Deep hole machining example: Proper use of the BTA deep hole drill

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The BTA deep hole drill bit represents a typical design for internal chip removal deep hole drills; it is an improvement over single-edge deep hole drills with internal chip removal. Its cutting edge features double-sided serrations, allowing chips to be cut from both sides and to be discharged outside the drill shaft through double-sided chip removal holes. The BTA deep hole drill features a uniform distribution of cutting forces, excellent chip separation and breaking capabilities, stable and reliable drilling performance, and well-aligned deep holes. 1. Structural features of the BTA deep hole drill: The BTA deep hole drill has the following structural features: (1) The drill body is equipped with outer cutting edges, middle cutting edges, inner cutting edges, guide blocks, and double-sided chip removal holes; it is connected to the hollow drill shaft through a shallow-threaded rectangular thread on the drill body. (2) In the core drilling section, the internal cutting edge replaces the flange of the twist drill, thereby overcoming the disadvantages of the twist drill’s long flange and high axial resistance ； Since the core is offset a certain distance from the axis of the drill hole, the cutting edge at the core is lower than that at the center during drilling; this results in the formation of a guiding core column that provides better guidance for the drill bit, preventing it from drifting during drilling. Once this guiding core column reaches a certain length, it will break off on its own and be discharged along with the chips. (3) The main cutting edge features an asymmetric, segmented and interlaced arrangement, which ensures reliable chip separation and prevents cracks from occurring when using a solid carbide blade to grind the chip flutes and chip separation grooves. (4) Several different grades of cemented carbide can be used for the blade material, in order to meet the varying requirements regarding wear resistance and strength that different parts of the structure have. For example, in the core drilling section, the cutting speed is low and the cutting force is high; under the pressure exerted by the chips, the blade is prone to chipping, so a cemented carbide blade with better toughness can be chosen ； For the outer edge of the drill bit, cemented carbide inserts with good wear resistance can be used. 2. Processing principle of the BTA deep hole drill: The operation of the BTA deep hole drill on a conventional lathe – the workpiece to be processed is positioned using a V-block on the lathe’s main slide and clamped with bolt plates. During drilling, the drill rod is clamped by a special chuck inside the spindle and rotates driven by the spindle, while the workpiece is fed through motion driven by the large slide. A fluid inlet is installed on the machine tool worktable, and it is hermetically connected to the left end face of the workpiece via an O-ring seal. The pressurized cutting fluid is injected through the inlet of the fluid feeder, flows into the cutting area via the gap between the outer diameter of the drill rod and the hole walls to cool the cutting tool, and the chips are discharged from the drainage tank together with the cutting fluid through the outlet of the special chuck located in the inner bore of the drill rod. A 5% concentration emulsion can be used as the cutting fluid ； The cutting parameters that can be used are: V=60～90m/min, S=0.035～0.23mm/r. Due to the slender shape of the drill rod, which makes it prone to deformation, a movable central support is installed on the machine tool guideways to provide support at any position of the drill rod. The feed unit and the large slide are connected by a connecting plate, and move forward together with the workpiece. 3. Key points for deep hole machining: Since deep hole machining has characteristics and disadvantages different from those of ordinary hole machining, the following points should be taken into account when using BTA deep hole drills: (1) It is not possible to directly observe the cutting condition of the tool during deep hole machining; therefore, it is necessary to judge the chip removal situation and tool wear by listening to sounds, observing the chips, checking the load on the machine tool, and monitoring the pressure of the cutting fluid. (2) Heat dissipation is difficult in deep hole machining; therefore, effective and reliable methods for cooling cutting heat must be employed. (3) Chip removal is difficult in deep hole machining; chip blockage can easily damage the tool, so it is necessary to select appropriate cutting parameters to ensure reliable chip breaking and smooth chip removal. On Taobao’s special sale site, what are the good hand care products and lotions? Are there any suitable day creams? The factory is for rent. Which brand is best for lotions and eye serums? Which eyeshadows work well? Which eye serums offer the best results? (4) During deep hole machining, the holes tend to deviate from their intended path; therefore, guidance mechanisms and measures should be taken into account in the design of the cutting tools and fluid supply systems. (5) During deep hole machining, the long drill rod has poor rigidity and is prone to vibration, which directly affects the machining accuracy and production efficiency; therefore, it is very important to select the cutting parameters appropriately. Among the above issues, chip removal, guidance, and cooling are the most important. Solving these issues properly can ensure drilling accuracy, extend tool life, and improve processing efficiency. Therefore, in deep hole machining, the following process measures can be adopted depending on the specific machining requirements: (1) Before drilling, a shallow hole with the same diameter as the drill bit is pre-drilled, which serves to guide and center the drill bit during drilling. This step is particularly necessary when machining small holes that require high straightness. (2) When installing and debugging the machine tool, make every effort to ensure that the central axis of the workpiece hole coincides with the central axis of the drill rod. (3) Select the cutting parameters appropriately based on the material of the workpiece, in order to control the degree of chip curling and obtain C-shaped chips that facilitate chip removal. When machining workpieces made of high-strength materials, the cutting speed V should be appropriately reduced. The magnitude of the feed rate has a significant impact on chip formation; a lower feed rate can be used as long as chip breaking is ensured. (4) To ensure effective chip removal and cooling, the cutting fluid should maintain an appropriate pressure and flow rate. High pressure and low flow rate can be used when machining small-diameter deep holes ； Low pressure and high flow rate can be used when machining large-diameter deep holes. (5) When starting drilling, the cutting fluid pump should be turned on first, after which the machine tool should be started up to begin cutting ； When drilling is completed or a fault occurs, the tool feed should be stopped first, then the machine should be shut down, and finally the cutting fluid pump should be turned off. 4. Example of deep hole machining: A deep hole with a diameter of 20+0.2 mm and a depth of 1500 mm was drilled in a workpiece made of 27SiMn with an outer diameter of 100 mm; the surface roughness of the hole walls was required to be Ra1.6 μm. A BTA deep-hole drill (diameter 20 mm) produced by Chengdu’s deep-hole machining centers was used for the processing; the blade grade is YT798, and the drill shank is made of 45 steel (quenched and tempered, HB240–280) ； Selected cutting parameters: V=60m/min, S=0.21mm/r ； A 5% emulsion is used as the cutting fluid, with a pressure of 2.5 MPa/m2; the hydraulic pump has a flow rate of 80 L/min. Processing precautions: ① The end face of the workpiece should be perpendicular to its axis line to ensure reliable sealing at the end face ； ②Before formal machining, a shallow hole with a diameter of 20 mm is pre-drilled at the hole location on the workpiece; this serves to guide and center the drill bit during drilling ； ③To ensure the tool’s service life, it is best to use automatic feed movement ； ④If the guide sleeves in the feed device and the movable central support are worn, they should be replaced promptly to avoid affecting the drilling accuracy. Long-term usage experience with BTA deep hole drills shows that the material of the workpiece has a significant impact on the quality of deep hole drilling and the processing efficiency; among various materials, 27SiMn offers the best machinability, followed by 45 steel ； 40Cr material is difficult to machine; it does not form C-shaped chips easily, which can lead to blockages in the drill rods and damage to the drills. Therefore, when machining 40Cr material, quenching and tempering treatment (HB240–260) can be carried out in advance to reduce the difficulty of processing. During drilling, it is important to monitor the wear of the clearance angles on each edge of the cutting tool. When the wear amount of these clearance angles exceeds 0.3 mm, the tool should be re-sharpened or replaced with a new one; continuing to use it will result in increased cutting forces, blockage of the drill rod, and damage to the tool.