Taby, Sweden -With only three axes to play with, most conventional milling machines suffer from limited space maneuverability. This limitation in recent years has led to the development of hexapod and tripod parallel kinematic machines (PKMs). With a machining head that tilts, a PKM offers up to five axes of maneuverability. One problem, however, is that conventional controls provide only limited performance with PKMs due to the complex movement of the strut-type structure. Another factor is that a controller should enable the complicated machine movements to be programmed just like a conventional CNC machine.
To get around these problems, JMC Technology Group uses a Siemens Sinumeric 840D controller on its latest Tricept 805 machine. The controller's program transforms conventional machine commands defined in the NURBS system into commands the Tricept can under- stand. This takes place in real-time while the machine is cutting metal. For both operator and CNC programmer, the unit behaves just like a conventional machine.
The controller also includes the Direct Measurement System (DMS) software that pro- vides automatic calibration. As in conventional machines, feed- back from encoders helps control the machine movements. In these indirect measurement systems, the controller assumes that the encoder signals represent true machine positions. The extent to which this assumption is correct depends on the position of the encoders-usually fairly deep in the machine and remote from the actual point of machining.
On the Tricept 805, the controller takes signals from encoders mounted on the central tube of the strut configuration, at a point close to the machining head. These sensors measure the bending and twisting of the central Tricept tube that cannot be detected by conventionally placed encoders. A linear scale on the central tube signals the length of the tube while two rotary encoders, integrated into the central joint, signal the tube orientation. Volker Kreidler, of Siemens Automation & Drives, claims, "The Direct Measuring System supplies ten to fifteen times better precision than the indirect axial encoders on the machine."
For auto-calibration, the controller moves the machine through a sequence of movements determined by a special program stored on the host PC. Signals from the primary indirect encoders are recorded along with signals from the secondary DMS encoders. A comparison of the two sets of data gives the offsets required to obtain true position data. Kreidler again: "The real benefit here is that the operator does not need any external measuring device for calibration. After a crash or a change in environmental conditions, auto-calibration takes just fifteen minutes."
Ulrika Liiv, Marketing Director at JMC Technology Group, thinks PKMs have a lot of untapped potential: "One must understand that a PKM is a non-Cartesian, non-linear machine which needs a different control approach. But it is worth it-the Tricept mechanical arrangement coupled with the new controller software can really cut processing times."
The actual machining time is controlled by the cutting data which is the same regardless of the type of machine. Liiv points out, though, that time can be saved on the transfers and other non-machining steps and that's where the Tricept advantages become apparent. For example, managers at Boeing's Wichita, Kansas plant have recognized these advantages and have installed a new Tricept 805 to back up the earlier 600 model for producing floor beams on their aircraft.
Additional details on Tricept...Contact JMC Technology Group; 4555 Independence Square, Indianapolis, IN 46203; Tel: (317) 791-4450; Fax: (317) 781-2466;
or Circle 501.
Additional details on the Sinumeric 840D controller...Contact Siemens Automation & Drives Group; Postfach 3240, Erlangen, Germany; Tel: +49 911 433 6388; Fax: +49 911 433 5511.