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23.1 Introduction

Chapter 23 Drilling and Related Hole-Making Processes (Review) EIN 3390 Manufacturing Processes Spring, 2012. Drilling is most common single machining operation Drilling makes up 25% of machining Most drilling tools have two cutting edges , or lips .

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23.1 Introduction

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  1. Chapter 23 Drilling and Related Hole-Making Processes (Review) EIN 3390 Manufacturing ProcessesSpring, 2012

  2. Drilling is most common single machining operation • Drilling makes up 25% of machining • Most drilling tools have two cutting edges, or lips. • Cutting action takes place inside the workpiece • The only exit for chips is the hole that is mostly filled by drill • Friction between the margin and the hole wall produces heat which is additional to that due to chip formation 23.1 Introduction

  3. FIGURE 23-1 Nomenclature and geometry of conventional twist drill. Shank style depends upon the method used to hold the drill. Tangs or notches prevent slippage: (a) straight shank with tang, (b) tapered shank with tang, (c) straight shank with whistle notch, (d) straight shank with flat notch. Nomenclature and Geometry of a Drill

  4. FIGURE 23-1 Nomenclature and geometry of conventional twist drill. Shank style depends upon the method used to hold the drill. Tangs or notches prevent slippage: (a) straight shank with tang, (b) tapered shank with tang, (c) straight shank with whistle notch, (d) straight shank with flat notch. Nomenclature and Geometry of a Drill

  5. A conventional two-flute drill, with drill of diameter D, has two principal cutting edges rotating at an rpm rate of N and feeding axially. • The rpm of the drill is established by the selected cutting velocity or cutting speed with V in surface feet per minute and Din inches. 23.2 Fundamentals of the Drilling Process

  6. FIGURE 23-2 Conventional drill geometry viewed from the point showing how the rake angle varies from the chisel edge to the outer corner along the lip. The thrust force increases as the web is approached. Conventional Drill Geometry

  7. Four actions take place a the drill tip • 1. A small hole is formed by the web and chips are not cut here in the normal sense. • 2. Chips are formed by the rotating lips. • 3. Chips are removed from the hole by the screw action of the helical flutes. • 4. The drill is guided by lands or margins that rub against the walls of the hole • New drill-point geometry and TiN coating have resulted in improved hole accuracy, longer life, increased feed-rate capabilities. • US manufacturing companies consume 250 million twist drills per year. 23.2 Fundamentals of the Drilling Process

  8. A conventional two-flute drill with diameter D, has two principal cutting edges rotating at rpm rate of Ns and feed fr. Ns = (12v)/(p D) Where V – cutting speed at the outer cornerof the cutting lip (point X in Fig. 23 -2)in surface, feet per minute, D – diameter of drill in inch. The depth of cut in drilling is a half of the feed rate, or t = fr/2 (see section A – A in Fig. 23-2), where is in inches per revolution. 23.2 Fundamentals of Drilling process

  9. The length of cut in drilling equals the depth of the hole, L, plus an allowance for approach and for the tip of drill, usually A = D/2. • In drilling, the speed and feed depend upon the material of workpiece, the cutting tool material, and the size of drill. • Table 23-1 gives some typical values for V and for carbide indexable insert drills. • The maximum velocity is at the extreme ends of the drill lips. The velocity is very small near the center of the chisel end of the drill. • Cutting time: Tm = (L + A)/(frNs)= (L +A)/fm where fm is the feed rate inches per minute. 23.2 Fundamentals of Drilling process

  10. The material removal rate (MRR) for drilling is: • Which reduces to or Where Tm is cutting time, fr is feed rate, and L is depth of the hole. /min Material Removal Rate

  11. A cast iron plate is 2” thick, and needs 1” diameter hole drilled in it. An indexable insert drill has been selected. 1) Select cutting speed and feed; 2) the spindle Ns and feed rate (in/min.); 3) the maximum chip load (depth of cut); 4) MRR; and 5) the total motor horsepower (HPs = 0.33). Solution: • From table 23-1, select a cutting speed of 200 fpm, and a feed of 0.005 ipr. • Ns = (12v)/(p D) = (12 x 200)/ (3.14 x 1) = 764 rpm, pick 750 rpm. 23.2 Fundamentals of Drilling process

  12. 2) Feed rate fm = fr x Ns = 0.005 x 750 = 3.75 in /min, and pick fm = 3.5 in/min. 3) the maximum chip load (depth of cut) t = fr/2 = 0.005/2 = 0.0025 in. 4) MRR = (p/4) (D2) fm = (3.14/4) x 12 x 3.5 = 2.75 in3/min. 5) HP =HPs x MRR = 0.33 x 2.75 = 0.9(horsepower) The value would typically represent 80% of the total motor horsepower needed, so in this case a horsepower motor greater than 1.5 or 2 would be sufficient. HP = 1.6 x 0.9 = 1.5 (horsepower) 23.2 Fundamentals of Drilling process

