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The size, precision, surface quality and structure of deep hole parts encountered in production are different. There are many methods for deep hole processing, such as general deep hole processing, precision deep hole processing and electric deep hole processing; conventional deep hole processing and ultra-slender deep hole processing; small diameter deep hole processing, medium diameter deep hole processing and large diameter deep hole processing; deep hole processing of rotating parts and deep hole processing of non-rotating parts. Different types of deep holes have different processing methods, processing systems, processing tools and processing processes. 1.2.1 Classification of deep hole processing 1. Classification by processing method (1) Solid material deep hole drilling method: The blank has no hole, and the deep hole is processed by deep hole drilling method, as shown in Figure 1.2. (2) Deep hole boring method: The blank has a deep hole, and the hole expansion method is used to correct the defects of the existing deep hole and expand the hole size to improve the processing accuracy of the existing deep hole, as shown in Figure 1.3. (3) Deep hole reaming method: The blank has a deep hole. In order to further improve the dimensional accuracy of the existing deep hole and reduce the surface roughness of the hole, a small back cutting amount expansion method is used, as shown in Figure 1.4. (4) Deep hole drilling method: The blank has no hole. A hollow drill is used to drill a deep hole. After drilling, the center of the rough hole is broken and a core rod remains. This is a deep hole drilling method, as shown in Figure 1.5. (5) Deep hole honing method: The blank has a deep hole. In order to improve the surface quality, surface stress state and dimensional accuracy of the existing deep hole, a low-speed grinding principle is used to correct the surface defects of the inner hole of the deep hole part. This is a deep hole processing method, as shown in Figure 1.6. (6) Deep hole rolling method: The blank has a deep hole. In order to refine the size and surface quality of the existing deep hole and strengthen the deep hole surface, a plastic deformation principle is used to correct the surface defects of the inner hole of the deep hole part. This is a deep hole processing method, as shown in Figure 1.7. (7) High-energy beam deep hole processing method: The blank has no holes, and the energy of high-energy beam (laser beam, ion beam) is used to drill deep holes by photothermal effect, as shown in Figure 1.8. (8) Deep hole vibration drilling method: The blank has no holes, and the drill bit (or workpiece) vibrates regularly while feeding normally, so that the drill bit cuts during the vibration, forming a pulsed cutting force waveform, so that the cutting amount changes according to a certain rule, so as to achieve a deep hole drilling method with improved cutting efficiency, as shown in Figure 1.9. 2. Classification by motion form As shown in Figure 1.10(a). (1) Workpiece rotation, tool feed processing method: The workpiece performs the main motion, and the tool performs the feed motion, as shown in Figure 1.10(b). (2) Workpiece stationary, tool rotation and feed processing method: The tool performs the main motion and feed motion, as shown in Figure 1.10(b). (3) Workpiece and tool rotation in opposite directions, tool feed processing method: The workpiece and tool perform the main motion at the same time, and the tool also performs the feed motion. This processing method inherits the advantages of single rotational motion and increases the relative main motion speed, as shown in Figure 1.10(c). (4) Workpiece rotation and feed, tool stationary processing method: The workpiece performs both main motion and feed motion, as shown in Figure 1.10(d). 3. Classification by chip removal method (1) External chip removal processing method: During deep hole processing, the cutting fluid enters the cutting zone from the inside of the drill rod, and the chips are discharged from the outside of the tool rod, as shown in Figure 1.11. (2) Internal chip removal processing method: During deep hole processing, the cutting fluid enters the cutting zone from the annular space formed by the outside of the drill rod and the processed hole, and the chips are discharged from the inside of the tool rod, as shown in Figure 1.12. 4. Classification by processing system (cooling, chip removal system) (1) Deep hole external chip removal system. During machining, the drill box drives the drill bit to rotate at high speed to achieve cutting motion, and the servo motor drives the screw to push the drill box to achieve feed motion. The oiler injects high-pressure cooling oil from the tail of the drill bit, directly from the inner hole of the drill bit to the cutting part of the workpiece. The chips are flushed by the coolant along the chip guide groove of the drill bit to the chip collector and enter the chip collection bucket. The coolant is filtered by filter paper and magnetic filtration and then returns to the oil tank for further use. Figure 1.13 shows a typical deep hole external chip removal system – gun drilling processing system. (2) BTA processing system. The BTA processing system is one of the deep hole internal chip removal processing methods and is a typical deep hole internal chip removal processing system. High-pressure cutting fluid is pressed into the cutting area of ​​the workpiece hole through the oiler installed in the guide frame, driving the chips through the inner hole of the drill rod and the main shaft hole of the drill rod box, and then flows to the internal chip collection cover and chip collection car installed at the rear end of the drill rod box main shaft to the collection pool. The chips are then removed by the magnetic separator, filtered and precipitated multiple times in the sedimentation box at irregular intervals, and then flow into the oil pump tank. The screw pump then sucks the oil into the oil feeder and continuously recycles. Figure 114 shows a typical BTA processing system. (3) Ejector Drilling System. The ejector drilling system belongs to the deep hole internal chip removal processing method. It is invented based on the ejector effect principle of fluid mechanics and is also a double-tube internal chip removal deep hole processing method, as shown in Figure 1.15. The jet-suction drilling system uses a double-tube tool bar. The cutting fluid enters from the inlet after being pressurized. 