A recent survey conducted in the German metalworking industry showed that drilling is the most time-consuming process in a machine shop. In fact, 36% of all machining time is spent on hole machining operations. Correspondingly, the turning time is 25% and the milling time is 26%.
Therefore, the replacement of high-speed steel and ordinary carbide drills with high-performance solid carbide drills can significantly reduce the man-hours required for drilling and reduce the cost of hole machining.
In the past few years, cutting parameters (especially cutting speed) have been increasing, especially for high-performance solid carbide drills. Twenty years ago, the typical cutting speed of solid carbide drills was 60-80 m/min. Today, it is not surprising that the machine can drill steel at a cutting speed of 200 m/min with sufficient power, stability and coolant delivery. Despite this, drilling has a significant potential for improvement in machining efficiency compared to the general cutting speeds of turning or milling.
Solid carbide drills require high toughness of the substrate, and bit wear is acceptable under controlled and uniform stability. Therefore, typical drilling tool grades contain more cobalt than turning or milling tools.
The drill material is usually made of fine-grained carbide to increase the strength of the cutting edge and ensure uniform wear without chipping. Water-based cutting fluids are often used when machining with carbide drills, so the temperature at the cutting edge is not too high, but the drill is required to have thermal shock resistance. The best performing drill grade is a typical pure tungsten carbide material without the need to add a large amount of tantalum carbide or titanium carbide.
For solid carbide drills, the coating must perform more than just increase the surface hardness and wear resistance. The coating must provide a thermal barrier between the tool and the workpiece material and remain chemically inert; the bond between the workpiece material and the coating must be minimized to reduce friction; the coating surface must be as smooth as possible; The coating of the twist drill must also have resistance to crack propagation. The dynamics of the drilling process may cause microcracks, and in order to maintain tool life, crack propagation must be prevented. By selecting the correct coating process and generating the appropriate coating microstructure, the coating material can be placed under compressive stress, thereby significantly extending tool life.
Good results can be obtained with a multi-layer coating. The multi-layer coating prevents microcracks from spreading between the layers of the coating, and even if individual coatings are damaged and peeled off, the other coatings can protect the carbide substrate. For drilling tools, nano-coating and precision-custom coatings also have great potential for development.
For example, a new TiAlN nanocoating with TiN on the top layer can solve many of the problems encountered in drilling stainless steel. The smooth TiN top coat reduces the bond and friction of the tool to the workpiece material, while the lower TiAlN nanocoat provides hardness and wear resistance to the tool. This coating has excellent crack propagation resistance and thermal shock resistance. The cutting speed can reach 70-80m/min when drilling stainless steel, which is almost twice that of conventional drills.
In order to give full play to the excellent performance of modern cemented carbide substrates and surface coatings, it is necessary to optimize the geometry and drill type of the drill bit. The drill tip, the drill angle, the edge shape, the cutting edge preparation, Reasonable adjustments are made to the number of flutes, flutes and land.
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