Case Study - HPB4³ - Custom Design
There are competing manufacturers worldwide who could be considered able to deliver a commodity machine tool that would meet some, but not all of the geometric specifications. The choice narrows dramatically if compromise is not permitted and if the operational manufacturing needs are considered.
A custom design allows the machine performance to be optimised to a narrower set of manufacturing targets and in this particular instance to deliver the ultimate in roundness and cylindricity of bores for hydrostatic spindles. Calculations of tool forces for medium material removal of 600mm cast iron bores indicates spindle torques approaching 600 Nm. Smaller bores will need higher speeds and less torque. If the two specifications are considered together, then the spindle has to have a power capability of 60 kW. This is not a simultaneous requirement, but it does set the range and hence the size of the motor.
The target roundness of large and small bores produced by the machine needs to be better than a micron. Reacting tool forces and delivering extremely good roundness cannot be achieved with traditional rolling element spindle designs. The solution to the conflicting performance requirements is to custom build a hydrostatic, high power spindle based on the ultra precision five pocket designs already in service on Cranfield Precisionís diamond turning lathes.
The design of the machine structure and the linear position axes becomes demanding when the workpiece bores become longer than the reach of the spindle and boring bar extensions. The repeatability of the positioning systems has to be good enough to match the diameters when opposite ends of long bores are machined by 180 degree rotation of the part with the worktable. Cranfield Precision have been using high performance linear motor technology for more than ten years.
The early applications were limited to the high dynamic axes on orbital cam and crank grinding machines, but more recently, with slotless technology, axes can be positioned with nanometre accuracy and smoothness good enough for diamond machining. In this instance, the motor needs to provide a combination of high motive force and micron level positioning. The use of motor pairs, symmetrically mounted about the centre of mass of each carriage, provides the optimum drive arrangement obeying one of the most critical rules of precision engineering design.
Symetrical Dual Motor Drive