A. Twist Drills

I Definition:

Drill: 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.

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II. General Classifications
 

A. Classification Based on Construction
1. Solid Drills: Those made of one piece of material such as high speed steel
2. Tipped Solid Drills: Those having a body of one material with cutting lips made of another material brazed or otherwise bonded in place
3. Composite Drills: Those having cutting portions mechanically held in place

B. Classification Based on Methods of Holding or Driving

1. 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
2. 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
3. Taper Shank Square Drills: Those having tapered shanks with four flat sides for fitting a rachet or brace
4. Shell Core Drills: Core drills mountable on arbors specifically designed for the purpose; commonly used with shell reamer arbors
5. Threaded Shank Drills: Those made with threaded shanks generally used in close center multiple spindle applications or portable angle drilling tools
6. Beaded Shank Bits: Drills with flat shanks having raised beads parallel to the axis


C. Classification Based on Number of Flutes

1. Two-Flute Drills: The conventional type of twist drill used for originating holes
2. Single-Flute Drills: Those having only one flute sommonly used for originating holes
3. Three-Flute Drills (Core Drills): Drils commonly used for enlarging and finishing, drilled, cast, or punched holes; they will not produce original holes
4. Four-Flute Drills (Core Drills): Used interchangeably with three-flute drills; they are of similar construction except for the number of flutes
D. Classification Based on Hand of Cut
1. 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"
2. Left-Hand Cut: When viewed from the cutting point the clockwise rotation of a drill in order to cut

III. Nomenclature of Twist Drills and Other Terms Relating to Drilling
 

Axis: The imaginary straight line which forms the longitudinal center line of the drill
Back Taper: A slight decrease in diameter from front to back in the body of the drill
Body: The portion of the drill extending from the shank or neck to the outer corners of the cutting lips
Body Diameter Clearance: That portion of the land that has been cut away so it will not rub against the walls of the hole
Built-Up Edge: An adhering deposit of nascent material on the cutting lip or the point of the drill
Cam Relief: The relief from the cutting edge to the back of the land, produced by a cam actuated cutting tool or grinding wheel on a relieving machine
Chip Breaker: Nicks or Grooves designed to reduce the size of chips; they may be steps or grooves in the cutting lip or in the leading face of the land at or adjacent to the cutting lips
Chip Packing: The failure of chips to pass through the flute during cutting action
Chipping: The breakdown of a cutting lip or margin by loss of fragments broken away during the cutting action
Chisel Edge: The edge at the end of the web that connects the cutting lips
Chisel Edge Angle: The angle included between the chisel edge and the cutting lip, as viewed from the end of the drill
Clearance: The space provided to eliminate undesirable contact between the drill and the workpiece
Clearance Diameter: The diameter over the the cut away portion of the drill lands


Crankshaft or Deep Hole Drills: Drills designed for drilling oil holes in crankshafts, connecting rods and similar deep holes; they are generally made with heavy webs and higher helix angles than normal
Cutter Sweep: The section formed by the tool used to generate the flute in leaving the flute
Double Margin Drill: A drill whose body diameter clearance is produced to leave more than one margin on each land and is normally made with margins on the leading edge and on the heel of the land
Drift: A flat tapered bar for forcing a taper shank out of its socket
Drift Slot: A slot through a socket at the small end of the tapered hole to recieve a drift for forcing a taper shank out of its socket
Drill Diameter: The diameter over the margins of the drill measured at the point
Exposed Length: The distance the large of a shank projects from the drive socket or large end of the taper ring gage
External Center: The conical point on the shank end of the drill, and the point end on some sizes of core drills
Flat Drill: A drill whose flutes are produced by two parallel or tapered flats
Flat (Spade) Drill: A removable cutting drill tip usually attached to a special holder designed for this purpose; generally used for drilling or enlarging cored holes
Flutes: Helical or straight grooves cut or formed in the body of the drill to provide cutting lips, to permit removal of chips, and to allow cutting fluid to reach the cutting lips
Flute Length: The length from the outer corners of the cutting lips to the extreme back end of the flutes; it includes the sweep of the tool used to generate the flutes and, therefore, does not indicate the usable length of the flutes
Gage Line: The axial position on a taper where the diameter is equal to the basic large end diameter of the specified taper
Galling: An adhering deposit of nascent work material on the margin adjacent to the leading edge at and near the point of a drill
Guide: A cylindrical portion, following the cutting portion of the flutes, acting as a guide to keep the drill in proper alignment; the guide portion may be fluted, grooved, or solid
Gun Drill: Special purpose straight flute drills with one or more flutes used for deep hole drilling; they are usually provided with coolant passages through the body; they may be either solid or tipped
Half-Round Drill: A drill with a transverse cross-section of approximately half a circle and having one cutting lip
Heel: The trainling edge of the land
Helical Flutes: Flutes which are formed in a helical path around the axis
Helix Angle: The angle made by the leading edge of the land with a plane containing the axis of the drill
Land: The peripheral portion of the body between adjacent flutes
Land Width: The distance between the leading edge and the heel of the land measured at a right angle to the leading edge
Lead: The axial advance of a leading edge of the land in one turn around the circumference
Lips: The cutting edges of a two flute drill extending from the chisel edge to the periphery
Lip Relief: The axial relief on the drill point
Lip Relief Angle: The axial relief angle at the outer corner of the lip; it is measured by projection into a plane tangent to the periphery at the outer corner of the lip
Margin: The cylindrical portion of the land which is not cut away to provide clearance
Multiple-Margin Drill: A drill whose body diameter clearance is produced to leave more than one margin in each land
Neck: The section of reduced diameter between the body and the shank of a drill Oil Grooves: Longitudinal straight or helical grooves in the shank, or grooves in the lands of a drill to carry cutting fluid to the cutting lips Oil Holes or Tubes: Holes through the lands or web of a drill for passage of cutting fluid to the cutting lips
Overall Length: The length from the extreme end of the shank to the outer corners of the cutting lips; it does not include the conical shank end often used on straight shank drills, nor does it include the conical cutting point used on both straight and taper shank drills
Periphery: The outside circumference of a drill
Peripheral Rake Angle: The angle between the leading edge of the land and an axial plane at the drill point
Pilot: A cylindrical portion of the drill body preceding the cutting lips; it may be solid, grooved, or fluted
Point: The cutting end of a drill, made up of the ends of the lands and the web; in form it it resembles a cone, but departs from a true cone to furnish clearance behind the cutting lips


