Hole Machining
The most common tools used for hole machining are various types of drills, boring tools, countersinks, and reamers. The most common drill type is the so-called twist drill, named after the helical shape of the drill’s flutes. Twist drills are generally made either of high-speed steel or carbide, and modern CNC machine tools primarily use solid-carbide drills with through-tool coolant. Indexable-tip drills and exchangeable-head drills are also used for larger holes. Thanks to their replaceable inserts and drill tips, these tools have lower operating costs than solid-carbide drills. Rough and finish boring tools are typically used for machining large, precision-diameter holes. Rough boring tools are generally equipped with two inserts, while finish boring tools have one insert. The diameter of a finish boring tool can be mechanically adjusted with accuracy of up to 0.002 mm, enabling the machining of precision holes. Reamers are typically used for small, precision-diameter holes, producing accurate holes with a high-quality surface finish for various types of pins and guide pins, for example.
High-speed steel (HSS, HSS-E, and HSS-E-PM) twist drills are the most common drill bits. HSS drills are available with either cylindrical shanks or Morse taper shanks and are equipped with a 118-degree point angle and a long chisel edge, which generally means that they require a center-drilling operation. HSS drill bits are available with various coatings, the most common being steam tempering and TiN coating. HSS drills are an economical solution for drilling, but on automated machine tools they are now mainly used only for small production batches and unstable conditions. HSS twist drills are the most common choice for hand drilling and manual machine tools. Cylindrical-shank HSS twist drills are typically clamped in CNC drill chucks or collet chucks, whereas Morse taper holders are the only suitable option for mounting taper-shank drill bits.
Carbide is a powder-metallurgically produced composite material in which tungsten carbide particles are combined with a cobalt-containing binder. Solid-carbide drills resemble cylindrical-shank HSS drill bits, but carbide drills generally have a 140-degree point angle and through-tool coolant channels. Solid-carbide drills are available in diameters from 0.1 to 25 mm, and their drilling depths range from 3 to 70×D. Cylindrical-shank carbide tools are generally used with hydraulic chucks, shrink-fit holders, or collet chucks. When using through-coolant tools with collet chucks, sealed collets must be used. Solid-carbide drills are a highly effective way to produce holes on modern machine tools and are often the most cost-effective solution for diameters under 14 mm. Compared with HSS drills, carbide drills also provide more accurate holes and a better surface finish on the hole walls.
Exchangeable-head drills, also known as “indexable-tip drills,” are tools in which the drill body is made of tool steel and the drill tip is a replaceable carbide component. The most common diameters for exchangeable-head drills are 12–37 mm, and their drilling depths range from 1.5 to 12×D.
Indexable-insert drills, also known as U-drills, are a cost-effective solution for holes with diameters of 10–59 mm and drilling depths of 2–5×D. Like exchangeable-head drills, U-drill bodies are made of tool steel and feature replaceable indexable inserts. U-drills have a 180-degree point angle and are also highly suitable for unstable conditions. Exchangeable-head drills and indexable-insert drills are generally clamped in Weldon holders.
During drilling, attention must be paid to the lubricating properties of the cutting fluid and its suitability for the material being machined. Through-tool coolant should be used whenever possible, and a high-pressure cutting-fluid system is recommended for deep-hole drilling. When using small through-coolant drills, the cleanliness of the cutting fluid must also be monitored, as small particles can block the tool’s internal coolant channels. The machine tool’s cutting-fluid tank should be cleaned regularly, and contaminants should be removed from the cutting fluid, for example, using a separate cutting-fluid filtration unit.
