**Shenyang Machine Tool Complete Equipment Co., Ltd. Haas Qingge**
The crankshaft rotates at high speed within the engine, converting the fuel energy in the cylinder into rotational motion. When the crankshaft rotates at high speed, it creates relative motion and friction between the bearing seat. To ensure smooth sliding friction between the main journal of the crankshaft and the crankshaft neck, oil lubrication is used. The external oil supply to the bearing seat can only reach the main journal, while the lubricating oil for the connecting rod journal is delivered from the nearest main journal through an oil hole. This places special demands on the structure of the crankshaft oil hole—straight holes on the main journal and inclined holes on the neck, with the inclined holes needing to connect to the straight ones (some crankshaft oil holes are directly drilled through the spindle neck).
Figure 1: Crankshaft processing site
Crankshaft machining typically follows a production line model. The efficiency of each machine directly impacts the overall productivity and equipment utilization. Therefore, manufacturers focus heavily on the coordinated use of all production equipment. Due to the long length of the crankshaft oil hole, the oil hole machining process tends to be time-consuming and has a higher production cycle compared to other parts.
**Crankshaft Lubrication Hole Machining Methods**
Crankshaft lubrication hole machining is considered deep hole drilling (L/d > 5), especially for inclined oil holes, which often have a depth of 15d to 30d. Common deep hole machining methods include flat drills, twist drills, gun drills, BTA internal chip removal systems, suction drilling, DF single-tube double-inlet oil method, deep hole nesting drills, and MQL (Minimum Quantity Lubrication) inner-cooled twist drills. In China, gun drills and MQL twist drills are widely used, along with older techniques like rocker drill presses with turning fixtures.
**Gun Drill**
A gun drill consists of a shank, a body, and a cutting head, all welded together. The shank is typically made of chrome-molybdenum steel pipe. During drilling, coolant is delivered to the tool tip through the pipe, and a V-shaped groove on the shank helps remove chips. The cutting head is usually made of cemented carbide. Gun drills offer high precision and good surface quality but require oil cooling and chip removal, as well as a dedicated cooling system. The cutting fluid used must meet strict pressure and filtration requirements, leading to significant economic and environmental costs. Additionally, the poor balance of the gun drill necessitates the use of a guide sleeve, which is a consumable part requiring different sizes for various drill bits. The machining efficiency of gun drills is relatively low, with cutting speeds ranging from 40 to 120 m/min and feed rates between 0.005 to 0.07 mm/r.
**MQL Technology**
MQL technology involves mixing compressed air with a small amount of lubricating oil, atomizing it into micron-sized droplets, and spraying it into the cutting zone via a conveying pipe and the inner hole of the twist drill. This method provides lubrication between the tool and workpiece, significantly reducing friction and improving cutting performance.
**Advantages of MQL Technology:**
1. Minimal lubricant consumption—only 0.03 to 0.3 L/h.
2. Improved cutting efficiency and surface quality, reduced cutting force and temperature, and extended tool life.
3. Environmentally friendly, using vegetable-based oils that minimize pollution.
4. Almost oil-free process, resulting in clean workpieces and simplified cleaning.
5. Reduced complexity in managing cutting fluids.
6. Easy installation and maintenance.
**MQL Twist Drill Processing Equipment**
The SUC8149 series crankshaft oil hole machining CNC machine developed by our factory uses MQL twist drill technology, offering unique advantages in crankshaft oil hole processing. The machine features a five-axis structure (X, Y, Z, A, B). The A-axis is the workpiece rotation axis, with the workpiece clamped by a hydraulic chuck and supported by a tailstock center. The turret and cutter are mounted on the column slide, moving vertically on the Y-axis guide to handle oil holes of different heights. The column moves along the Z-axis guide rail, enabling feeding and retraction actions. The Z-axis turntable is mounted on a semi-circular base via a B-axis hinge, allowing angular adjustments for oil hole processing.
The oil mist delivery system of the SUC8149 machine’s spindle is shown in Figure 2. The MQL micro-lubrication pump sends the oil-gas mixture to the hollow shaft of the main motor through a pneumatic hose, connected via a rotary joint. The oil mist is then transferred to the six-worker turret and finally reaches the shank and inner-cooled twist drill. The machine typically uses cold solid carbide drills, with two capillary holes in the tool center to deliver the MQL oil mist to the cutting edge and workpiece surface. The inner-cooled twist drill is held by a hydraulic shank, with an inner cooling tube ensuring proper sealing. Testing and real-world production have shown that standard expansion cores may not seal effectively under MQL conditions due to lower pressure, making MQL-specific expansion cores with end O-ring seals the best choice for reliable oil mist transmission.
**Case Study**
Taking the actual oil hole machining of the Chongqing Xiaokang 474 crankshaft using the SUC8149 machine as an example:
1. Milling cutter for straight oil hole: 2800 r/min, 0.12 mm/r, 5 mm depth, 4 cuts, 3.57 s.
2. Deep hole drilling for straight oil hole: 3000 r/min, 0.23 mm/r, 47 mm depth, 4 cuts, 16.35 s.
3. Milling cutter for inclined oil hole: 2500 r/min, 0.10 mm/r, 10 mm depth, 4 cuts, 9.6 s.
4. Deep hole drilling for inclined oil hole: 3000 r/min, 0.23 mm/r, 90 mm depth, 4 cuts, 31.3 s.
5. Chamfer drill for both straight and inclined oil holes: 1500 r/min, 0.1 mm/r, 5 mm depth, 12 cuts, 24 s.
Total cutting time: 84.82 seconds. It is evident that the combination of twist drill and MQL significantly improves efficiency in deep hole drilling of crankshafts. In addition to oil hole processing, the machine also supports chamfering, milling webs, and marking surface operations, enabling flexible and precise machining at various angles.
**Conclusion**
In summary, MQL twist drill machining for crankshaft oil holes offers high efficiency, minimal pollution, excellent surface quality, and eliminates the need for complex cooling systems or support components. These benefits have been consistently demonstrated in our factory's production over the years and make this technology highly recommendable for widespread adoption.
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