Possible geometries of an HSS drill

# Possible geometries of an HSS drill 

High-speed steel (HSS) drill bits are a must-have tool for drilling a wide variety of materials, from wood to hard metals like stainless steel or titanium alloys. The geometry of HSS drill bits plays a crucial role in their performance, as the different angles and characteristics directly influence the quality of the drilling, the life of the tool, and the type of materials it can drill effectively. This article explores the various geometries available for HSS drills, including tip angles, helix angles, and special modifications, highlighting their importance and optimal use. 

The tip angle

The tip angle is probably the most important parameter of a drill bit. It designates the angle between the two lips of the drill bit at its tip, and it determines the aggressiveness of the drilling and the efficiency of the material removal. 

The two most commonly used tip angles in HSS drills are 118° and 135°, each of which is suitable for specific materials and conditions: 

• 118°: This angle is the standard for general-purpose drill bits. It is sharper, which means that the drill bit penetrates faster and removes more material per rotation, making it ideal for softer materials like aluminum, wood, and some plastics. However, this angle can cause the drill bit to "walk," which is a shift in the tip of the drill bit when it comes into contact with harder or smoother surfaces. To avoid this, it is often recommended to use an awl to center the hole before drilling.

• 135°: This angle is flatter and is better suited for harder materials, such as stainless steel and other metal alloys. A wider angle distributes drilling pressure over a larger area, reducing tool wear and allowing for smoother, more controlled drilling. The downside is that drill bits with this angle can struggle to penetrate the material quickly, often requiring prior pointing to prevent shifting. However, this geometry offers better longevity and is often accompanied by a "split" tip to improve self-centering and reduce the effort required to drill.

The helix angle

The helix angle, also known as the spiral angle, determines the inclination of the grooves in the drill, which are used to remove chips during drilling. This angle usually varies between 30° and 40° on standard drill bits, but can be adjusted depending on the material being drilled.

Standard Helix Angle (30-40°): This angle is ideal for ferrous materials like mild steel. It allows for good chip evacuation while maintaining sufficient rigidity to withstand drilling pressure. 

High Helix Angle  (Beyond 40°): A higher helix angle  is often used to drill softer materials, such as aluminum or plastics, where fast chip evacuation is crucial to prevent jamming in the drill groove. This angle is also used for deeper drillings, as it helps keep the chips away from the drilling area.

Low helix angle (below 30°): Conversely, for very hard or brittle materials, such as cast iron, a lower helix angle is often preferred. This prevents the chips from being too large and damaging the workpiece, while maintaining the rigidity of the drill bit.

The split point

HSS drill bits can be equipped with a split tip, an additional geometry that improves the self-centering of the drill. This geometry reduces the axial pressure required to initiate drilling, which is especially useful for hard and slippery materials. Drill bits with a split point are often used for drilling in stainless steels and other high-strength materials, as they minimize the chance of deflection and extend the life of the drill bit.

Flute geometry

The drill flutes, i.e. the grooves that run along the shank, play a vital role in removing chips and reducing heat. A special geometry, parabolic flutes, is often used for deep drilling.

Standard Flute : Straight or helical flutes are the standard for most HSS drills, allowing for proper chip evacuation when drilling shallow to medium depths.

Parabolic flute : This geometry is commonly used for deep drilling, as it allows chips to be evacuated more efficiently without jamming. This is especially important when drilling deep holes in hard metal materials, as chip buildup can cause overheating and reduce tool life

Clearance and attack angles

In addition to tip and helix angles, clearance and attack angles are also crucial for the proper functioning of an HSS drill bit. These angles influence the efficiency of the drill edge and its behavior in terms of friction and wear.

Clearance Angle : This is the angle formed by the back surface of the drill bit edge and the surface of the workpiece. A proper clearance angle reduces friction between the drill bit and the workpiece, minimizing wear. Too little an angle can increase friction and quickly damage the bit, while too much angle could weaken the cutting edge.

Angle of attack : The angle of attack is the angle at which the lip of the drill bit cuts through the material. A larger angle of attack allows for more aggressive material removal, but at the cost of faster wear. A reduced angle of attack allows for finer control and is generally preferred for hard materials

The geometry of HSS drills, with its different variants of tip angles, propeller and flutes, offers great flexibility to adapt to a wide range of materials and drilling situations. 118° drill bits are great for general applications on soft materials, while 135° drill bits with a split point are essential for drilling through harder materials. Optimizing the helix angle and flute geometry also allows for efficient chip and heat management, increasing productivity and tool life. Choosing the right geometry for an HSS drill bit is therefore a key step in achieving optimal results in terms of drilling quality and tool durability.