Regular Drill Bit Processing
Conventional drill bits often face great problems when machining stainless steel and heat-resistant alloys. During machining, there will be a sharp whistling sound, rapidly increasing wear or chipping of the tool cutting edge. The typical phenomenon is the chipping of the secondary cutting edge, also known as the guide edge. If this phenomenon occurs when drilling alloys, the most likely result is a shortened tool life or even tool scrapping.
Hard Metal Tool Regrinding
Regrinding high-quality hard metal tools is usually not economical or even impossible. The chipping of the guide edge on hard metal drill bits is a typical phenomenon when cutting stainless steel and heat-resistant alloys.
The main cause of this phenomenon is the vibration of the drilling tool, which has various reasons. One reason is the rebound of the tool due to the material being cut. When the vibration occurs, the head of the tool moves in an elliptical trajectory, while the blade or the tip of the tool moves in a polygonal (mostly triangular) trajectory. This movement has an adverse effect on the cutting path of the tool. Whether a drilling tool vibrates and how much it oscillates depends mainly on the grinding form of the tool head, the type of guide edge, the grinding accuracy and the precision of the grinding work.
The grinding of hard metal tools usually adopts the 4-sided and conical grinding process. Compared with the conventional grinding process, this unique process requires deep grinding of the cutting edge into the center of the drill. The tool head form will ensure high precision and the grinding is carried out as far as possible according to the latest research results in cutting technology. If the centering accuracy is not high at the beginning of drilling, the tool may swing to a large extent, which may also lead to a decrease in precision during processing.
Grinding errors such as low concentricity or low tool symmetry may aggravate the above phenomenon. Errors in peripheral links may further affect the processing accuracy. Therefore, it is necessary to first merge the deviations and tolerances of the clamping system and the machine tool spindle, such as concentricity deviation and inclination. Finally, torsional and axial vibrations and low-frequency bending vibrations (oscillating motion) generated between the drill bit and the machine tool can lead to angular or out-of-round apertures.
Milling Tools Increase Oscillating Motion
Stainless steel and heat-resistant alloys place high demands on drilling tools. Due to the high hardness of the tool material, high cutting forces are required. The machinability of steel is adversely affected by the high tendency to cold hardening, the low thermal conductivity and the low toughness. The ductility of the material means that the drilled hole diameter is usually smaller than the nominal diameter due to material rebound.
Deviations in diameter and roundness increase the pressure on the guide edge, leading to increased contact between the drill bit and the hole wall and even possible drill breakage. The increased pressure on the guide edge is mainly related to friction and local temperature rise, and can also cause damage to the material edge. The load on the tool tip caused by squeezing or oscillation can be found, which can indicate in advance which areas will break before the standard service life.
Cutting Parameters
Cutting parameters also have an impact on drilling quality, including not only cutting speed, but also feed rate, which is also a decisive factor. At present, the maximum cutting speed of quenched and tempered steel is about 200 m/min, and the feed rate can generally be much higher than 0.1 mm/turn. For example, a drill with a diameter of 8.5 mm can withstand a feed rate of 0.25 mm/turn or even higher. Higher feed rates stabilize the drill bit and slightly eliminate the tendency to swing, so the quality of the drilling process can be appropriately improved.
However, stainless steel and nickel-based alloys cannot use such high cutting speeds and feed rates due to the limitations of the material properties themselves, otherwise the drill bit will be overloaded or even damaged. The feed rate under normal circumstances needs to be kept at a low level, far below the feed rate of 0.1 mm/turn. Since the lateral cutting edge of the drill bit not only cuts the workpiece when cutting in, it will also squeeze the workpiece. Therefore, using such parameters is conducive to avoiding swinging motion. The drill will squeeze the workpiece surface. If the workpiece interferes with the guide edge of the drill, then the drill with better symmetry can basically maintain a stable cutting process, and the swinging movement will also follow the spiral line.
The chips generated during the chipping process need to be quickly discharged from the chip flute. In addition, the chip generation rate needs to be controlled so that it can be discharged more smoothly to avoid damaging the inner wall of the aperture. The adjusted chip flute profile and optimized chip shape can make the chips curl as much as possible. The chips need to be curled together as much as possible according to different materials. In addition, it is necessary to avoid uncontrolled short chips from entering the chip flute as much as possible, which will cause damage to the inner wall of the aperture. Using a Y-type drill can achieve better surface quality while ensuring the same service life, and at the same time ensure that the chips are quickly and smoothly discharged into the chip flute.
Conical Cutter Head
The conical cutter head shape is more conducive to centering. The first impression left by the Y-shaped drill bit is that the angles between different chip flutes are inconsistent. The three guide edges are arranged in the shape of the letter Y, even though the drill has only two cutting edges. The Y-type drill has a conical head structure and is precision ground to ensure accurate centering. The TiAlN coating brings high wear resistance and production efficiency, and has a very wide range of applications. Drills around the world can be reground and re-coated in a very short time.
Non-uniform Arrangement of Chip Flutes
With the help of non-uniform arrangement of chip flutes, directional cutting forces can be achieved. In the direction of the force, there is a guide edge on the cutting edge and a further guide edge at the end of the drill blade. The Y-shaped structure supports this additional guide edge. The load on the edge arranged opposite the two edges is reduced accordingly. During the cutting process, the three guide edges play different roles: the guide edge is responsible for cutting, the guide edge is responsible for cutting and supporting, and the guide edge or sliding edge is responsible for supporting.
With this structural arrangement, tool swing can be basically eliminated, especially when drilling, the roundness tolerance and cylindricity tolerance can be guaranteed. If the cutting edge is further optimized, wear can be minimized. The high quality requirements of the drilling process and the “pressure” exerted on the drill, especially the cutting edge and the guide edge, are reduced.
The above-mentioned technology can appropriately extend the cutting distance of the tool. There is a certain regularity between the hole and the drilling depth, for example, the finished hole diameter is slightly larger than the nominal diameter of the drill. This means: the drill no longer gets stuck in the hole. Under good conditions, a hole quality of IT8 can be achieved. The first and last hole diameters drilled with the same drill remain continuous and stable. The tool life of subsequent processes such as reamer and tapping can also be increased.
Y-drills have been successfully used in many applications. For example, good results can be achieved even when machining stainless steels such as 1.3916, 1.4350 or 1.4542 that have not yet rusted. During the service life, the efficiency increase can often reach more than 100%. Even when machining thermally stable and even hardened steels with a hardness of 55HRC, Y-drills still give satisfactory results.
