A twist drill bit is characterized by many attributes, two of which are the flute and helix. This article investigates why they are essential and how they influence the drill bit’s efficiency. Read on to find out.
What Is Flute on a Drill Bit?
The flute of a drill bit is the groove cut into its profile that typically corkscrews along the drill bit’s length. On a typical metalworking twist drill bit, you can see two spiral grooves (flute) starting from the cutting edges to the shank.
These come in many angles and widths and should match the material in question.
Purpose of Flute
A twist drill bit flute is meant for chip removal and flow of coolant. As the drill cuts into the workpiece, the material is removed in the form of chips. These chips should be removed from the hole to enable drilling further into the material.
A twist drill bit’s flute provides a passage for the chips to travel up the drill bit’s flute length and out of the hole that is being drilled. Wider flute groove profiles provide better chip evacuation.
Chip build-up inside the hole could damage the drill bit or workpiece since the hole gets clogged. It could lead to excessive heat generation as friction between the chips, workpiece, and drill bit increases. This could further damage both drill bit and workpiece through annealing heat treatment. Here, the drill bit could become brittle and break.
Flute Length
Flute length refers to the portion of the drill where the flute is, from the cutting tip’s leading edge to where the flute tapers off. For continuous drilling, the flute length should always be longer than the intended borehole depth. Since chips are evacuated through the flute, a portion of the flute should remain outside the borehole to allow effective chip evacuation.
Chip build-up will occur if the entire flute enters the workpiece, and the borehole will be jammed with chips. The drill bit and borehole would be damaged here, and the drill bit may even snap off.
To avoid this, you can utilize a technique called peck drilling. Here the drill bit cuts for a small depth and is lifted out of the hole frequently before the chips are filled in the entire flute length. This technique used on CNC drills or milling machines allows for easy chip removal.
On a bench drill press, you have to do this manually to avoid breaking the drill bit and/or damaging the hole.
Types of Flutes
Twist drill bits have flutes of all shapes and depths. Here, the extremes are high helix and straight.
Here’s what I mean:
Helical Flute
Helical flutes are spiral grooves that corkscrew along the drill bit shaft’s length.
These range from high helix, where the flute twists tightly around the shaft at a high angle, to low helix, where the angle is so high that the spiral groove winds lazily around the shaft, barely there.
Straight Flute Bits
A straight flute drill bit has a helix angle of 0°. Here, the flutes don’t “pull in” the material being drilled. This enables drilling of tricky materials that bend easily during drilling operations.
Examples include sheet metal or other soft materials with a thin cross-section.
Helix Angle of a Drill Bit
The helix angle refers to the spiral angle of a twist drill bit, measured between the center shaft and helix edge. This feature determines how chips are formed during the drilling process. Soft, long-chipping materials require a large helix angle, while hard, short-chipping materials require smaller helix angles.
Drill bit helix angles determine the drill bit’s cutting efficiency since it determines the force with which chips are pushed from the borehole. Higher angles mean chips are pushed out with greater force. The downside is that a higher helix angle decreases the drill’s cross-sectional area. This makes the drill weaker and less stable. Balancing these two aspects and matching them to the application is essential for effective drilling.
Helix angles typically range from 10° to 45°. Standard twist drill with a helix angle is 25° to 35° is considered normal spiral angle.
Slow spiral drill bits have their helical grooves machined to a low angle of 10° to 20°, while a high helix angle of 30° to 45° are deemed as fast spiral. Everything in between is categorized as a normal spiral.
Small helix angles result in long spirals, while large helix angles go with short spirals. Deeper holes also require higher helix angles for more effective chip removal. Harder materials require higher cutting forces, which go with lower helix angles.
The other critical dimension on a drill bit is the point angle which is usually ground to 118° or 135°.
Drill Bit Helix Angle for Different Materials
Most people focus on the type of coating (black oxide vs titanium) and the drill bit material (HSS, Cobalt, or carbide); but fail to recognize the importance of point and helix angles.
The following list of materials and the suitable helix angle should help you choose the right drill bit for your work.
Aluminum
Aluminum is classified as a soft material and thus requires a large helix angle, also known as a fast spiral drill. This allows chips to flow off easily, preventing heat build-up and borehole clogging.
Ferrous Metals
Ferrous metals are classified as hard and thus require a small helix angle. Here, the chips forms would be short, rough, and hard.
Soft Materials
Soft materials, such as soft plastics, hardwood, and chipboard, require a large helix angle, also known as a slow spiral drill. Here, the chips formed are long.
Wood
There are many types of wood, and some are pretty hard. That said, even hardwood is classified as a soft material for drilling applications. So, to drill hardwood, you need a brad point bit or twist drill bit with a large helix angle.
Please note that many wood drill bits have a different design compared to metal drill bits. While the wood bits range from flat blade spade bits to multi-tooth Forstner bits, the metalworking is usually performed with twist drills.
Dan Miller is a professional toolmaker with extensive experience in the metalworking and tooling industry.
An avid DIY enthusiast and passionate hobbyist woodworker, Dan shares his expert knowledge here.