool-change cost is obtained by the use of a specific rate of tool changing assigned to the operation. Tool-use cost includes the cost of new tools and the cost of regrinding. Total cost per hole drilled for each test condition is the sum of the following: • Operating cost times the number of minutes required to drill one hole • Tool-change cost divided by tool life in holes per tool • Tool-use cost divided by tool life in holes per tool Dividing this total by the volume of metal removed per hole gives the drilling cost in dollars per cubic inch of metal removed for each test condition. Testing to Evaluate Drilling Conditions Laboratory tests are usually conducted to develop and evaluate drilling techniques and drill designs and materials, without concern for machines and fixtures. This requires close control over test conditions (especially rigidity) and the provision of adequate, smooth power to the drill. Production tests are used to evaluate drill performance in combination with drilling machines and fixtures. Accelerated tests may lead to erroneous conclusions because the wear or failure may be different in practice. Variation Among Drills. There is considerable variation in performance among drills of the same type and even on resharpening a single drill. Therefore, enough drills should be tested to allow for this variation, and drills should be sharpened often enough to represent shop practice. Sharpening should reproduce the original point of the drill and should include removal of the part of the drill that has metal pickup on the margin or has reverse back taper. The web should be thinned to its original dimensions, and the resharpened drill should be free from burns and checks. Criteria for Drill Life. Preliminary testing is often necessary to determine realistic criteria for drill life. Workpiece material, cutting fluid, and operating conditions influence the type of failure. Useful criteria include drill noise, drill wear, inability to cut, total failure of drill, inaccurate hole size, poor hole finish, burrs, drill breakage, and increase in the amount of torque required for a given drilling operation. Interpretation of Results. Because of variations in drills, test conditions, and work material, a substantial number of tests usually must be made. As an illustration, differences of 30% or more between groups of identical drills can be detected reliably with about six drills of each type run through several sharpenings. Statistical analysis can indicate whether observed differences are significant. Caution must be exercised in interpreting test results because of the wide discrepancies shown by machining tests in general. For example, consider the data shown in Fig. 30(a). Results were essentially the same in drilling 4130 and 4340 steel at 0.13 mm/rev (0.005 in./rev) feed, but speed for a given drill life was 40 to 50% higher for 4130 at 0.051 mm/rev (0.002 in./rev) feed. It could validly be concluded from these results that this lot of 4130 could be drilled at higher speeds than 4340 at a feed of about 0.051 mm/rev (0.002 in./rev). However, no conclusion could be drawn about 4130 and 4340 steels in general without additional testing. Drilling Steel Having 48 to 55 HRC Hardness Successful drilling of steel that has been heat treated to a hardness of 48 to 55 HRC depends mainly on the design of the drill, the rigidity of the machine setup, and the choice of tool material. As a rule, improvements in drill design and in machine and workpiece rigidity are of more direct benefit