Drilling & Related Operations

LECTURE-03 DRILLING & RELATED OPERATIONS

NIKHIL R. DHAR, Ph. D. DEPARTMENT OF INDUSTRIAL & PRODUCTION ENGINEERING BUET BANGLADESH

Introduction

Drilling is most common single machining operation Drilling makes up 25% of machining Drilling occurs at the end of a tool within the material, four actions take place a the drill tip 

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A small hole is formed by the web— chips are not cut here in the normal sense. Chips are formed by the rotating lips. Chips are removed from the hole by the screw action of the helical flutes. The drill is guided by lands or margins that rub against the walls of the hole

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Definition Drill can be defined as a rotary end cutting tool having one or more cutting lips, and having one or more helical or straight flutes for the passage of chips and the admission of a cutting fluid.

Nomenclature and geometry of conventional twist drill Department of Industrial & Production Engineering

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Nomenclature and Geometry of a Twist Drill Three main components are the:  Shank  Body  Point 

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Shank - This is the portion which is “clamped” to provide the drive. Straight shank for drill up to ½”. Shank is equal to body diameter. Above ½”, shank can be tapered or reduced. Body  Flutes - Helical grooves cut around the body which form the cutting edges Allow coolant to flow to the cutting edge Allow chips to be withdrawn  Margin- Narrow raised section of the body. Provides full body to hole support to help keep it aligned as it drills.  Body clearance - Reduced section of the drill between the flutes and margin. Used to reduce friction between drill and workpiece

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Web- Thin section in the center of the drill which forms a “core” for the drill. This feature increases as it extends to the shank. Forms the chisel edge of the drill. Point - Chisel edge of the drill.  The spiral, or rate of twist in the drill, controls the rate of chip removal in a drill. A fast spiral drill is used in high feed rate applications under low spindle speeds, where removal of a large volume of swarf is required. Low spiral drills are used in cutting applications where high cutting speeds are traditionally used, and where the material has a tendency to gall on the drill or otherwise clog the hole, such as aluminum or copper. 

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The point angle is determined by the material the drill will be operating in. Harder materials require a larger point angle, and softer materials require a sharper angle. The correct point angle for the hardness of the material controls wandering, chatter, hole shape, wear rate, and other characteristics. The lip angle determines the amount of support provided to the cutting edge. A greater lip angle will cause the drill to cut more aggressively under the same amount of point pressure as a drill with a smaller lip angle. Both conditions can cause binding, wear, and eventual catastrophic failure of the tool. The proper amount of lip clearance is determined by the point angle.

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Classifications Classification Based on Construction  

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Solid Drills: Those made of one piece of material such as high speed steel Tipped Solid Drills: Those having a body of one material with cutting lips made of another material brazed or otherwise bonded in place Composite Drills: Those having cutting portions mechanically held in place

Classification Based on Methods of Holding or Driving 

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Straight Shank Drills: Those having cylindrical shanks which may be the same or different diameter than the body of the drill; the shanks may be made with or without driving flats, tang, grooves or threads Taper Shank Drills: Those having conical shanks suitable for direct fitting into tapered holes in machine spindles, driving sleeves or sockets; tapered shanks generally have a driving tang Taper Shank Square Drills: Those having tapered shanks with four flat sides for fitting a rachet or brace Shell Core Drills: Core drills mountable on arbors specifically designed for the purpose; commonly used with shell reamer arbors

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Threaded Shank Drills: Those made with threaded shanks generally used in close center multiple spindle applications or portable angle drilling tools Beaded Shank Bits: Drills with flat shanks having raised beads parallel to the axis

Classification Based on Number of Flutes  

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Two-Flute Drills: The conventional type of twist drill used for originating holes Single-Flute Drills: Those having only one flute commonly used for originating holes Three-Flute Drills (Core Drills): Drills commonly used for enlarging and finishing, drilled, cast, or punched holes; they will not produce original holes Four-Flute Drills (Core Drills): Used interchangeably with three-flute drills; they are of similar construction except for the number of flutes

Classification Based on Hand of Cut 

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Right-Hand Cut: When viewed from the cutting point the counterclockwise rotation of a drill in order to cut; the great majority of drills are made right hand Left-Hand Cut: When viewed from the cutting point the clockwise rotation of a drill in order to cut