Point Angle: THe angle included between the cutting lips projected upon a plane parallel to the drill axis and parallel to the two cutting lips
Relative Lip Height: The difference in indicator reading on the cutting lip of the drill; it is measured at a right angle to the cutting lip at a specific distance from the axis of the tool
Relief: The result of the removal of tool material behind or adjacent to the cutting lip and leading edge of the land to provide clearance and prevent rubbing (heel drag)
Shank: The part of the drill by which it is held and driven
Sleeve: A tapered shell designed to fit into a specified socket and to receive a taper shank smaller than the socket
Socket: The tapered hole in a spindle, adaptor, or sleeve, designed to receive, hold, and drive a tapered shank
Step Drill: A multiple diameter drill with one set of drill lands which are ground to different diameters
Straight Flutes: Flutes which form lands lying in an axial plane
Subland Drill: A type of multiple diameter drill which has independent sets of lands in the same body section for each diameter
Tang: The flattened end of a taper shank, intended to fit into a driving slot in a socket
Tang Drive: Two opposite parallel driving flats on the extreme end of a straight shank
Taper Drill: A drill with part or all of its cutting flute length ground with a specific taper to produce tapered holes; they are used for drilling the original hole or enlarging an existing hole
Taper Square Shank: A taper shank whose cross section is square
Web: The central portion of the body that joins the lands; the extreme end of the web forms the chisel edge on a two-flute drill
Web Thickness: The thickness of the web at the point, unless another specific locationis indicated
Web Thinning: The operation of reducing the web thickness at the point to reduce drilling thrust

B. Combined Drills and Countersinks
 

I. Definition

Combined Drill and Countersink: Single or double-end cutting tool, having helical or straight flutes, and having a drill portion and an adjacent integral countersink portion, primarily used to produce center holes in work that will be held between machine centers


 
 

II. General Classifications
A. Classification Based on Construction
1. Solid Combined Drills and Countersinks: Those made of one piece of material such as high speed steel
2. Tipped Solid Combined Drills and Countersinks: Those having a body or drill portion of one material with cutting edges or lips, or both, made of another material brazed or otherwise bonded in place
B. Classification Based on Type
1. Plain Type Combined Drills and Countersinks: Those having a drill portion and a single adjacent integral countersink portion
2. Bell Type Combined Drills and Countersinks: Those having a drill portion and an adjacent integral countersink portion, plus an additional secondary conical section to provide clearance for the bearing surface
C. Classification Based on Hand of Cut
same as twist drills

          III. Nomenclature of Combined Drills and Countersinks
 

Axis: The imaginary straight line which forms the longitudinal center line of the combined drill and countersink
Back Taper: A slight decrease in diameter from the front to back in the drill length
Bell Angle: The included angle of the secondary conical section providing clearance or protection for the countersink angle conical surface (normally 120 degrees)
Bell Diameter: The diameter at the intersection of the countersink portion and the bell portion at the leading edge of the land
Body: The central portion of the tool by which it is held or driven
Countersink Angle: The included angle of the countersink portion (normally 60 degrees)
Countersink Relief Angle: The angle between a plane at right angles to the axis of the tool, and a line tangent to the surface of the countersink portion at hte intersection of the countersink portion and the body and at the leading edge of the land; for the bell type, the relief angle is measured at the intersection of the bell portion and the body

Process Parameters:
 
 

Depth of cut: The depth of the hole generated by the drilling process
Feed: The rate that the drill advances into the material, generally measured in distance per flute
Speed: The cutting speed is usually measured at the periphery of the drill in surface feet or meters per minute
Thrust: The axial force required to drill
Torque: The twisting moment required to drill
Surface Finish: The roughness of the walls of the drilled hole; a measure of the hole quality

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Drill Press:

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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 motro. 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.).

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REFERENCE: Above shown figures and many other definitions are referred from Metal Cutting Tool Handbook published by the Metal Cutting Tool institute , N.Y

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