A twist drill bit is a cutting tool designed to make round holes by shearing material as it rotates and advances into a workpiece. Its helical flutes, sharp lips, and pointed tip work together to cut, guide, and clear chips from the hole. Understanding drill bit geometry matters because it affects accuracy, heat, tool life, and the quality of the finished hole.
In a workshop, the same physics connects a small hand drill to a large drill press.
Understanding Tools & Workshop Machines: Twist Drill Bits
Most of the useful cutting happens at the two cutting edges near the outside of the drill. The centre section, called the chisel edge, has very little sideways speed because it is close to the axis of rotation. It mainly pushes material aside instead of slicing it cleanly.
This is why drilling needs a downward force, often called thrust. A small centre punch mark in metal helps the drill start in the right place.
For a large hole, a smaller pilot hole can reduce the force needed at the centre. It can improve control, though the pilot hole must not be so large that it removes support for the chisel edge.
Speed and feed must suit each other. The outside edge of a large drill travels farther in one turn than the edge of a small drill. A larger drill therefore needs fewer revolutions each minute to avoid excessive heat.
If the feed is too light, the edges rub instead of making proper chips. Rubbing creates heat and quickly dulls the drill. If the feed is too heavy, the drill can grab, stall, or break.
In a drill press, a steady firm feed usually produces curled chips in metal. Very fine dust or blue, discoloured chips can show that the cutting zone is too hot. Cutting fluid can cool the work and reduce friction when drilling many metals.
Different materials give very different chip behaviour. Mild steel makes tough chips and needs controlled speed. Aluminium cuts easily, but it can stick to the cutting edges if heat builds up.
A suitable lubricant helps prevent this sticking. Plastics can melt when a drill spins too fast or pauses while rubbing. A slower speed with a smooth feed gives a cleaner hole.
Wood often drills easily, but the drill can tear fibres as it breaks through the far side. A scrap backing board supports the fibres and reduces splintering. Deep holes need occasional withdrawal of the drill.
This is called peck drilling. It clears packed chips and lets air or fluid reach the cutting area.
Good drilling depends on holding the work securely. A rotating drill can catch a loose piece and spin it without warning. Use a vice, clamps, or a proper fixture instead of holding small work by hand.
Keep hands away from chips, since fresh metal chips are sharp and hot. A brush is safer than fingers for clearing them. Stop the machine before measuring a hole or changing the workpiece.
Students should watch for drill wobble, a squealing sound, smoke, rough hole edges, and chips that stop coming out. These signs can point to a blunt drill, poor clamping, incorrect speed, or poor alignment. The same habits matter when making holes for shelves, repairing a bicycle part, building a model, or preparing metal for bolts and screws.
Key Facts
- Cutting speed is the surface speed at the drill edge: v = πDN, where D is drill diameter and N is rotational speed.
- For rotational speed in revolutions per minute: N = v/(πD), using consistent units.
- Feed rate is the distance the drill advances per revolution or per minute: feed per minute = feed per revolution × rpm.
- Twist drill flutes remove chips and allow cutting fluid or air to reach the cutting zone.
- A common general-purpose point angle is 118°, while harder metals often use about 135° split points.
- Torque increases with drill diameter, feed, and material strength, so large holes require lower speed and more rigid holding.
Vocabulary
- Twist drill bit
- A rotating cutting tool with helical flutes used to drill circular holes in materials such as metal, wood, and plastic.
- Flute
- A spiral groove in the drill bit that carries chips away from the cutting edges and out of the hole.
- Cutting lip
- The sharpened edge near the tip of the drill bit that shears material from the workpiece.
- Point angle
- The included angle at the tip of the drill bit that affects centering, cutting force, and suitability for different materials.
- Web
- The solid central core of a twist drill bit that gives strength but does little cutting at the very center.
Common Mistakes to Avoid
- Using too high an rpm on large drill bits, which overheats the cutting lips because the outer edge surface speed becomes too large.
- Pressing harder instead of sharpening a dull bit, which increases heat and wandering but does not restore proper cutting geometry.
- Failing to clamp the workpiece, which is wrong because the drill can grab the material and spin it dangerously.
- Ignoring chip removal in deep holes, which traps chips in the flutes and causes friction, poor hole finish, and possible bit breakage.
Practice Questions
- 1 A 10 mm diameter twist drill is run at 1200 rpm. Calculate the cutting speed at the outside edge in m/s using v = πDN.
- 2 A drill press is set to 800 rpm with a feed of 0.12 mm per revolution. What is the feed rate in mm/min?
- 3 A student drills steel with a large bit and sees blue chips, smoke, and a squealing sound. Explain what is likely wrong with the speed, lubrication, or sharpness, and describe two corrections.