Holemaking

The most common tools used in hole machining are various drills, reamers, countersinks, and broaches. The most common drill type is the so-called twist drill, which gets its name from the spiral shape of the drill's chip flute. Twist drills are usually made of either high-speed steel or carbide, and today CNC machines primarily use through-coolant solid carbide drills. For large holes, indexable drills (u-drill) and replaceable tip drills (insert drill) are used, which offer cost advantages due to their replaceable cutting inserts and drill tips. For machining large and precise holes, roughing and fine boring tools are typically used. Roughing reamers are generally equipped with two cutting inserts, while a fine boring tool is equipped with one insert. The diameter of a fine boring tool can be adjusted mechanically up to an accuracy of 0.002 mm, enabling the machining of precise holes. For small, precise holes, broaches are most commonly used to create accurate and high-quality holes, for example, for various pin and guide applications.

High-speed steel (HSS, HSS-E, and HSS-E-PM) twist drills are the most common drill bits. HSS drills come with either cylindrical shanks or Morse taper shanks and are equipped with a 118-degree point angle and a long chisel edge, hence they typically require centering drilling. HSS drill bits are available with various coatings, the most common being steam treatment and TiN coating. HSS drills offer a cost-effective solution for drilling but are today used primarily for small series and unstable conditions when using automatic machine tools. HSS twist drills are the common choice when drilling manually or with manual machine tools. Cylindrical shank HHS twist drills are typically mounted in a CNC drill chuck or a collet chuck, whereas only Morse taper holders are suitable for mounting taper shank drill bits.

Carbide is a composite material made through powder metallurgy with tungsten carbide particles mixed with a cobalt-containing binder. Solid carbide drills resemble cylindrical shank HSS drill bits, but carbide drills typically have a 140-degree point angle and through-cooling channels. TKM drills are available in sizes 0.1-25mm, with drilling lengths ranging from 3-70xD. Cylindrical shank carbide tools are generally used with hydraulic chucks, thermal shrink-fit chucks, or collet chucks. When using through-coolant tools, sealed collets should be used with collet chucks. TKM drills provide a very efficient way to make holes with modern machine tools and often offer the most cost-effective solution for hole diameters under 14mm. Compared to HSS drills, carbide drills assure more accurate holes and a better surface finish on the hole walls.

Replaceable tip drills, or so-called "insert drills," are tools where the drill body is made of tool steel, and the drill tip has a replaceable carbide tip. The most common diameters for replaceable tip drills are 12-37mm, and the drilling depths range from 1.5-12xD.

Indexable drills, or so-called U-drills, offer a cost-effective solution for 10-59mm drill holes with drilling depths of 2-5xD. The U-drill body resembles that of a replaceable tip drill, made from tool steel, and features replaceable indexable inserts. U-drills have a 180-degree point angle and are well-suited for unstable conditions too. Replaceable tip drills and indexable drills are typically mounted in Weldon chucks.

In drilling, attention should be paid to the lubrication capacity of the cutting fluid and its suitability for the material being machined. Whenever possible, use tool through-cooling, and a high-pressure cutting fluid system is recommended for deep drilling. When using small through-coolant drills, attention should also be paid to the cleanliness of the cutting fluid, as small particles might clog the tool's through-cooling channels. The cutting fluid tank of the machine tool should be cleaned regularly, and impurities should be removed from the cutting fluid, for instance, using a separate cutting fluid purification unit.