Hole making
The most common tools in hole machining are various drills, reamers, countersinks, and boring tools. The most common type of drill is the so-called twist drill, named after the helical shape of the drill's chip groove. Twist drills are typically made from either high-speed steel or carbide, and today, CNC machining often employs through-coolant solid carbide drills. For large holes, indexable insert drills (U-drills) and replaceable tip drills (cartridge drills) are used, which have replaceable cutting inserts and drill tips, making them more cost-effective than solid carbide drills. For machining large and precisely-sized holes, rough and fine reamers are typically used. Rough reamers generally have two cutting inserts, while fine reamers have one insert. The diameter of a fine reamer can be adjusted mechanically to an accuracy of up to 0.002 mm, allowing for the machining of precisely-sized holes. For small, precisely-sized holes, burnishing tools are typically used, which create accurate and high-quality holes, for example, for various locking and guiding pins.
High-speed steel (HSS, HSS-E, and HSS-E-PM) twist drills are the most common drill bits. HSS drills are either cylindrical shank or Morse taper shank and feature a 118-degree tip angle with a long chisel edge, which usually requires spot drilling. HSS drill bits come in various coatings, the most common being steam-tempered and TiN coating. HSS drills are a cost-effective solution for drilling but are nowadays mainly used in small batches and unstable conditions on automatic machining machines. HSS twist drills are the most common choice for hand drilling or manual machining. Cylindrical shank HSS twist drills are typically mounted in CNC drill chucks or collet chucks, whereas Morse taper shank drills are suitable only for Morse taper holders.
Carbide is a powder metallurgically manufactured composite material in which tungsten carbide particles are mixed with a cobalt-containing binder. Solid carbide drills resemble cylindrical shank HSS drill bits, but carbide drills usually have a 140-degree tip angle and through-coolant channels. TKM drills are available in sizes from 0.1-25 mm, and their drilling lengths vary between 3-70xD. Hydraulic chucks, shrink-fit chucks, or collet chucks are usually used with cylindrical shank carbide tools. Sealed collets must be used with through-coolant tools. TKM drills are a highly efficient way to produce holes with modern machining tools, often being the most cost-effective solution for diameters under 14 mm. Compared to HSS drills, carbide drills provide more accurate holes and better surface quality on the hole walls.
Replaceable tip drills, also known as "cartridge drills," are tools in which the drill body is made of tool steel, and the drill tip is a replaceable insert made of carbide. The most common drill diameters for replaceable tip drills are 12-37 mm, and the drilling depths are 1.5-12xD.
Indexable insert drills, also known as U-drills, are a cost-effective solution for 10-59 mm holes with drilling depths of 2-5xD. Like replaceable tip drills, the U-drill body is made of tool steel and features replaceable indexable inserts. The U-drill is equipped with a 180-degree tip angle and is also suitable for unstable conditions. Replaceable tip drills and indexable insert drills are typically mounted in Weldon holders.
In drilling, attention must be paid to the lubrication capability of the cutting fluid and its suitability for the material being machined. Whenever possible, the tool's through-cooling system should be used, and in deep drilling operations, a high-pressure system for the cutting fluid is recommended. When using small through-coolant drills, attention should also be paid to the cleanliness of the cutting fluid, as small particles can clog the tool's through-cooling channels. The cutting fluid tank of the machining tool should be cleaned regularly, and impurities removed from the cutting fluid, for example, using a separate cutting fluid purification unit.
