The fourth axis of CNC is an important machine tool accessory of a vertical machining center. Its application provides a rotary coordinate for CNC machine tools, through the rotary shaft drive table to complete equal, unequal or continuous rotary processing, expand the scope of adaptation of CNC machine tools processing parts. In this paper, we will introduce its structures and applications.

Typical Structures Of The Fourth Axis

The Fourth Axis Of Worm Gear Transmission

Worm gear pair transmission is the main structure of the fourth shaft, generally using servo motor to drive and indexing shaft. Mainstream brand general minimum indexing unit can reach 0.001°, indexing accuracy between 15 "~ 40", repeat accuracy 5 "~ 8", using closed-loop control after the indexing accuracy can reach ±3 ", repeat positioning accuracy can reach ±2 ". The quality and reliability of the fourth shaft are determined by the material selection, manufacturing accuracy, heat treatment level and adjustable clearance structure of worm gear and worm gear pair. At present, most brands of worm materials are made of alloy steel, worm gear materials are made of aluminum copper alloy.

The Fourth Axis Of End - Tooth Disc Indexing

The fourth axis of the end gear plate is driven by servomotor or hydraulic motor, worm gear or gear drive, hydraulic or pneumatic lock. Accurate positioning of end-tooth disc, indexing can reach 1°, indexing accuracy up to ±3 ", repeat positioning accuracy up to ±1.5 ".

Strengths: good stiffness, large load capacity, high repeatable positioning accuracy, good precision retention performance;

Weakness: end tooth plate manufacturing precision requirements are very high, can not be arbitrary Angle indexing.

The Fourth Axis Driven By The CAM  

It adopts globoidal CAM mechanism to drive the rotating shaft, and realizes fixed Angle equal division and mechanical locking.

Strengths: fast rotary indexing speed, strong bearing capacity, good structural rigidity.

Weakness: CAM indexing mechanism of high processing accuracy, the need for specialized processing equipment.

The Fourth Axis Driven Directly Torque Motor  

The fourth spindle uses torque motor direct drive, hydraulic or air brake, absolute encoder feedback. The minimum dividing unit can reach 0.001°, and the speed of mainstream brands can reach 300r/min.

Advantages: arbitrary indexing, high speed, high reliability. Simple structure, easy to achieve mass, specialized production.

Disadvantages: relatively poor rigidity, small driving torque, suitable for light cutting.

Applications Of The Fourth Axis

A Clip A Processing

General shaft parts can use three claws to hold the outer diameter, processing long shaft workpiece, can be matched with chuck and top tailstock.

Single Turntable Processing

For the processing of various side holes of the opposite parts, especially suitable for the application of high precision of each space hole Angle, using the fourth axis for accurate and fast indexing.

Bridge Plate Structure Processing

For the machining of the parts with angles on the workpiece or with the plane, the bridge plate structure can be clamped at one time, or multiple workpiece can be installed at the same time, so as to reduce the tool changing time.

Cooperation With Numerical Control System

After the fourth axis is connected to the system, it is necessary to carry out debugging and parameter setting, and zero setting of the fourth axis. According to the requirements of numerical control system, set the clamping and loosening M instruction, M11 is loosening, M10 is clamping. At the same time cooperate with other instructions, completed the semi-automatic production process.

In Conclusion

The application of the fourth axis solves the problem that the traditional process needs a lot of design fixtures and repeated positioning and clamping. The reduction of workpiece positioning and clamping times and the improvement of rotary accuracy guarantee the quality of processed products. 

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