Presentation on theme: "Chapter 23 Drilling and Related Hole-Making Processes (Review) EIN 3390 Manufacturing Processes Spring, 2012 1."— Presentation transcript:
1 Chapter 23 Drilling and Related Hole-Making Processes (Review) EIN Manufacturing Processes Spring, 20121
2 23.1 Introduction Drilling is most common single machining operation Drilling makes up 25% of machiningMost drilling tools have two cutting edges, or lips.Cutting action takes place inside the workpieceThe only exit for chips is the hole that is mostly filled by drillFriction between the margin and the hole wall produces heat which is additional to that due to chip formation
3 Nomenclature and Geometry of a Drill FIGURE 23-1 Nomenclature andgeometry of conventional twist drill.Shank style depends upon themethod used to hold the drill. Tangsor 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.
4 Nomenclature and Geometry of a Drill FIGURE 23-1 Nomenclature andgeometry of conventional twist drill.Shank style depends upon themethod used to hold the drill. Tangsor 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.
5 23.2 Fundamentals of the Drilling Process 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 D in inches.
6 Conventional Drill Geometry FIGURE 23-2 Conventionaldrill geometry viewed from thepoint showing how the rakeangle varies from the chisel edgeto the outer corner along the lip.The thrust force increases as theweb is approached.
7 23.2 Fundamentals of the Drilling Process Four actions take place a the drill tip1. 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 holeNew 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.
8 23.2 Fundamentals of Drilling process 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 )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 ), where is in inches per revolution.
9 23.2 Fundamentals of Drilling process 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)/fmwhere fm is the feed rate inches per minute.
10 Material Removal Rate The material removal rate (MRR) for drilling is: Which reduces toorWhere Tm is cutting time, fr is feed rate, and L is depth of the hole./min
12 23.2 Fundamentals of Drilling process 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 ipr.Ns = (12v)/(p D) = (12 x 200)/ (3.14 x 1) = 764 rpm, pick 750 rpm.
13 23.2 Fundamentals of Drilling process 2) Feed rate fm = fr x Ns = 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 = in.4) MRR = (p/4) (D2) fm = (3.14/4) x 12 x 3.5 = 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)
14 23.3 Types of Drills The most common drills are twist drills Twist drills have three partsBody: 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.
15 Types of Twist Drills FIGURE 23-3 Types of twist drills and shanks. Bottom to top:Straight-shank, three-flute coredrill; taper-shank; straight-shank;bit-shank; straight-shank,high-helix angle; straight-shank,straight-flute; taper-shank,subland drill.
16 Drill Walking Standard drills have a straight line chisel point. This point caused drills to “walk” along the surfaceThis effect is counter by using centering techniquesCenter punchesPre-drilled guide holes for large holesSpecialized methods of grinding the point address walking
17 Specialized TipsSpecialized tips are used to produce self centering holes where hole position is critical.Helical tipsFour-facet tipsRaconBickfordCenter core, or slot drillsUsed in machining centers and high speed automatic NC systems where manual center punching is impractical
18 Drill Point Geometry FIGURE 23-4 As the drill advances, it produces a thrustforce. Variations in the drill-pointgeometry are aimed at reducingthe thrust force.
19 Center Core Drill FIGURE 23-5 Center core drills can greatly reduce the thrust force.
21 Depth-to-Diameter Ratio Standard drills typically are used to produce holes with a depth to diameter ratio of 3:1Deeper holes result in drift of the tool decreasing hole straightnessSpecialized drills called deep-hole drills or gundrills are used for greater ratiosGundrills are single tipped tools with a coolant channel delivering coolant to the tip and flushing chips to the surfaceRatios of 100:1 are possible with gundrills
22 Steps to High Accuracy Holes with Conventional Drills FIGURE To obtain ahole that is accurate as to sizeand aligned on center (located),this 4 step sequence ofoperations is usual.
23 Specialty Drills Hole cutters: used for holes in sheet stock Subland drills: used for multi diameter holesSpade drills: used for holes over 1 inchIndexable drills: used for high speed shallow holes in solid stockMicro drills (pivot drills): used for holes to inch diameter where grain boundaries and inclusion produce non-uniform material properties
24 Subland DrillFIGURE Special purpose subland drill (above), and some of the operations possible with other combination drills (below).
27 23.4 Tool Holders for Drills Straight-shank drills are typically held in chucksThree-jaw jacobs chucks: used on manual drill presses, require used of a keyCollet chuck: used with carbide tools where high bearing thrust is usedQuick change chucks: used were rapid change is neededTapered shank drills held in mores taper of the machine spindle
28 Correct Chucking of Carbide Drills FIGURE Here are some suggestions for correct chucking of carbide drills.
29 23.5 Workholding for Drilling For prototype pieces, stock material is held in simple clamping visesFor high production rates, custom jigs are usedStock material is never to be held on the work table by hand
30 23.6 Machine Tools for Drilling Drilling can be performed on:LathesVertical millsHorizontal millsBoring machinesMachine centersSpecialized machines designed specifically for drilling called “drill presses”
31 Requirements of a Drill Press Drill presses must have sufficient power and thrust to perform cutDrill presses must be rigid enough to prevent chatterDrill press consist of a base, a work table, and a column that supports the powerhead and spindle
33 23.9 ReamingReams remove small amounts of material to ensure exact hole size and improve hole surface finishReams are either hand operated or machined at slow speedReam typesShell reamsExpansion reamsAdjustable reamsTapered reams
34 Summary 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 onlyDrills come in a wide variety of types and tip geometries depending upon production rate and accuracy neededHole geometries can be adjusted through the use of counterboring, countersinking and reaming
35 HW for Chapter 23Review Questions:1, 2, 10, and 25 (page )Problems (page 654):3, 5, 7, 9