Manufacturers and machine tool builders are being asked to achieve tighter and tighter part tolerance objectives. The degree of precision available in a modern machining center is now at a point where consideration of thermal effects must now be factored into the machining process.
All materials are affected by temperature deviations. While the rate and degree of change varies from material to material, demand for tighter tolerances means that thermal compensation is increasingly finding its way into general manufacturing.
Thermal variation can significantly impact part accuracy. In a manufacturing process, ambient and created heat produced primarily by the spindle, axis drive systems and workpiece action can distort related machine components increasing the burden of producing accurate parts on a repeatable basis.
The most dramatic effect of thermal distortion is movement of the spindle away from the table. This drift makes achieving consistent tolerances next to impossible. Complicating potential solutions are the different rates of expansion of the various axes based on geometry, mass and temperature gradations.
The combined effect of numerous volumetric error components—squareness; linear, vertical and horizontal errors; roll, pitch and yaw errors in the X, Y and Z axes—induced by thermal distortion pose an enormous design challenge for machine builders. Recognition of these effects is also an opportunity to combat them.
In general, air conditioning and perhaps 24/7 machining, which create stable external and internal thermal operation, will help overcome thermal interference and contribute to more consistent part output. However, full process control can come from an individual machine's innate capability to consistently meet tolerance objectives. The design of a more thermal tolerant machine is a necessary step in true part accuracy improvement.
Bridgeport Machines Inc. considers controlling volumetric accuracy of both the machine and workpiece in real time, in varied environments, essential. “Control” and “real time” are the operative words when thermal monitoring is factored into the total productivity and accuracy of any close tolerance machining. To this end, the company has developed a thermal compensation system, which is now being fitted as standard on all models in its CNC vertical machining center lines.
As a first step, Bridgeport has minimized errors due to heat generation by fitting high-efficiency, digital spindle motors from Fanuc and Siemens. These motors reduce heat transfer by 25 percent by isolating it from the main structure of the machine. Heat generation has been reduced by re-designing the spindle itself with the introduction of ceramic bearings as standard on 10,000 rpm spindle assemblies and a hybrid bearing on 12,000 rpm spindles.
On the portal frame Bridgeport machines, refrigerated spindle motor chillers have been incorporated to reduce thermal effects generated by the in-line spindle motor assemblies. This allows most of the heat within the spindles to be regulated and kept constant, which is an important factor.
While all of these measures have reduced thermal effects significantly, residual heat is further compensated for by the introduction of an innovative temperature measuring system, which uses a mapping technique to check thermal growth in the Y and Z axes in every standard machine frame.
These data are then fed into the computer control, which automatically effects mechanical compensation for any heat generation. The system works under the basis that heat generated in the spindle head will vary according to spindle speeds. It also recognizes that the heat generated in the ballscrew will also differ depending on duty cycles. To deal with such variability, it is important to have a dynamic system of measurement that can take into account these differing heat sources.
For input, the system uses a thermal strip to measure heat along the spindle head while calibrating it against a changing ambience. This produces dynamic data that is fed back to the Fanuc, Heidenhain or Siemens control.
From temperature rises monitored by the thermal strip (called a thermistor), a calculation in the control is made based on the mathematical model unique to the particular machine type and consequent levels of distortion determined. The values are then fed through the CNC to the relevant axes as offsets to counter any unwanted thermal effects.
Bridgeport's dynamic thermal compensation system creates real time monitoring and feedback of temperature changes, output as axis compensation. It's a closed loop system designed to take thermal variation out of the machining process to help shops produce close tolerance workpieces consistently throughout the part run.