Thursday, June 13, 2019

Introduction to the motor spindle structure of cnc machining center

The electric spindle is a new technology that integrates the machine tool spindle and the spindle motor in the field of cnc machine tools in recent years. Together with the linear motor technology and high-speed tool technology, it will push high-speed machining to a new era. Cnc machining electric spindle is a set of components, including the electric spindle itself and its accessories: electric spindle, high frequency inverter, oil mist lubricator, cooling device, built-in encoder, tool changer.

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The high-speed spindle is the most important key technology for high-speed cutting technology and the most important part of high-speed cutting machine tools. It requires high dynamic balance, good rigidity, high rotation precision, good thermal stability, can transmit enough torque and power, can withstand high centrifugal force, with accurate temperature measuring device and efficient cooling device. High-speed cutting generally requires a spindle speed capability of not less than 40,000 r/min and a spindle power of more than 15 kW. Usually, the electric spindle components integrated with the spindle motor are used to realize direct transmission without intermediate links. Most of the motors use induction integrated spindle motors. At present, most commonly used are hot-pressed silicon nitride (Si3N4) ceramic bearings, hydrodynamic and hydrostatic bearings, and air bearings. Lubrication uses oil-gas lubrication, jet lubrication and other technologies. Spindle cooling is generally performed by water cooling or air cooling inside the spindle.

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1. The figure above shows the working principle diagram of high-speed spindle of ceramic bearing. It adopts C or B-level precision angular contact ball bearing. The bearing arrangement is similar to that of traditional grinding machine spindle structure. It adopts “bead ball” structure, ball material Si3N4; adopts electric spindle (The motor is integrated with the main shaft); the bearing speed characteristic value (= shaft diameter (mm) × speed (r/min)) is 1.2 to 2 times higher than ordinary steel bearings, up to 0.5 to 1×106. The rotation precision is high, the hydrostatic bearing rotation error is below 0.2μm, the aerostatic bearing rotation error is below 0.05μm; the power loss is small; the hydrostatic bearing speed characteristic value can reach 1×106, the aerostatic bearing speed characteristic value Up to 3 × 106. Aerostatic bearings have a low load carrying capacity.
The advantages of ceramic bearings compared to steel balls are:
(1) The ceramic ball density is reduced by 60%, so that the centrifugal force can be greatly reduced;
(2) The ceramic modulus of elasticity is 50% higher than that of steel, which makes the bearing have higher rigidity;
(3) The ceramic friction coefficient is low, which can reduce bearing heat, wear and power loss;
(4) The ceramic has good wear resistance and long bearing life.

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2, magnetic bearing high-speed spindle
The figure above shows the working principle diagram of the high-speed spindle of the magnetic bearing. The main shaft is supported by two radial and two axial magnetic bearings. The gap between the stator and the rotor of the magnetic bearing is about 0.1 mm. High stiffness, about 10 times the stiffness of the ball bearing spindle. The speed characteristic value can reach 4×106. The accuracy of the rotation depends mainly on the accuracy and sensitivity of the sensor, as well as the performance of the control circuit, which is currently up to 0.2 μm. The cnc processing mechanical structure and circuit system are more complicated; and because of the high heat, the performance of the cooling system is relatively high.cnc machining
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