When a mechanical machining process engineer selects a drill bit for a specific hole machining task, the first consideration is the depth of the hole to be machined. The deeper the hole to be machined, the greater the amount of chips that need to be discharged during the machining process. If the chips generated during machining cannot be discharged in a timely and effective manner, they may block the chip removal groove of the drill bit, thereby delaying the machining process and ultimately affecting the machining quality of the hole. Therefore, effective chip removal is a key factor in successfully completing any material's hole machining task.

Long diameter ratio of the drill bit

    When a craftsman selects the appropriate type of drill for a specific drilling task, it is necessary to calculate the length-to-diameter ratio of the drill. The length-to-diameter ratio is the depth of the hole being machined divided by the diameter of the drill, for example, if the diameter of the drill is12.7mm and the depth of the hole to be machined is38.1mm, then its length-to-diameter ratio is3:1. When the length-to-diameter ratio is approximately4:1 or less, the chip flute of most standard twist drills can smoothly discharge the chips removed by the cutting edge of the drill. However, when the length-to-diameter ratio exceeds this range, special deep-hole drills must be used to achieve effective machining.

   Once the length-to-diameter ratio of the machined hole exceeds4:1,the standard twist drill finds it difficult to lift the chips away from the cutting zone and discharge them out of the hole, and the chips quickly block the chip removal grooves on the drill, necessitatingstoppingdrilling, withdrawing the drill from the hole, clearing the chips from the chip removal grooves, and then reaming to continue cutting, this operation needs to be repeated many times to achieve the required hole depth, thisdrillingmethod is usually called"peck drilling.Peck drillingis adopted to machine deep holes, which will shorten the tool life, reduce the machining efficiency, and affect the quality of the machined hole. Every time the drill is withdrawn from the hole, cleared of chips, and reinserted into the hole, there is a possibility of deviating from the center line of the hole, thus making the hole diameter larger, exceeding the specified size tolerance.

  To solve the problem of deep hole machining, in recent years, drill manufacturers have developed two new types of deep hole machining drills——the ordinary parabolic drill and the wide-blade parabolic drill.

Ordinary parabolic drill bit

  The chip flute of parabolic drill bit is parabolic, which is specially designed for continuousdrillingoperation with a length-to-diameter ratio of up to15:1, and a material hardness of no more than25～26HRC(including low carbon steel, various aluminum alloys, copper alloys, etc.)for deep hole drilling. For example, a parabolic drill bit with a diameter of12.7mmcan successfully machine a hole depth of up to190mm

   The spiral angle of parabolic drill bits is 36°～38°, greater than the spiral angle of standard twist drills(28°～30°). The spiral angle can represent the“twist” degree of the drill bit, the larger the spiral angle, the faster the&chip removal speed of the drill bit. Another feature of ordinary parabolic drill bits suitable for deep hole machining is that the drill core is thicker(the drill core of the drill bit refers to the central part of the drill bit that is not ground after the chip removal groove is formed). The drill core part of the standard twist drill accounts for about20%of the entire finished drill bit, while the drill core of the parabolic drill bit can account for about40%of the entire drill bit. In deep holedrilling, a thicker drill core can increase the rigidity of the drill bit and improve the stability ofdrillingmachining. The parabolic drill bit has a slot at the tip, so a larger drill core diameter can be used, and it can also prevent the phenomenon of drill bit

   Parabolic drill bits are made of high-speed steel material, and in order to enhance the cutting performance, the drill bit can also be surface coated.

Broad-blade parabolic drill bit

  To adapt to difficult-to-machine materials(cold work hardening materials)deep holedrillingthe need for processing, some tool manufacturers have developed a wide-blade parabolic drill. Many of the features of this drill are similar to ordinary parabolic drills, such as a large spiral angle(36°～38°), easy chip removal; the drill core is thick, and the drill has good rigidity and stability during deep hole machining, etc. The difference between it and the ordinary parabolic drill is in the shape of the chip removal groove and the land. The land of the wide-blade parabolic drill smoothly transitions to the chip removal groove, giving the cutting edge of the drill a high degree of strength and rigidity, and the chips can also be smoothly discharged through the chip removal groove.

  In deep hole drilling, the high temperature generated by friction may cause the cutting edge of the drill bit to become soft or retempered, thus accelerating the wear of the drill bit. The ability of the cutting edge of the drill bit to maintain its hardness during processing can be expressed by the "red hardness". Wide-blade parabolic drill bits are usually made of high-speed steel and cobalt high-speed steel materials. Since cobalt high-speed steel parabolic drill bits have higher red hardness, the tool life is longer and the wear resistance is stronger.

Drill bit surface coating

  Ordinary parabolic drill bits and wide-blade parabolic drill bits often use the following surface coatings:①Titanium nitride(TiN)coating: This coating can significantly improve the service life of the drill bit and the quality of the machined hole, compared to coated drill bits,TiNcoated drill bits are more suitable for high-speed drilling of a variety of material workpieces(especially various steel parts).②Titanium carbon nitride(TiCN)coating: At the appropriate cutting temperature,TiCNcoating is harder, more ductile, and more wear-resistant thanTiNcoating, and is also suitable for high-speed drilling of a variety of workpiece materials(especially steel parts). However, special care should be taken when processing non-ferrous metal materials, becauseTiCNcoating has a high chemical affinity with non-ferrous metals and is very easy to wearaway.③Titanium aluminum nitride(TiAlN)coating: This coating can improve the service life of the drill bit, especially for cutting operations with high cutting temperatures. LikeTiCNcoating, TiAlNcoating is also not suitable for processing non-ferrous metal materials.

DrillingOptimization of parameters

  In deep hole machining, in order to maximize the cutting performance of the drill, it is necessary to optimize and adjust the cutting speed and feed rate according to the specific length-to-diameter ratio. When the length-to-diameter ratio of the drilling process is 4:1, the cutting speed should be reduced by 20% and the feed rate should be decreased by 10%; when the length-to-diameter ratio is 5:1, the cutting speed should be reduced by 30% and the feed rate should be decreased by 20%; when the length-to-diameter ratio reaches 6:1～8:1, the cutting speed should be reduced by 40%. In addition, when the length-to-diameter ratio is 5:1～8:1, the feed rate should be decreased by 20%.

   Although the price of parabolic twist drill is 2～3 times that of standard twist drill, its excellent performance in deep hole(long diameter ratio greater than4:1) processing greatly reduces the cost of each hole, and thus it has become the tool for mechanical engineers to process deep holes.