Drilling

Drilling is a precise cutting process used to create holes in solid materials. Most holes produced this way have a circular cross-section. To achieve this, a tool called a drill bit is pressed against a workpiece. The bit rotates at high speeds, often ranging from hundreds to thousands of revolutions per minute. This rotation forces a cutting edge against the material to slice it. As the material is cut, small pieces called chips or swarf are removed from the hole.

The mechanism of drilling depends on the material being cut. In standard metalworking, the bit rotates and cuts through the surface. For rock drilling, the process often involves a hammering action. The bit is struck with quickly repeated, short movements to break the stone. This hammering can occur from above the hole, known as top-hammer drilling. It can also happen from within the hole, which is called down-the-hole or DTH drilling.

Engineers use many different types of drills for specific tasks. A subland drill can create holes of more than one diameter. Spade drills are used when larger hole sizes are required. Indexable drills are helpful for managing the flow of chips. For extremely deep holes, gun drilling is a common method. These bits are self-centering because they use bearing pads that slide along the hole surface. This allows gun drilling to create holes with a depth-to-diameter ratio greater than 300:1.

Another specialized method is trepanning, which is used for very large diameter holes. Instead of turning the material into dust, trepanning cuts out a solid disk. This works much like a drafting compass would. It is often used on flat products like sheet metal, plates, or I-beams. For much smaller tasks, microdrilling is used to create holes less than 0.1 millimeters wide. These tiny holes require very high spindle speeds, often exceeding 10,000 RPM.

Precision is a major concern in all drilling operations. If a drill bit is not perfectly straight, it may "walk." Walking is when the bit is deflected away from the intended center-line. To prevent this, workers often use spot drilling to create a guide hole first. They might also use a drill jig with bushings to constrain the bit's position. For deep holes, specialized equipment is needed to maintain straightness and roundness. Some methods even rotate the workpiece while the tool stays still to ensure accuracy.

Modern technology has introduced vibration drilling to solve common problems. This method was first studied in the 1950s by Professor V.N. Poduraev at Moscow Bauman University. Vibration drilling uses small, controlled axial oscillations to break up chips. Breaking the chips into smaller pieces makes them easier to remove through the flutes of the bit. This is very useful for drilling through multi-material stacks used in aeronautics.

There are also advanced ways to move the cutting tool, such as orbital drilling. Also called circle interpolating, this process rotates a tool around its own axis. At the same time, the tool orbits around a center axis that is offset from the tool itself. This allows a small tool to create much larger holes. It also reduces thrust force, which helps create holes without burrs. This technique is very effective for composite materials because it prevents delamination.

Finally, heat management is vital for successful drilling. Cutting fluids are often used to cool the drill bit and the workpiece. These fluids can be applied as a spray mist or by flooding the area. The liquid helps increase tool life and makes it easier to eject chips. In some cases, oil-hole drills are used to pump lubricant directly through the bit. This helps prevent the metal from grabbing the bit and causing "chatter."