Precision forming

With the development of precision forming technology, the contour of the blank is getting closer and closer to the finished product size, and the machining allowance of the part is significantly reduced. At present, in mass production, a blank with a dimensional accuracy of (0.3 to 1.0) mm has been obtained by precision thermoforming (precision, precision forging and sintering); it can be obtained by precision cold forming (cold rolling and cold drawing) IT7 ~ IT11 precision blank. Such blanks are often directly finished by finishing without roughing or roughing. For blank dimensional tolerances within the grinding allowance, quenching can be performed without the need for soft cutting, followed by grinding or hard turning (or hard milling).

Simplifying the manufacturing process through precision forming is a commonly used method in mass production.

Hard processing

Hard cutting (45HRC to 62HRC) with a tool with a certain geometry significantly simplifies the process. For example, when machining a bearing ring in a conventional process, five processes of hot rolling, annealing, turning, quenching, and grinding are required. Instead of using the hard car process instead of grinding, only three processes of hot rolling, quenching (62HRC) and hard car (Rz1μm to Rz2μm) are required, which reduces the number of processes. This reduces cycle time by 65%, reduces energy consumption by 45%, and reduces parts manufacturing costs by 35%. Another example is the Kassel plant in DaimlerChrysler, which uses the rear axle half shaft of a truck (CK45, 61HRC, 300,000 pieces per year) to replace the previous grinding process and use a CNC. Instead of the past three grinding machines, the lathe reduced the cost per piece of the workpiece by more than 50%.

At present, the process of simplifying mold manufacturing through hard milling is a technological development trend in the mold manufacturing industry. The traditional manufacturing process for molds is milling (soft milling) and electrolytic machining. In recent years, high-speed hard milling has replaced soft milling and electrolytic machining in many occasions (except for slit and deep groove machining), eliminating the need for electrode fabrication and polishing processes, and hard milling for complete machining of the mold. For example, the conventional machining process of a connecting rod forging die (56NiCrMoV7) is finish milling, quenching, electrode manufacturing, electrolytic machining and polishing. At present, after hard milling, the workpiece can be quenched (47HRC) under one clamping. It can be processed into finished products by rough milling and finishing milling. The rough surface is less than Ra 0.8μm. It can be seen here that the new process eliminates the conventional process of soft milling, electrode manufacturing, electrolytic machining and polishing, which greatly simplifies the process manufacturing process. The processing time of the mold can be shortened by 75% and the processing cost can be saved by 50%.

The application of hard milling also creates conditions for the integration of CAD-CAM-HSC in mold manufacturing. It can be considered that high-speed hard milling is bringing a major change to mold manufacturing technology.

It should be pointed out that hard-cylinder and hard-milling are high-temperature cutting. In order to avoid premature failure of the tool due to the sudden change of temperature caused by the use of cooling lubricating fluid, hard machining uses dry cutting, which is beneficial to the protection of the ecological environment.

High speed grinding

With the development and application of super-hard CBN grinding wheels, the grinding speed and feed rate have been greatly improved. This high-speed grinding (circumferential speed 90m/s to 250m/s) process not only has a high material removal rate, but also achieves high processing quality, which makes the grinding process from pure finishing process to A general processing technology. For example, the electroplated CBN grinding wheel is used to grind the alloy gray cast iron camshaft. The grinding allowance on the diameter is 4.1mm, the grinding speed is 90m/s, the grinding time is only 12s, and the material grinding rate of the unit grinding wheel width reaches 20mm3/mms. In this way, the high-speed grinding with the CBN grinding wheel eliminates the pre-machining process such as turning or milling in the conventional process, and directly realizes the rough and finishing of the workpiece by grinding. For example, a company is processing the engine crankshaft of a lawn mower. In the past, it used six turning processes and three grinding processes. At present, it uses three high-speed grinding processes, using electroplated CBN grinding wheels, and the grinding speed is 123m/s. Eliminating all turning operations, the processing time is reduced by 65%.

High speed turning

The turning and milling process is a new composite machining process that is synthesized by turning and milling. When turning and milling, the workpiece is rotated slowly, and the workpiece is milled by a disc milling cutter mounted on the carriage drive shaft. Due to the high milling speed (300m/min~500m/min), small cutting force, short machining time, high machining accuracy and good surface quality, the application of this new process can save processing steps. When the high-speed turning and milling process is used to machine the connecting rod neck, main journal, shoulder and undercut of the crankshaft, it saves two processes than the conventional process.

Fracture splitting process

Fracture splitting is a process in which a monolith is separated into two pieces by impact rupture. At present, this process has been applied to the processing of connecting rods (Fig. 7) and crankcases. Through the fracture splitting, the broken link body and the connecting rod top cover, the crankcase bearing seat and the bearing cap can be directly assembled accurately, eliminating the traditional process for realizing the connecting rod body and its top cover and crankcase bearing. A large number of cutting operations required for the precise assembly of the seat and bearing caps (such as the traditional connecting rod processing process, generally 14 processing steps, and the new process of fracture splitting requires only 6 processing steps). This greatly simplifies the process flow, and also significantly improves the assembly accuracy of the two components and improves the performance of the engine.

Multi-tools and compound tools

In medium and large-volume production (such as the automotive industry), composite tools are often used to complete multiple parts of the workpiece or to process multiple processes on composite machine tools and automatic lines to reduce the processing station and shorten the length of the automatic line. Rationalize production.

The use of multi-tools and compound tools and the use of the interpolation function of the machining center control system simplifies the machining process.

When milling a hole with a multi-tool (such as a blade cutter) on a machining center, the rotary cutter performs a helical interpolation motion around the Z-axis, and a hole of a desired size can be machined in one working stroke. For example, machining a hole with a diameter of 285 mm and using a 160 mm diameter milling cutter can complete the machining task in one working stroke, which saves five reaming operations and saves 73% of machining time compared to conventional processes.

Milling holes with multi-tools on the machining center

In recent years, the use of thread milling cutters in machining centers has also required threaded holes to be milled in one working stroke (Fig. 5). According to the conventional process, the processing of the threaded holes is often performed by using two or three tools for drilling, chamfering and tapping, respectively. This not only shortens the machining time, but also increases the tool magazine capacity of the machining center in disguise. This threaded hole milling process is very commonly used for threaded hole machining on automotive engine cylinder heads, cylinder blocks and gearbox bodies.

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