2/3 of the cutting fluid enters the annular space between the inner and outer drill bars, flows to the cutting part for cooling and lubrication, and pushes the chips into the inner cavity of the drill bar. The remaining 1/3 of the cutting fluid is sprayed into the inner drill bar at high speed from the crescent-shaped nozzle on the inner drill bar, forming a low-pressure area in the inner cavity of the inner drill bar, which produces a suction effect on the cutting fluid carrying the chips. Under the dual effects of spraying and suction, the chips are quickly discharged from the outlet. (4) DF system. The DF system belongs to the deep hole chip removal processing method and is a further development of the jet-suction drilling system. It is also called the double-inlet single-tube chip removal system. The cutting fluid of the DF system is divided into two branches, the front and the back, which enter from two inlets respectively, as shown in Figure 1.16. The first 2/3 of the cutting fluid flows to the cutting part through the annular area formed by the drill rod and the machined hole wall, pushing the chips into the chip outlet on the drill bit and then into the drill rod, flowing to the chip extractor; the second 1/3 of the cutting fluid directly enters the chip extractor, and is accelerated after passing through the narrow conical gap between the front and rear nozzles, generating a negative pressure suction effect, thereby achieving the purpose of accelerating chip removal. The structure of the first half of the DF system that plays a “pushing” role is similar to the BTA processing system, and the structure of the second half that plays a “suction” role is similar to the ejector drilling system. Because the DF system uses a dual oil inlet device, only one drill rod is used to complete the pushing and suction chip cutting method, so the drill rod diameter can be made very small, and smaller holes can be processed. (5) SIED system. The SIED system belongs to the internal chip removal method and is a single-tube internal chip removal ejector drilling processing system. Based on three internal chip removal drilling technologies: the BTA machining system, the ejector drilling system, and the DF system, a separately adjustable power-boosted chip extraction device has been added to enable independent control of the cooling and chip removal fluid flows. As shown in Figure 1.17, the cutting fluid, after being discharged from the hydraulic pump, branches into two streams: the first stream flows into the oil feeder, then flows through the annular space between the drill pipe and the hole wall to the cutting part, pushing the chips into the chip outlet on the drill bit; the second stream flows into the chip extractor, then enters the rear nozzle cavity through the gap between the tapered nozzle pair, generating a high-speed jet and negative pressure.

The SIED system features independent pressure regulating valves for each of the two fluid streams, allowing them to be adjusted for optimal cooling and chip extraction conditions.

1.2.2 Characteristics of Ultra-Slender Deep Hole Machining in Difficult-to-Machine Materials
Difficult-to-machine materials exhibit excellent properties such as high strength, good wear resistance, good hardenability, good corrosion resistance, and high-temperature resistance. Consequently, they are increasingly used in industries such as aerospace, petrochemicals, and marine development. However, this type of material has high toughness, low thermal conductivity, large plastic deformation during cutting, severe work hardening, high cutting heat and difficulty in heat dissipation. The chips adhere to the cutting edge seriously, easily form built-up edge, and are difficult to remove. It is very difficult to process ultra-slender deep holes on them. Ultra-slender deep hole processing of difficult-to-process materials has the following characteristics compared with general cutting processing: (1) The ultra-slender deep hole processing process is basically in a fully closed or semi-closed state. The cutting state, tool feed and tool wear of the deep hole tool cannot be directly observed. The only way to judge whether the cutting process is normal is to listen to the sound, observe the chips, observe the machine tool load, pressure gauge, touch the vibration, etc. (2) Since the hole is relatively long, the path of the chips in the deep hole is relatively long, which is not easy to remove. It is easy to cause chip blockage and make the tool ineffective. Therefore, it is very important to control the length and shape of the chips and to perform forced chip removal. (3) Due to the slenderness of the drill rod, its rigidity is poor, and the axis of the hole is easily deflected during processing. Vibration will occur during the processing, and the vibration will increase with the increase of processing depth, making it difficult to ensure the accuracy and surface roughness of the processed hole. Therefore, support and guidance are extremely important. (4) Cutting heat is not easy to dissipate in the cutting area, and heat is easily accumulated, causing serious wear of the drill bit. Generally, 80% of the cutting heat is carried away by the chips during the cutting process, while in deep hole drilling, only 40% is carried away. The tool accounts for a larger proportion, which is slow to diffuse and easy to overheat. The cutting edge temperature can reach 600°C, and forced and effective cooling methods must be adopted.