1 Definition of CNC deep hole drilling machine
Academic definition: A drilling machine that uses a special deep hole drill to make a linear feed motion, and the workpiece rotates to drill deep holes.
Technical definition: Relying on specific drilling technology, special machine tools for drilling deep hole systems, and precision shallow holes with a length-to-diameter ratio greater than ten are collectively called deep hole drilling machines.
2 Features of CNC deep hole drilling machine
The CNC deep hole drilling machine represents advanced and efficient hole processing technology, and precision processing results can obtain through one pass. The machined holes have precise positions and good dimensional accuracy; high straightness, coaxial, and high surface roughness.
Various forms of deep holes can process conveniently, and different particular forms of deep holes, such as cross holes, inclined holes, blind holes, and flat bottom blind holes, can also be solved well. The CNC deep hole drilling machine can process deep holes with a large aspect ratio (up to 300 times) and process precision shallow holes.
3 The structure of the CNC deep hole drilling machine
CNC deep hole drilling machines mostly divide into horizontal and three-coordinate structures. The machine tool has an independent and complete cutting oil high pressure, cooling, and filtering system to ensure sufficient, clean, and moderate temperature cutting oil supply.
For ease of operation, a high-pressure coolant provides from the machine tool’s back—the drill bit fix on the spindle through a rotating clamping sleeve. You can choose the form and position of workpiece rotation and tool rotation, and you can also select the way of spindle movement or table movement.
The machine tool is equipped with a chip recovery box. When the chips are discharged, the coolant will be recovered and cooled for recycling. The front end of the recovery box is equipped with a guide sleeve to guide the drill bit into the workpiece’s surface. If the length of the drill bit exceeds, and there is no support, increase the support appropriately.
4 Classification of CNC deep hole drilling machines
1. According to the layout of the spindle, it can divide into:
Horizontal deep hole drilling machine, vertical deep hole drilling machine, and three-coordinate deep hole drilling machine.
2. Classification according to chip removal method:
The cutting fluid passes through the hollow drill pipe and reaches the drill head for cooling and lubrication, and discharges the chips from the drill bit and the V-groove outside the drill pipe as an external chip removal method, such as gun drills.
The cutting fluid enters between the outer wall of the drill pipe and the machined surface of the workpiece, reaches the tool head for cooling and lubrication, and pushes out the chips from the inside of the drill pipe, such as BTA drilling.
3. Classified according to the form of exercise:
When the workpiece rotates, the tool performs a feed motion; the workpiece does not move, and the device rotates and then conducts a feed motion; the workpiece rotates, and the device performs a reverse drilling rotation and then performs a feed motion.
The specific method used depends on the characteristics of the workpiece and the condition of the processed hole.
4. According to the current commonly used deep hole drilling processing system, and it divides into:
1. Gun drill system (small and medium-sized)
It mainly uses to drill small diameter (generally less than 35mm) deep holes. It requires high cutting fluid pressure and is the most common method of deep hole drilling. It belongs to the internal cooling and external chip removal method.
The cutting fluid passes through the hollow drill rod to reach the drill head for cooling and lubrication and discharges the chips from the drill bit and the V-shaped groove outside the drill rod. The high-pressure gas-oil mist provides cooling for it And the power of chip removal.
The gun drill structure divide into three parts:
- Drill bit: made of cemented carbide;
- Drill pipe: made of special-shaped high-quality steel pipe;
- Drill shank: made of high-quality medium carbon structural steel.
The gun drill consists of a V-shaped cutting edge and one (or two) drill bits that can pass cutting fluid, a crescent-shaped drill rod, and a drill shank for clamping. It is mainly suitable for deep hole processing.
The ratio of hole depth to hole diameter is more significant than 100 times, especially for deep hole processing of diameter 2 to diameter 20.
The gun drill shank clamps on the machine tool’s main shaft, and the drill bit enters the surface of the workpiece through the guide hole or the guide sleeve. After entering, the 2-3 blades on the drill bit’s arc surface play a guiding and squeezing role.
