3 Factors to Consider When Choosing a Twist Drill
Carbide twist drill is a solid carbide tool used for hole machining after the carbide bar is ground and formed by a professional grinder, and the drill tip and cutting edge are TIALN coated. It is suitable for steel, stainless steel and cast iron.
Materials can be roughly divided into 3 types: high-speed steel, cobalt-containing high-speed steel and solid carbide.
✔️High Speed Steel (HSS)
Since 1910, high-speed steel has been used as a cutting tool for more than a century, and it is currently the most widely used and cheapest cutting tool material. A more stable environment such as a drilling machine. Another reason that makes high-speed steel enduring may be that high-speed steel tools can be repeatedly ground. Due to the low price, they are not only used for grinding drills, but also widely used in turning tools.
✔️Cobalt High Speed Steel (HSSE)
Cobalt-containing high-speed steel has better hardness and red hardness than high-speed steel, and the increase in hardness also improves its wear resistance, but at the same time sacrifices part of its toughness. Like high-speed steel, they can be reground to increase the number of times of use.
Cemented carbide is a metal-based composite material. Among them, tungsten carbide is used as the matrix, and some materials of other materials are used as a binder to be sintered by a series of complex processes such as hot isostatic pressing. Compared with high-speed steel, it has a huge improvement in hardness, red hardness and wear resistance. But the cost of carbide tools is also much more expensive than high-speed steel. Carbide has more advantages than previous tool materials in tool life and processing speed. In the repeated grinding of tools, professional grinding tools are required.
Coatings can be roughly divided into the following 5 types according to the scope of use:
Uncoated knives are the cheapest and are usually used to process softer materials such as aluminum alloys and mild steel.
✔️Black oxide coating
Oxidized coatings can provide better lubricity than uncoated tools, and are also somewhat better in terms of resistance to oxidation and heat, and can increase service life by more than 50%.
✔️Titanium nitride coating
Titanium nitride is the most common coating material and is not suitable for processing materials with relatively high hardness and high processing temperature.
✔️Titanium carbonitride coating
Titanium carbonitride is developed from titanium nitride and has higher high temperature and wear resistance, usually purple or blue. Used to machine cast iron workpieces in the Haas workshop.
✔️Aluminum Nitride Titanium Coating
Aluminum titanium nitride is more resistant to high temperatures than all the above coatings, so it can be used in higher cutting environments. For example, processing superalloys. It is also suitable for the processing of steel and stainless steel, but due to the elements containing aluminum, chemical reactions will occur when processing aluminum, so avoid processing aluminum-containing materials
In general, a cobalt-containing drill with a titanium carbonitride coating or a titanium nitride coating is a more economical solution.
Geometric features can be divided into the following 3 parts.
The ratio of length to diameter is called double diameter, and the smaller the double diameter, the better the rigidity. Selecting a drill with the edge length just for chip removal and the short overhang length can improve the rigidity during machining, thereby increasing the service life of the tool. Insufficient blade length is likely to damage the drill.
✔️Drill tip angle
A tip angle of 118° is probably the most common in machining and is typically used for soft metals such as mild steel and aluminum. The design of this angle is usually not self-centering, which means that it is inevitable to machine the centering hole first. The 135° drill point angle is usually self-centering, which saves a lot of time by eliminating the need to drill the centering holes separately, as there is no need to machine the centering holes.
A 30° helix angle is a good choice for most materials. But for environments that require better chip evacuation and a stronger cutting edge, a drill with a smaller helix angle can be selected. For difficult-to-machine materials such as stainless steel, a design with a larger helix angle can be selected to transmit torque.