Case Study - DT40 - Design Concept
The most obvious difficulty with the specification is the repeatability and accuracy of the part. A traditional machine design has a large metrology and machining loop which is subjected to changes in size with variations in ambient temperature. Parts made from steel expand by as much as 15 microns per metre per degree Celsius. It would be unreasonable to expect a production lathe to be located in a temperature controlled workshop to better than 1.0 deg C or to maintain an air shower of similar capability. The machining loop had to be made smaller by inventing a radical axis configuration.
The axes are reflected from the normal view of the operator so that the rotation of the workhead is reversed along with the direction of action of the toolslide. The Z axis is compounded with the workspindle and placed centrally between the hydrostatic guide bars. A stiff, light delta frame holds the Z bearing bars to the X bearing bars. The design utilises two key enabling technologies. The first is a two axis grating based encoder. The reading head of the encoder is attached to the X axis and the measuring plate is attached to the Z axis.
The 4 micron pitch grating plate encoder produces sinusoidal output signals which are interpolated by more than 8000 in the Cranfield CNC6400 controller to a useable resolution of 2 nm. The signals then form the input to the second key enabling technology which is a digital, direct acting force motor driven servo system. The actuators are extremely simple voice coil motors producing thrust proportional to the current through the stator coils. The magnets are attached to the ends of the axes and the coils are secured to the support frame. The voice coil forcers do not have the complication of bandwidth limiting commutation typical of brushless DC linear motors.