Why Parallel Kinematics Machine (PKM)?

Updated: 2008/9/30

 

Agile Machining

      A well known problem in conventional 3-Axis machining is the fact that the coordinate system of a part most of the times has reference points that are either inside the part or not consistence with the desired centre line of the part, therefore it is a costly and time consuming work to align a part to the coordinate system of the machine. In conventional machines this is solved either by using very expensive and un-flexible servo controlled fixtures or by adjusting each part manually using advanced offline measuring equipment.

      A PKM machine can easily carry a laser probe or conventional probe and in combination with a machine that has 5-Axis capability and extreme acceleration and rapid motion, generates the possibility to fly over the part in a couple of seconds and measure everything that needs to be analyzed and adjust relevant programming data accordingly before machining. One example could be an engine block where the cylinder liners are not consistent and has to be probed individually, calculated and put into the machine coordinate system before machining, all in a few seconds. In the same way a frame for a car, plane, train or construction machine, can be probed in production and all features that are important can be measured and taken in consideration before the final machining. This technology has been proven and is in production at Boeing.

                       

One Setup Machining

      Another problem in 3-Axis conventional machining is that to be able to machine all six sides of e.g. a cylinder head, a minimum of two and normally three fixtures has to be used. This technology requires a number of machine setups in a transfer line concept which creates a lot of problems like if one of the machines goes down the whole line stops. Another problem is tolerance stack-ups coming from moving the part from fixture to fixture with consequential CPK problems, and finally the cost of designing and manufacturing several different fixtures is substantial.

      With the unique capability of a PKM machine, that allows the spindle to actually point all the way backwards, the machine literary can machine all sides of a cylinder head or a bumper in only one setup. If this technology is used with the above described probe concept it creates the possibility to use really low cost fixtures with no repeatability or build in accuracy as long as they are stable and rigid. As a consequence all tolerance stack-up problems disappears and the CPK values of the parts increases. This technology has been proven and is in production at numerous PKM machine users. 

                       

Compound Angle Machining

      Due to the more and more advanced design of cars, mostly in the bodies, but also in advanced frames and engines, the requirements for complex machining and compound angles becomes higher every day. To perform this type of machining with a 3-Axis machine that by default is designed for cubical machining, a lot of add-on equipment is required. This equipment could be everything from an additional full 2-Axis table to an additional angular head or an offline special machine. In all cases the additional equipment is expensive, difficult to calibrate and maintain, especially if the part only requires e.g. one compound angle hole and nothing more.

      A PKM machine with its capability does not notice any differences between a perpendicular hole or surface compared to a compound angle one and therefore it is very suitable for all complex machining that is expected to be performed on the cars of the future. Also, it is easy to imagine that a part for an airplane, train, or construction machine requires even more advanced machining that today can not be done with conventional machines other than a PKM machine. This technology has been proven and is in production at numerous PKM machine users.

                       

Multi path blending and tool-drag elimination

      An old problem in 3-Axis conventional machining is the fact that to be able to machine a perfect flat surface, with multi paths, it requires a very rigid and accurate machine and even then it is in reality almost impossible to avoid a small hatch between the passes. There are mainly two ways to solve this with conventional machines and one is to use a cutter that cover the whole surface, which required a lot of horse power and stability, or to mechanically lean the spindle a couple of thousands of degrees in a so called attack angle but this requires that all passes comes from the same direction which cost time.

      As earlier mentioned, a PKM machine does not care whether the spindle is perpendicular to the surface or not so it is very easy to pre-program the a PKM machine paths with a defined attack angle and just let it use this angle whenever it machines a surface. Another advantage is that the rear side of the cutter does not touch the part and therefore the tool life time is increased, especially in the foundry industry where remaining sand otherwise comes up behind the front of the cutter and jams between the rear of the cutter and the part and kills the tool quickly. This technology has bee tested at several users.