Correcting thermal errors: Turning Performance

In my October column, I pointed out that machine tools are poor substitutes for coordinate measuring machines partly because of their thermally induced errors. End users generally operate machine tools in poor thermal environments, and the machines have heat sources, such as from the cutting process, the spindle and the drives. Those heat sources change machine geometry.

At least three options exist for attempting to correct these thermal errors:

• “Brute-force” temperature control;

• Thermal modeling and temperature measurement; and

• Direct measurement of position errors.

The brute-force option requires removing the heat before it can change the geometry of the machine. Some of the techniques are surprisingly simple and inexpensive, such as controlling coolant temperature. Generally, machining centers have a large coolant sump that has a long thermal time constant. A coolant control system consists of a temperature sensor (a thermocouple, for example), a heat exchanger (using chilled water, for example) and a method to regulate the flow through the heat exchanger.

Holding the coolant temperature to within a couple of degrees is relatively easy. The spray of the coolant over the work zone, tool and workpiece helps keep temperatures more constant.

Air conditioning for the shop environment is an obvious option because the machine tool and workpiece expand and contract following changes in ambient temperature. While holding the ambient temperature constant is a clear benefit for accuracy, many machine shops are essentially uncontrolled. Why? One reason is the cost of air conditioning is explicit, but the cost of not air conditioning is hidden in the cost of out-of-tolerance parts, rework, scrap and the periodic NC programming tweaks required to keep parts in tolerance. Those programming changes are needed because the machine is not thermally stable, and therefore tweaked programs require further adjustments.