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When inspecting various products around us we realize that the vast majority have several holes in them. Holes are generally used either for assembly with fasteners such as bolts screws and rivets each requiring a hole or to provide access to the inside of a part. Note for example the number of rivets on an airplane s fuselage or the bolts in engine blocks and various components under the hood of an automobile.

Holemaking is among the most important operations m manufacturing. In automotive engine production the cost of holemaking is one of the largest machining costs. Drilling is a major and common holemaking process other processes for producing holes are punching and various advanced machining processes.

Because drills usually have a high length-to-diameter ratio the next figure they are capable of producing relatively deep holes. They are somewhat flexible however depending on their diameter and should be used with care in order to drill holes accurately and to prevent the drill from breaking.

Furthermore the chips that are produced within the workpiece have to move in the direction opposite to the axial movement of the drill. Consequently chip disposal and the effectiveness of cutting fluids can present significant difficulties in drilling.

Generally the hole diameters produced by drilling are slightly larger than the drill diameter oversize as one can note by observing that a drill can be easily removed from the hole it has just produced. The amount of oversize depends on the quality of the drill and of the equipment used as well as on the practices employed.

Depending on their thermal properties some metals and nonmetallic materials expand significantly due to the heat produced by drilling so the final hole could be smaller than the drill diameter.

Also drilled holes may be subjected to subsequent operations for better surface finish and dimensional accuracy such as reaming and honing. Drills generally leave a burr on the bottom surface upon breakthrough necessitating deburring operations.

The most common drill is the conventional standard-point twist drill Fig. (a) the main features of which are the point angle lip-relief angle chisel-edge angle and helix angle. The geometry of the drill tip is such that the normal rake angle and velocity of the cutting edge vary with the distance from the center of the drill.

Generally two spiral grooves flutes run the length of the drill and the chips produced are guided upward through these grooves. The grooves also serve as passageways to enable the cutting fluid to reach the cutting edges. Some drills have internal longitudinal holes through which cutting fluids are forced improving lubrication and cooling as well as washing away the chips.
Drills are available with a chip-breaker feature ground along the cutting edges. This feature is important in drilling with automated machinery where disposal of long chips without operator assistance is necessary.

Small changes in drill geometry can have a significant effect on the drill performance particularly in the chisel-edge region which accounts for about 50% of the thrust force in drilling. For example too small a lip relief angle Fig. (a) increases the thrust force generates excessive heat and increases wear. Conversely too large an angle can cause chipping or breaking of the cutting edge. Consequently in addition to conventional point drills several other drill-point geometries have been developed to improve drill performance and increase the penetration rate Fig. (c) . Special grinding techniques and equipment are used to produce these geometries.

Fig. (a) Standard chisel-point drill indicating various features. The function of the pair of margins is to provide a bearing surface for the drill against walls of the hole as it penetrates into the workpiece. Drills with four margins double-margin are available for improved drill guidance and accuracy. Drills with chip-breaker features are also available. (b) Crankshaft-point drill. (c) Various drill points and their manufacturers 1. Four-facet split point by Komet of America. 2. SE point by Hertel. 3. New point by Mitsubishi Materials. 4. Hosoi point by OSG Tap and Die. 5. Helical point.

A step drill produces holes of two or more different diameters. A core drill is used to make an existing hole larger. Counterboring and countersinking drills produce depressions on the surface to accommodate the heads of screws and bolts. A center drill is short and is used to produce the hole at the end of a piece of stock so that it may he mounted between centers in a lathe between the headstock and the tailstock. A spot drill is used to spot to start a hole at the desired location on a surface. Spade drills have removable tips or bits and are used to produce large and deep holes. They have the advantages of higher stiffness because of the absence of flutes in the body of the drill ease of grinding the cutting edges and lower cost. Crankshaft drills have good centering ability and because chips tend to break up easily these drills are suitable for producing deep holes.

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Drill Doctor Drill Bit Sharpener - 3/32in. Dia. to 1/2in. Dia. Bits, Model# DD350X

$59.99

The Drill Doctor DD350X is a value-priced and easy-to-use drill bit sharpener, ideal for the shop that uses the most common bit styles and works mainly in wood and light metals. Bit Sizes (SAE): 3/32 to 1/2, Angle Sharpening (deg.): 118, Carbide-Tip Bits: Yes, High Speed Steel Bits: Yes, Masonry Bits: Yes, Titanium-coated Bits: Yes Easy front-loading chuck Simplified alignment and sharpening procedures Replaceable diamond sharpening wheel [Read more]