to drilling performance than is a change in tool material. A number of other conditions may also affect drill performance, and in certain cases, they may become critical. Among these conditions are feed and speed, the efficiency and adaptability of power equipment (portable or stationary), and the accuracy of working surfaces of the drill. The following three examples illustrate the effects of operating variables in drilling hardened steel. Example 5: Effect of Nitriding on the Drill Life of a High-Speed Tool Steel Drill. Blind holes, 6.91 mm (0.272 in.) in diameter and 15.75 mm (0.620 in.) deep, were drilled in 4335 steel hardened to 48 to 50 HRC. The drills had a life of only one to four holes per sharpening, and a breakage rate of 1 drill for every 16 to 20 holes. When the drills were nitrided, drill life was increased to 18 to 20 holes per sharpening, and drill breakage was reduced to 1 drill for every 50 to 60 holes. All drills were standard heavy-duty drills ground from solid heat-treated high-speed steel. Each had a split point ground with a positive axial rake of 3 to 4° on the split portion, an included point angle of 135° and a lip relief angle of 5 to 7°, ground flat 0.8 mm ( in.). Drilling was done in a radial-arm drill press with multiple speeds and automatic feed. Speed was 147 rev/min (3.20 m/min, or 10.5 sfm), and feed was 0.05 mm/rev (0.002 in./rev). Example 6: High-Speed Tool Steel Versus Carbide Drills. For H11 steel at 50 HRC or below, M33, M34, and M36 high-speed tool steel drills of heavy-duty construction gave good results. Positive-drive equipment was used, with drill speeds of 7.6 to 9.1 m/min (25 to 30 sfm) and feeds of 0.013 to 0.018 mm/rev (0.0005 to 0.0007 in./rev). Material harder than 50 HRC required solid-carbide drills with short flutes. Speeds of 12 to 15 m/min (40 to 50 sfm) and feeds of 0.013 to 0.025 mm/rev (0.0005 to 0.001 in./ rev) produced satisfactory results. The design of the solid-carbide twist drills used for holes of less than 6.4 mm ( in.) diameter in this material is illustrated in Fig. 32(a) and 32(b); the design of straight-flute solid-carbide drills, for holes 6.4 mm ( in.) in diameter or larger, is shown in Fig. 32(c). [...]... C-2(c) - 185365 - 1 4 9- 7 5 1 -3 1 9- 3 0 Cast iron 1 3236.5 1 1 3847.5 1 1 4 9- 7 5 1 Stainless steels (Types 303, 304 and 316, 416) -3 1 9- 3 0 1 3236.5 1 - 1 3847.5 1 - 1 4 9- 7 5 1 -3 1 9- 3 0 Nitralloy 135 4 5-1 20 - 4 5-1 20 1 3236.5 1 - 1 3847.5 1 - 1 4 9- 7 5 1 -3 1 9- 3 0 Inconel 100 1 3236.5 1 - 1 3847.5 1 - 1 4 9- 7 5 1 Aluminum: 2017T 2024T, 6061T and -3 1 9- 3 0 1 3236.5 1 1 3847.5 1 1 4 9- 7 5 1 -3 185365 6001200 185 490 ... 6 0-1 85 200600 200400 6 0-2 00 200650 300600 0.150.25 0.080.13 0.0060.010 0.0030.005 SNMMND SPGM C-5(b) C-5(b) 200400 9 0-1 85 300600 0.100.15 0.0040.006 SNMG C-5(b) 1 3236.5 1 - 1 3847.5 1 - 1 4 9- 7 5 1 Free -machining steels (1100 and 1200 series) -3 1 9- 3 0 1 3236.5 1 1 3847.5 1 1 4 9- 7 5 1 Alloy steels (4000, 5000, 8000 and 90 00 series) -3 1 9- 3 0 1 3236.5 1 - 1 3847.5 1 - 1 4 9- 7 5 1 Tool steel -3 1 9- 3 0 6 0-1 20... 0.0050.007 0.0030.004 C-5(b) 2 5-3 5 150200 7 5-1 10 SNMG 2 5-3 5 SPGM C-2(c) 2 5-3 5 7 5-1 10 2 5-3 5 7 5-1 10 0.080.10 0.0030.004 SNMG C-2(c) 2 5-3 5 7 5-1 10 2 5-3 5 7 5-1 10 0.080.10 0.0030.004 SNMG C-2(c) 2 5-3 5 1 7 5-1 10 2 5-3 5 7 5-1 10 C-2(c) 6001200 185 490 60 0160 0 0.0030.004 0.0080.010 SNMG 185365 0.080.10 0.200.25 SPGM C-2(c) 6001200 185 490 60 0160 0 0.200.30 0.0080.012 SNMMND C-2(c) - 185365 6001200 185 490 60 0160 0 0.250.38 0.0100.015... 