Drilling

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Drilling is the process of using a drill bit in a drill to produce cylindrical holes in solid materials, such as wood or metal. Different tools and methods are used for drilling depending on the type of material, the size of the hole, the number of holes, and the time to complete the operation.

[edit] Drilling in metal

Under normal usage, swarf is carried up and away from the tip of the drill bit by the fluting of the drill bit. The continued production of chips from the cutting edges produces more chips which continue the movement of the chips outwards from the hole. This continues until the chips pack too tightly, either because of deeper than normal holes or insufficient backing off (removing the drill slightly or totally from the hole while drilling). Lubricants and coolants (i.e. cutting fluid) are sometimes used to ease this problem and to prolong the tools life by cooling and lubricating the tip and chip flow. Coolant is introduced via holes through the drill shank (see gun drill).

Straight fluting is used for copper or brass, as this exhibits less tendency to "dig in" or grab the material. If a helical drill (twist drill) is used then the same effect can be achieved by stoning a small flat parallel with the axis of the drill bit.

For heavy feeds and comparatively deep holes oil-hole drills can be used, with a lubricant pumped to the drill head through a small hole in the bit and flowing out along the fluting. A conventional drill press arrangement can be used in oil-hole drilling, but it is more commonly seen in automatic drilling machinery in which it is the workpiece that rotates rather than the drill bit.

[edit] Drilling in wood

Wood being softer than most metals, drilling in wood is considerably easier and faster than drilling in metal. Cutting fluids are not used or needed. The main issue in drilling wood is assuring clean entry and exit holes and preventing burning. Avoiding burning is a question of using sharp bits and the appropriate cutting speed. Drill bits can tear out chips of wood around the top and bottom of the hole and this is undesirable in fine woodworking applications.

The ubiquitous twist drill bits used in metalworking also work well in wood, but they tend to chip wood out at the entry and exit of the hole. In some cases, as in rough holes for carpentry, the quality of the hole does not matter, and a number of bits for fast cutting in wood exist, including spade bits and self-feeding auger bits. Many types of specialised drill bits for boring clean holes in wood have been developed, including brad-point bits, Forstner bits and hole saws. Chipping on exit can be minimized by using a piece of wood as backing behind the work piece, and the same technique is sometimes used to keep the hole entry neat.

Holes are easier to start in wood as the drill bit can be accurately positioned by pushing it into the wood and creating a dimple. The bit will thus have little tendency to wander. In metal working, an accurate position needs to be marked with a punch to avoid the bit wandering from the desired position of the hole.

[edit] Microdrilling

Microdrilling refers to the drilling of holes less than 0.5 mm. Drilling of holes at this small diameter presents greater problems since coolant fed drills cannot be used and high spindle speeds are required.

[edit] Drilling as a Manufacturing Process

OPERATION DEFINITION Hole making is one of the most important machining operations in the manufacturing process. Holes serve a variety of functions including but not limited to: fasteners for assembly, weight reduction, ventilation, access to other parts, or simply for aesthetics. Hole making or drilling is used in the production of almost any part conceivable and those that aren't drilled are made with machines that have been drilled.

HOLE MAKING OPERATIONS On most workpieces it is vitally important that the hole be drilled precisely in reference to the x, y, z-axes. When possible drilled holes should be located perpendicular to the workpiece surface. This is due to the large length-to-diameter ratio which causes the drill bit to be easily deflected which can cause the hole to be misplaced, or the drill bit to break or fatigue. Because there are so many types of production operations that involve making a variety of holes in countless different materials, there are many methods for hole making.

CONSIDERATION FOR DRILLING Because drilling can often be such a critical process there are a number of considerations that should be taken in order to ensure the most accurate drill hole possible.

As mentioned before the hole and drill motion should be perpendicular to the surface of the workpiece to reduce the tendency to fatigue or break the drill bit. This also helps to reduce 'walking' of the drill bit over the workpiece surface.

• 'Walk' is common when drilling small diameter holes. It is advantageous to create a centering mark or feature during the casting or forging process. Creating a centering dimple with a centering punch will also reduce the tendency to 'walk'.

• The bottoms of the hole should match the standard drill point angles. Avoid flat bottom hole or odd shapes.

• Create through holes instead of blind holes when possible.

• If a blind hole must be drilled and tapped, it should be drilled deeper than the tapped depth.

• Holes that need to be reamed must also be initially drilled deeper than the reamed hole depth.

• A part should be designed such that it won't need to be repositioned or manually moved during the drilling process. This also reduces production time and overall cost.

• Drill speed should be another consideration. Some materials like plastics as well as other non-metals and some metals have a tendency to heat up enough to expand making the hole smaller than desired.

TWIST DRILL The most common type of drill is a standard-point twist drill. This type of drill is versatile and can be used on a variety of materials such as wood, plastic, masonry, ceramic, and metal. These drill bits have two spiral grooves running the length of the drill. These grooves aid in transporting cutting fluid to the drill tip and in removing the chips from the hole. These types of drill bits are held in chucks or collets on machines that are either hand-held or automated. This type of drilling can often cause burrs at both the entrance and the exit of the hole and parts will often need a subsequent deburring operation to smooth out the holes.

GUN DRILLING Another type of drilling operation is called gun drilling. This method was originally developed to drill out gun barrels and is used commonly for drilling smaller diameter deep holes. This depth-to-diameter ratio can be even more than 300:1. The key feature of gun drilling is that the bits are self-centering; this is what allows for such deep accurate holes. The bits use a rotary motion similar to a twist drill however; the bits are designed with bearing pads that slide along the surface of the hole keeping the drill bit on center. Gun drilling is usually done at high speeds and low feed rates.

PUNCHING AND TREPANNING These operations involve drilling but there are other methods such as punching and trepanning which don't necessarily use common drill bits. Punching essentially works just like a paper punch, it uses a punch that pushes the material through a die. The punch and die set can be in almost any shape. Punching can be cost effective because labor costs are low, however, the equipment costs can be high making punching most cost effective for high production parts that don't require high tolerances.

Trepanning is commonly used for creating larger diameter holes (up to 250mm or 10in) where a standard drill bit is not feasible or economical. Trepanning removes the desired diameter by cutting out a solid disk similar to the workings of a drafting compass. Trepanning is performed on flat products such as sheet metal, plates, or structural members like I-beams. Trepanning can also be useful to make grooves for inserting seals like O-rings.

GENERAL RECOMMENDATION FOR SPEEDS AND FEEDS IN DRILLING ^[1]

-Manufacturing Engineering and Technology, Kalpakjian, Schmid, 2006

HOW THIS VALUE-ADDING OPERATION HELPS ACHIEVE WORKPIECE FUNCTION Drilling and hole making is an indispensable step in the manufacturing process. Many other steps in the process can be done using a variety of methods. For example forming a part can be done forging, casting, machining, it can be shaped in a die or by other methods. In order to determine the best and most profitable method some considerations need to be taken. Drilling however isn’t something that is optional in most cases. Sometimes a hole may be built into a die or a casting mold but these features can’t usually meet tight tolerances and can’t be very complex. Anytime a fastener needs to be used a hole must be drilled and tapped. There are many instances when drilling is the only option for making hole and hollow features. These include gun barrels, fastener holes, and small precise venting holes.

[edit] References

This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed.

1. ^ Manufacturing Engineering and Technology, Kalpakjian, Schmid, 2006

[edit] See also

• Drills
• Drilling (oil)
• Gun drill
• Well drilling
• Borehole
• Boring
• Geotechnical investigation
• Cutting fluid
• Lubricant

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