At this time, the high-pressure cutting fluid is sent to the cutting area through the small hole in the middle of the drill bit for cooling and lubrication and helping chip removal—chips and cutting fluid discharge along the V-shaped groove of the drill bit.
Compared with traditional twist drills, gun drills have the characteristics of high machining accuracy, short machining time, long drill life, and good chip removal. However, conventional twist drills must retract to remove chips at a depth of about 1 to 2 times the diameter.
The processing accuracy is low, the surface roughness is insufficient, the processing efficiency is low, the operation labor intensity is high, and the quality is difficult to guarantee.
2. BTA single tube drilling system (medium size)
It belongs to the method of external cooling and internal chip removal. The cutting fluid enters between the drill pipe’s outer wall and the machined surface of the workpiece through the oil applicator and reaches the tool head for cooling and lubrication, and the chips push out of the drill pipe. In addition to the guiding function, the oil applicator also provides a channel for oil delivery to the cutting area.
This system is widely used, but limit by the chip removal space of the drill rod inner hole and is mainly used for deep hole drilling with a diameter of >12mm. Compared with jet drilling, the high cutting fluid pressure makes the single-tube drilling system more reliable, especially when drilling materials that are difficult to segment (such as low carbon steel and stainless steel). Compared with the jet suction drilling system, the BTA single tube drilling system is the first choice for large-scale, high-load continuous processing.
3. Ejector drilling system
The ejector drilling system is deep hole drilling with internal chip removal. The cutting fluid enters from the oil delivery port on the coupler.
Most of the cutting fluid enters the annular space between the inner and outer drill rods forward, reaches the tool head for cooling and lubrication, and pushes the chips into the inner drill rod’s internal cavity to discharge backward.
Besides, a small part of the cutting fluid utilizes the jet effect of fluid mechanics. The crescent nozzle on the drill rod is sprayed into the inner drill rod’s back at high speed, forming a low-pressure zone in the internal drill rod cavity to discharge the cutting fluid and chips from the cutting zone. The backward suction produces, and under the dual action of suction, the chips promptly bounced outward.
This relatively independent system requires lower cutting fluid pressure than the BTA system while also reducing the drilling system’s sealing requirements. Due to the inner tube, the ejector drill’s minimum diameter range is limited, generally not smaller than a hole with a diameter of 18mm.
5 Problems that should be paid attention to in the use of CNC deep hole drilling machine
When deep hole machining is in a closed and semi-closed state, the operator cannot directly observe the tool’s cutting condition.
At present, we can only judge whether the cutting process is normal by listening to the sound, watching the chips, observing the machine load and pressure gauge, touch vibration and other appearance phenomena based on experience.
Cutting heat is not easy to spread.
Generally, 80% of the cutting heat is taken away by the chips during the cutting process, while only 40% of the deep hole drilling. The tool accounts for a large proportion of the cutting heat spreads slowly and is easy to overheat. The knife-edge temperature can reach 600 degrees, which must enforce—the cooling method.
Chips are not easy to discharge.
Due to the depth of the hole and the chips’ long path, it is easy to block up and cause the drill to break. Therefore, the chips’ length and shape should control well, and mandatory chip removal should carry out during the work process.
The processing system has insufficient rigidity.
Due to the limitation of the aperture size, the long diameter of the hole is relatively large. The drill rod must make thin and extended so that the rigidity will be insufficient, vibration is easy to occur, and the drilling is easy to deviate. Therefore, support and guidance are significant. This is more prominent in CNC gun drilling machine tools.
6 Application range of CNC deep hole drilling machine
The deep hole processing of deep hole drilling machines covers various materials from glass fiber, Teflon, and other plastics to high-strength alloys (such as high-temperature heat-resistant alloys and titanium alloys).
CNC deep hole drilling machines have widely used in automobile manufacturing, mold industry, hardware industry, machinery industry, coal and petroleum industry, aerospace, hydraulic machinery, textiles, medical machinery, oil pump nozzles, electric steam turbines, and other industries.