6 0-1 20 - 9 0-1 85 1 3236.5 6 0-1 20 1 1 - 3847.5 6 0-1 20 200400 9 0-1 85 300600 0.130.20 0.0050.008 SNMMND C-5(b) 6 0-1 20 9 0-1 85 120275 300600 40 090 0 0.150.25 0.100.20 0.0060.010 0.0040.008 SNMMND SPGM C-5(b) 120245 200400 400800 C-2(c) 120245 400800 120275 40 090 0 0.130.25 0.0050.010 SNMG C-2(c) - 120245 400800 120275 40 090 0 0.150.30 0.0060.012 SNMMND C-2(c) - 120245 4 5-1 05 400800 150350 120275 4 5-1 50 40 090 0... titanium-coated carbide inserts Material Carbon steel (1000 series) Drill size mm 1 9- 3 0 Speed Standard length m/min sfm 9 0-2 00 300650 9 0-2 00 300650 9 0-2 45 9 0-2 00 300650 9 0-2 00 in Stub length m/min sfm 9 0-2 45 300800 Feed Insert style(a) Grade SPGM C-5(b) mm/rev 0.080.13 in./rev 0.0030.005 300800 0.080.15 0.0030.006 SNMG C-5(b) 9 0-2 45 300800 0.100.20 0.0040.008 SNMMND C-5(b) 9 0-2 45 120275 300800 40 090 0 0.130.25... SNMMND SPGM C-2(c) C-2(c) 4 5-1 05 150350 4 5-1 50 150500 0.100.13 0.0040.005 SNMG C-5(b) 4 5-1 05 150350 4 5-1 50 150500 0.100.15 0.0040.006 SNMMND C-5(b) 4 5-1 05 150350 150200 4 5-1 50 150500 150200 0.130.20 0.130.18 0.0050.008 0.0050.007 SNMMND SPGM C-5(b) C-5(b) 4 5-1 20 150200 4 5-1 20 150200 0.130.18 0.0050.007 SNMG C-5(b) 4 5-1 20 150200 4 5-1 20 150200 0.130.18 0.0050.007 SNMG C-5(b) 4 5-1 20 150200 7 5-1 10 4 5-1 20 0.130.18... C-5(b) 120215 300650 400700 C-5(b) 400700 120275 40 090 0 0.100.15 0.0040.006 SNMG C-5(b) - 120215 400700 120275 40 090 0 0.080.20 0.0050.008 SNMMND C-5(b) - 120215 120215 6 0-1 85 400700 200600 120275 6 0-2 00 40 090 0 200650 0.150.30 0.080.13 0.0060.012 0.0030.005 SNMMND SPGM C-5(b) C-5(b) 6 0-1 85 200600 6 0-2 00 200650 0.100.15 0.0040.006 SNMG C-5(b) 6 0-1 85 200600 6 0-2 00 200650 0.130.20 0.0050.008 SNMMND C-5(b)... for each grade of carbide is plotted in the lower part Composition, hardness, and grain size of the three grades of carbide were as follows: Grade C-1 C-2 C-3 Composition % W Ta C 88.25 5.75 88.25 5.75 87.00 4.0 6.00 Co 6.0 6.0 3.0 Hardness, HRA 91 .2 91 .8 92 .2 Grain Size, m ( in.) 3 (120) 1-2 (4 0-8 0) 1-3 (4 0-1 20) Fig 33 Top: design of carbide-tip drills used to drill H11 steel sheet at 54 HRC... Reamer-blade life per grind 6 M2 high-speed steel 0.2 (0.008) 0.002 (0.002) 9 (30) 0. 89 (0.035) 0. 2-0 .25 (0.00 8-0 .01) Soluble oil:water (1:20) 0. 19 min 800 holes Fig 8 Reaming of six stud holes in a wheel hub with a straight-flute inserted-blade adjustable reamer Dimensions in figure given in inches Inserted-blade adjustable reamers with straight flutes were used Each reamer held six M2 high-speed... piece Example 4: Shell Versus Solid Reamers for Gray Iron In one plant, solid reamers were compared with shell reamers for reaming 29. 97/ 29. 95 mm (1.1 798 /1.1 790 in.) diam shaft holes in gray iron water-pump bodies (131 to 207 HB) Specifications called for maximum out-of-roundness of 0.0075 mm (0.0003 in.), maximum taper of 0.005 mm (0.0002 in.), and finish of 1.5 m (60 in.) Figure 12 shows the workpiece . 9 0-2 45 30 0- 800 0.0 8- 0.13 0.00 3- 0.005 SPGM C-5 (b) 3 2- 36.5 1 - 1 9 0-2 00 30 0- 650 9 0-2 45 30 0- 800 0.0 8- 0.15 0.00 3- 0.006 SNMG C-5 (b) 3 8- 47.5 1 - 1 9 0-2 00 30 0- 650 9 0-2 45. 4 9- 7 5 1 -3 6 0-1 20 20 0- 400 9 0-1 85 30 0- 600 0.1 5- 0.25 0.00 6- 0.010 SNMM- ND C-5 (b) 1 9- 3 0 - 1 12 0- 245 40 0- 800 12 0- 275 40 0- 90 0 0.1 0- 0.20 0.00 4- 0.008 SPGM C-2 (c) . 316, 416) 4 9- 7 5 1 -3 4 5-1 05 15 0- 350 4 5-1 50 15 0- 500 0.1 3- 0.20 0.00 5- 0.008 SNMM- ND C-5 (b) 1 9- 3 0 - 1 4 5-1 20 15 0- 200 4 5-1 20 15 0- 200 0.1 3- 0.18 0.00 5- 0.007 SPGM C-5 (b)