Clamping control

Solutions for state-of-the-art machine tools

The clamping and fixing operations performed by CNC machine tools require a high precision level during their execution. The pressure supplied to the cylinders in order to keep the material fixed during the machining process requires to be adjusted according to the stock removal needed power. The removal size and the machine features stand out as the main characteristics affecting the tangential forces acting on the spindle. The hydraulic section of the clamping device has therefore to be designed in such a way as to meet these requirements with much accuracy and rapidity.

The key element that plays a major role in obtaining the required pressure is the 3-way pressure reducing valve. The valve maintains the set pressure in both ways reducing the pressure from line P to A (or B), while also allowing the return flow from A (or B) to discharge in T. The pressure switch inside the valve makes sure that the signal is triggered only when the machining pressure is reached in the monitored outlet port of the valve (when the machining conditions are settled). Thanks to the use of a special pressure switch with a low hysteresis it has been possible to monitor the real machining pressure with an error ranging from 3% to 5% of the set value and to connect the line signalling when the pressure has been reached. The 3-way construction also helps the flow return to the tank in case of exceeding pressure arising out of the forces on the actuator side.

Aiming at developing energy saving systems, the hydraulic system is supported by a variable displacement pump coupled with an accumulator: the pump simply inlets the flow rate needed during the movement while possible requests for higher flow rates are guaranteed by the accumulator. This soft motion extend the system lifetime by eliminating the pulses possibly generated by a fixed pump. Furthermore the accumulator reduces the minimum time required for the system to reach the set pressure.

For the speed regulatio of the actuator driving the tailstock, the system is equipped with two directional solenoid valves (4/2 and 4/3) positioned after the 3-way pressure reduction valve. The tailstock motion is achieved in two speed settings. One is for the rough approach while the other is for the final positioning which has to be slow and gentle. Each valve, when enabled, creates a different flow rate thanks to the openings placed on the inlets “A” or “B”, which can be optimized according to the different applications.

Upon request, the MZE*T3 proportional version of the pressure reducing valve is also available ensuring the precise, fast pressure adjustment with electronic control. Thanks to the proportional feature the clamping pressure can be changed in real time without interrupting the machining process.

The proportional option increases the productivity of the machine especially in case of series production thus obtaining important reductions in the cycle total duration period.