Less time from chip to chip
Continual improvements to the machining process mean that optimisation has already come close to physical limits. However, some major savings can still be achieved by looking at ways to shorten unproductive auxiliary process times, as GF AgieCharmilles have proved. They were able to reduce the times for frequent tool changes by 0.7 seconds – 12% of the entire process – in their MIKRON HPM 800U five-axis machining centre with the introduction of a high performance Series RHC high speed pneumatic cylinder, from world leaders SMC.
In automated machining, shortening unproductive auxiliary process times is worthwhile, even if they are only seconds long. The MIKRON HPM 800U five axis machining centre carries out an average of four tool changes per minute and Urs Hänni, Head of Milling Machine Project Management, and Frank Trepte, Development Engineer at GF AgieCharmilles in Nidau confirm that with the RHC high speed pneumatic cylinder from SMC that adds up to a time saving of at least four seconds per minute.
The number of tool changes per unit time is not the same for every work piece, and there may be other processes running during the chip-to-chip time such as tool feeding, driving the axis into position, starting the spindle motor, etc. However, investigation of longer timeframes and the corresponding analysis of the machining operations are impressive as Hänni and Trepte report, “the RHC cylinder is significantly quicker than the previously used ISO cylinder and results from the machining centres put into service with the new high speed pneumatic cylinders confirm these time savings.”
Owing to their extensive power spectrum and their high potential for automation, five-axis machining centres such as the MIKRON HPM 800U are normally used for machining processes involving frequent tool changes. Sometimes tools are only in use for a few seconds before being replaced by another tool. And the more frequently the tool changes occur, the more the saving in the shortening of the auxiliary process times.
The high speed pneumatic cylinder opens and closes a sliding door between the machining area and the tool magazine during a tool change at a significantly higher speed than the ISO cylinder that has been used up until now.
High speed – smoothly braked
The MIKRON HPM 800U five-axis machining centre is designed with travels of 800 x 800 x 550 mm in the X-, Y- and Z directions, for machining awkwardly shaped work pieces with long tools (HPM – High Performance Milling). The machines are used to produce high-precision, high-tech components withfirst class surface qualities, including complicated surface combinations that have to be executed true to shape such as turbine blades for jet engines, compressor discs for gas turbines, impellers, injection moulding tools, forging dies for crankshafts, bearing shells, etc. The milling operations are carried out with simultaneous movements in all five axes. Time and again different geometries and materials have to be machined and to do this the tools also have to be changed frequently.
As the physical limits of the actual machining process have largely been exhausted, the potential for increasing productivity is now focused on shortening the chip-to-chip time including speeding up the tool changes.
In the case of the MIKRON HPM 800U, the tool magazine can hold up to 210 tools, which are automatically positioned for lightning fast tool changes. The automatic tool changer, which handles the exchange of the two tools, consists of a double arm gripper with two opposing arms which rotate about a central vertical axis. At their outer ends the two arms have gripping devices for picking up the tools.
At the beginning of the tool change, the pneumatic cylinder pushes the sliding door between the work area and the tool magazine open. Immediately thereafter the double arm gripper rotates its dual arm by 90°, which until then has been located parallel to the outer side of the sliding door. As a result, one arm is positioned to remove the tool from the machining spindle, whilst the other is simultaneously collecting the new tool from the tool magazine. To complete the process, the double arm gripper then swings both arms round by 180°, in order to insert the new tool into the spindle and simultaneously replace the removed tool in the magazine chain.
Almost immediately after the exchange, the supply to the cutting fluid jet, which is directed onto the machining zone, is turned on and the newly changed tool starts the next machining operation. However as a result of the rapid rotation of the tool coming in contact with the cutting fluid, the entire machining booth becomes sprayed in fluid. This is where the high speed cylinder really comes into its own by keeping lubricant out of the tool magazine and the surrounding area by reliably sliding the door tightly closed before any machining takes place.
In order to keep the time required for the tool change as short as possible, the pneumatic cylinder carries out the opening and closing of the sliding door at high speed. During this process, it is especially important that the cylinder’s high kinetic energy is kept under control so that it does not strike the end stops but approaches them in a smooth and controlled way.
High kinetic energy under control
GF AgieCharmilles´s Urs Hänni and Frank Trepte specifically selected a 32mm RHC cylinder with a stroke of 750 mm which fits exactly with the circular path traced out by the arms of the double gripper unit when opening and closing the sliding door. The cylinder is accelerated from the start in either direction at speeds reaching 3000 mm/sec - three times faster than the standard ISO/VDMA cylinder – and is braked in a smooth and controlled manner when stopping at the end positions. This pneumatically controlled braking absorbs a kinetic energy of 21 joules which is 10 to 20 times more than the previously used pneumatic cylinder and the significantly longer damping path is controlled by damping sleeves and special venting valves instead of traditional screw-in throttles.
To explain this further SMC’s Manuel Hänni says “when compared to conventional cylinders, our RHC is fitted with larger end fittings to help achieve rapid acceleration and high speeds in each direction. These bigger fittings ensure that a large volume of compressed air can enter the cylinder to help drive the piston and they also allow rapid venting on the other side of the piston.
Precisely 80 mm before the piston’s end position, the damping sleeve pulls into the damping seal, which effectively blocks the venting air through the fitting. The escaping air can now only pass through its own damping line and the controlled venting valve and, as a result, the piston is smoothly braked.
When the piston moves in the other direction, a further feature of the damping seal comes into play, as it functions like a non-return valve, only allowing the compressed air to pass through from the end of the cylinder. So the compressed air entering the cylinder has free passage through the damping seal into the piston space to accelerate the piston. As soon as the damping sleeve emerges from the damping seal, the cross-section of the passageway increases and the full acceleration of the piston occurs”.
The RHC cylinder range is available in diameters between 20 and 100 mm and its main strengths are fast acceleration to high speeds combined with smooth, jolt-free braking. Additionally these cylinders can also be used to break heavy loads travelling at low or medium speeds smoothly without jolting and they have been successfully used as a drive for high-speed Z-axes, where masses of several kilograms have to be moved. In a recent application in a conveyor system, the RHC has moved masses of 40 kilograms at speeds up to 1000 mm/sec.
Both Urs Hänni and Frank Trepte fully agree “the RHC cylinder’s outstanding performance, coupled with its low maintenance needs have really made an outstanding contribution to the success of our MIKRON HPM 800U and the decreased chip-to-chip time has resulted in even faster production performance.”