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May 2017

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High Production CNC Crankshaft Grinding Machines

AZ SpA offers its DU line of high production CNC crankshaft grinding machines. Main features include:

  • Single (DU) or double (DU2W) grinding wheel carriage
  • Works main pins, crankpins and flange in one set-up
  • Double grinding spindle to reduce cycle time
  • Grinding operation with CBN grinding wheels.

The crankshaft orbital grinding machine features a welded base structure made of composite material specifically designed by finite element method to obtain the best compromise between structural deformations and permitted natural frequencies. "The multicomponent composite material offers advantages compared to other traditional solutions, particularly in terms of dynamic performance, which translates into a better vibrational response and considerable advantages in terms of weight," said a company spokesperson. The base is ready to connect with an optional temperature control system to control and stabilize temperature by heating ducts/cooling embedded in composital. This system reduces warm-up time and thermal problems caused by downtime, production breaks or changes of use. The fixed table on the machine table base is made of monolithic cast iron that is high strength and thermally stabilized.

"The crankshaft machining process includes grinding of all mains and crankpins including radius and thrust surface, as well as external diameter of the flywheel," said the spokesperson. "Assuming that a crankshaft was previously machined at the sides and the shoulder of the flange, we have identified the use of a grinding machine with two independent grinding wheels carriage as the best solution for high production."

The linear and angular movements of the grinding head are driven by linear and torque motors. The position of these is continuously monitored by closed loop with absolute encoders, linear and angular, that provide a positioning accuracy of +/-0.0005 mm and +/-0.005°.

The vitrified CBN grinding wheels have a standard diameter of 610 mm and an abrasive thickness of 5 mm. The grinding wheel body is made of aluminum to help ensure lightness and stiffness and to avoid acoustic sensor problems. The standard cutting speed is 125 m/s. The grinding wheels supplied can be radius, contouring and tapered according to the needs of the customer and component work cycle.

The grinding wheel heads are constructed of a single or double pair of orthogonal carriages made of cast iron and thermally stabilized. They are connected to the base through recirculating preloaded roller guideways to help ensure linearity and flatness in all carriage positions and in all loading situations. The carriage movement is provided by a pair of linear motors, which are independent and sized to work in the field of application required. Each upper carriage houses a stationary turret, which is designed to be configured with different types of spindles for different types of processing, such as internal or external grinding.

The grinding wheel turrets are set with two opposing electro-spindles. Lubrication is with permanent grease and seals are non slippery pneumatic (filter 5 µm). In the electro-spindle are arranged two thermocouples for control and regulation of the cooling system. Rotation is controlled by an angular encoder. The spindle is designed to work with CBN wheels. The coupling system between the flanges and spindle is conical type. The dynamic grinding wheel balancing is ensured by an internal dynamic balancer with GAP and crush control sensor.

The headstock is made of high-strength cast iron that is thermally stabilized. It is developed for the permanent grease lubrication and pneumatic non-contact seals (filter 5 µm). Two thermocouples provide control and regulation of the cooling system. Rotation is controlled by an angle encoder with 10,000 pulses/rev to ensure the accuracy of the orbital system. On the headstock spindle is installed an auto-clamping and auto-compensating hydraulic three jaw chuck system with a clamping force of 20 (kN). This provides accuracy and repeatability of automatic workpiece clamping.

The automatic cycle of shaping and dressing of the grinding wheels is performed with electric spindles. Both are pressurized and capable of working under continuous emulsion jet. The dressers are placed on the table, the first unit behind the headstock and the second behind the tailstock. On them are installed two roller diamond dressers, which allow all dressing operations on a CBN grinding wheel: front, radial and taper. Diameter, longitudinal and geometric dimensions of CBN grinding wheels are checked and updated by touch probe.

The tailstock is divided into two parts made of high strength thermally stabilized cast iron. In this casing are housed bushings that ensure the proper sliding of the quill, arranged with coupling Morse 5. The load on tailstock is adjusted by means of a spring system compressed, manually adjustable and controlled continuously by a load cell connected to the CNC. The movement of the sleeve is hydraulic. The tailstock is equipped with an air suspension system that eases the manual movement along the table. Cooling is forced externally by thermo regulated liquid.

The grinding machine is equipped with two hydraulic automatic steady rests that support the workpiece on the main pins selected during processing. The machine is equipped with a kit for gripping the main diameter of the crankshaft. Steady rests are installed on the table using a support with an automatic system of engagement/disengagement, orthogonal to the workpiece axis, with hydraulic movement. Clamping and positioning along the table is manual.

Both grinding spindles are equipped with in-process measurement, dedicated for crankshaft applications.

Angular position measurement on the first crankpin can be detected automatically after zero longitudinal axis of the piece, before the beginning of grinding cycle. A system with a pneumatic rotary arm, positioned on the wheel carriage, engages a touch probe. Using a special program with double touch, the angle is calculated without any need of reading the diameter of the pin.

AZ uses the Siemens Sinumerik 840D digital CNC system for complex tasks. A special parametrical program developed for orbital and special shapes, like camshafts, permits programming and monitoring of all grinding work.

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