  13. The most common drills are twist drills • Twist drills have three parts • Body: consisting of two or more spiral grooves called flutes, separated by lands. Flutes serve as channels through which chips are withdrawn from hole and coolant gets to cutting edges. • Point: a wide variety of geometry are used, but typically have a cone angel of 118°, and a rake angle of 24° • Shank: a straight or tapered section where the drill is clamped. 23.3 Types of Drills

  14. FIGURE 23-3 Types of twist drills and shanks. Bottom to top: Straight-shank, three-flute core drill; taper-shank; straight-shank; bit-shank; straight-shank, high-helix angle; straight-shank, straight-flute; taper-shank, subland drill. Types of Twist Drills

  15. Standard drills have a straight line chisel point. • This point caused drills to “walk” along the surface • This effect is counter by using centering techniques • Center punches • Pre-drilled guide holes for large holes • Specialized methods of grinding the point address walking Drill Walking

  16. Specialized tips are used to produce self centering holes where hole position is critical. • Helical tips • Four-facet tips • Racon • Bickford • Center core, or slot drills • Used in machining centers and high speed automatic NC systems where manual center punching is impractical Specialized Tips

  17. FIGURE 23-4 As the drill advances, it produces a thrust force. Variations in the drill-point geometry are aimed at reducing the thrust force. Drill Point Geometry

  18. Center Core Drill FIGURE 23-5 Center core drills can greatly reduce the thrust force.

  19. Typical Causes of Drilling Problems

  20. Standard drills typically are used to produce holes with a depth to diameter ratio of 3:1 • Deeper holes result in drift of the tool decreasing hole straightness • Specialized drills called deep-hole drills or gundrills are used for greater ratios • Gundrills are single tipped tools with a coolant channel delivering coolant to the tip and flushing chips to the surface • Ratios of 100:1 are possible with gundrills Depth-to-Diameter Ratio

  21. FIGURE 23-10 To obtain a hole that is accurate as to size and aligned on center (located), this 4 step sequence of operations is usual. Steps to High Accuracy Holes with Conventional Drills

  22. Hole cutters: used for holes in sheet stock • Subland drills: used for multi diameter holes • Spade drills: used for holes over 1 inch • Indexable drills: used for high speed shallow holes in solid stock • Micro drills (pivot drills): used for holes 0.02 to 0.0001 inch diameter where grain boundaries and inclusion produce non-uniform material properties Specialty Drills

  23. Subland Drill FIGURE 23-11 Special purpose subland drill (above), and some of the operations possible with other combination drills (below).

  24. Straight-shank drills are typically held in chucks • Three-jaw jacobs chucks: used on manual drill presses, require used of a key • Collet chuck: used with carbide tools where high bearing thrust is used • Quick change chucks: used were rapid change is needed • Tapered shank drills held in mores taper of the machine spindle 23.4 Tool Holders for Drills

  25. FIGURE 23-17 Here are some suggestions for correct chucking of carbide drills. Correct Chucking of Carbide Drills

  26. For prototype pieces, stock material is held in simple clamping vises • For high production rates, custom jigs are used • Stock material is never to be held on the work table by hand 23.5 Workholding for Drilling

  27. Drilling can be performed on: • Lathes • Vertical mills • Horizontal mills • Boring machines • Machine centers • Specialized machines designed specifically for drilling called “drill presses” 23.6 Machine Tools for Drilling

  28. Drill presses must have sufficient power and thrust to perform cut • Drill presses must be rigid enough to prevent chatter • Drill press consist of a base, a work table, and a column that supports the powerhead and spindle Requirements of a Drill Press

  29. Reams remove small amounts of material to ensure exact hole size and improve hole surface finish • Reams are either hand operated or machined at slow speed • Ream types • Shell reams • Expansion reams • Adjustable reams • Tapered reams 23.9 Reaming

  30. Drilling is the most common machining operation • Drilling can be performed on a number of machine tools, drill presses are specialized machine tools for drilling only • Drills come in a wide variety of types and tip geometries depending upon production rate and accuracy needed • Hole geometries can be adjusted through the use of counterboring, countersinking and reaming Summary

  31. Review Questions: 1, 2, 10, and 25 (page 653-654) Problems (page 654): 3, 5, 7, 9 HW for Chapter 23

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