3. Core Material: The drill's 'teeth'.
The performance ceiling of a drill bit largely depends on its material:
High-Speed Steel (HSS): The most commonly used material, offering good toughness and affordability, with reduced susceptibility to breakage. Suitable for low-speed drilling and manual operations. Cobalt-containing HSS exhibits superior heat resistance, making it suitable for drilling difficult-to-machine materials such as stainless steel.
Carbide: Extremely hard and wear-resistant, capable of withstanding very high cutting speeds. However, it is brittle, susceptible to impact and vibration, and prone to chipping. Suitable for high-speed machining of hard materials on stable machine tools.
Powder Metallurgy High-Speed Steel (HSS-PM): Exhibits properties intermediate between high-speed steel and carbide, retaining the toughness of HSS while enhancing wear resistance.
Coating: Applies a thin film to the drill surface, such as TiN (gold-coloured) or TiAlN (blue-grey). Coatings increase surface hardness, heat resistance, and lubricity, significantly extending drill life.
4. Core Geometric Parameters
Grinding drill bits is a craft, and the following angles are crucial:
1) Point Angle: The angle between the two main cutting edges.
118°: Standard point angle, suitable for drilling common materials such as steel and cast iron.
135°: Increased point angle, suitable for drilling hard and tough materials like stainless steel, reducing cutting resistance.
2) Relief Angle: The angle preventing friction between the drill's rear face and the workpiece.
3) Chisel Edge: The transverse edge at the drill's very tip. This is the most stressed part during drilling, responsible for material displacement. Precision drills reduce this edge through grinding to minimise feed resistance.
5. Application Scenarios and Selection Guide
1) Domestic renovation: Use a hand-held drill or hammer drill.
Wood: Standard high-speed steel twist drills suffice. For smooth hole walls, use flat drills or woodworking three-point drills.
Metal: High-speed steel twist drills are essential, preferably with lubricant applied to the drill bit for cooling.
Tiles/Glass: Mandatory use of carbide drill bits, typically with a 90° angle. Mark position with tape, drill at low speed with light pressure in non-hammer mode.
Concrete/Brick Walls: Mandatory use of hammer drill bits featuring carbide-tipped blades in cross or straight configurations. Operate in hammer mode.
2) Machining, using lathes, milling machines or machining centres.
Rough machining: Employ index-able insert drills for high efficiency without regrinding.
Finish machining: Use solid carbide drills with high spindle speeds and coolant to ensure hole accuracy and surface finish.
Deep holes: Employ gun drills or deep hole drills with internal cooling.
6. Usage Techniques and Precautions
1) Centre Marking: Before drilling into metal, mark a dimple with a centre punch to prevent the drill bit from slipping.
2) Speed and Feed Rate: Use low speeds for large holes and high speeds for small holes; employ low speeds for hard materials and high speeds for soft materials. Maintain a consistent feed rate to avoid excessive force causing drill bit breakage.
3) Adequate Cooling: When drilling metals, particularly stainless steel, always employ cutting fluid or machine oil for cooling and lubrication. This extends drill life and enhances hole finish quality.
4) Chip Removal: During deep drilling, frequently withdraw the drill to clear chips from the flutes, preventing blockages that may cause jamming or breakage.
5) Regrinding: When the drill becomes blunt, promptly regrind it on a grinding wheel. Maintaining symmetrical cutting edges and consistent point angles is crucial for producing acceptable holes.
In summary, selecting a drill bit is akin to choosing a key: first consider the material of the lock you wish to open (wood, metal, or concrete), then assess the strength at your disposal (a hand drill or a machine tool), and finally select the appropriate key (the type and material of the drill bit).