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Cutting Tools for Drilling Operations Twist Drill: The twist drill is the most common type of drill. It has two cutting edges and two helical flutes that continue over the length of the drill body. The drill also consists of a neck and a shank that can be either straight or tapered. A tapered shank id fitted by the wedge action into the tapered socket of the spindle and has a tang that goes into a slot in the spindle socket, thus acting as a solid means for transmitting rotation. Straight shank drills are held in a drill chuck that is, in turn, fitted into the spindle socket in the same way as tapered shank drills.

Twist drill

 Core Drill: A core drill consists of the chamfer, body, neck and

shank. This type of drill may have three or four flutes and an equal number of margins, which ensures superior guidance, thus resulting in high machining accuracy.

 Spade Drill: A spade drill is used for drilling

large holes of 90 mm or more. The design of this type of drill results in a marked saving in tool cost as well as in tangible reduction in tool weight that facilitates its ease of handling. Moreover, this drill is easy to grind.

Core drilling: to increase diameter of existing holes

Spade drill: for large, deep holes

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Cutting Tools for Drilling Operations Step Drill: A multiple diameter drill with one set of drill lands which are ground to different diameters 

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Step drill: for stepped holes

Gun Drill: A gun drill is used for drilling deep hole. All gun drills are straight-fluted, and each has a single cutting edge. A hole in the body acts as a conduit to transmit coolant under considerable pressure to the tip of the drill.

Gun drill with holes for coolant

Center Drill: Center drill bits are used in metalworking to provide a starting hole for a larger-sized drill bit or to make a conical indentation in the end of a workpiece in which to mount a lathe center. In either use, as the drill is either establishing the center of a hole or making a conical hole for a lathe center. However, the true purpose of a center drill is the latter task, while the former task is best done with a spotting drill.

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Operations Related to Drilling Reaming: Reaming is used to slightly enlarge a hole, to provide a better tolerance on its diameter and to improve its surface finish. The tool is called a reamer and it usually has straight flutes.

 Tapping: This operation is performed

by an tap and is used to provide internal screw threads on an existing hole.

 Counterboring: Counterboring provides a stepped hole, in

which a larger diameter follows a smaller diameter partially into the hole. A counterboring hole is used to seat bolt heads into a hole so the heads do not protrude above the surface.

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Operations Related to Drilling Countersinking: This is similar to Counterboring, except that the step in the hole is cone-shaped for flat head screws and bolts.

 Centering: Also called center drilling,

this operation drills a starting hole to accurately establish its location for subsequent drilling. This tool is called a centerdrill.

 Spotfacing: Spotfacing is similar to milling. It is used to

provide a flat machined surface on the workpart in a localized area.

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Drills and Drilling Operations

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Drill Press A drill press is preferable to a hand drill when the location and orientation of the hole must be controlled accurately. A drill press is composed of a base that supports a column, the column in turn supports a table. Work can be supported on the table with a vise or hold down clamps, or the table can be swiveled out of the way to allow tall work to be supported directly on the base. Height of the table can be adjusted with a table lift crank than locked in place with a table lock. The column also supports a head containing a motor. The motor turns the spindle at a speed controlled by a variable speed control dial. The spindle holds a drill chuck to hold the cutting tools (drill bits, center drills, deburring tools, etc.). Department of Industrial & Production Engineering

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Specialized Drill Presses Gang-drilling machines: independent columns, each with different drilling operation, work piece slid from one column to next Turret-type, upright drilling machines: used when numerous drilling operation are required in rapid succession, turret rotates needed tool into position for each operation

Gang-drilling machines Department of Industrial & Production Engineering

Turret-type, upright drilling machines 18/15

Specialized Drill Presses Radial drilling machines: used on large workpieces, spindle mounts on radial arm, allowing drilling operations anywhere along the arm length Multiple-spindle drilling machines: Single powerhead operates multiple spindles enabling multiple holes at one time, each hole can be unique

Radial drilling machines Department of Industrial & Production Engineering

Multiple-spindle drilling machines 18/16

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 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

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Questions

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