Spiral Machining

There has been an increase in demand for scroll, or spiral, compressors as the accuracy of the methods used to make them have improved. Scroll compressors are used to compress refrigerant or air to create vacuums. A scroll compressor has two Archimedes spirals nested together. Figure 1 shows components of a scroll compressor, and figure 2 shows how it works. The gas enters at two places on the outside of the scrolls and exits through a hole at the center of the fixed scroll. Patented in 1905 by Leon Creux, this is an example of an inventor being way ahead of the curve. Creux could never make a unit that worked. The way that a scroll compressor seals in the gas is the conundrum. In figure 1, the open ends of the helixes have a groove to accept O-ring material that sets against the base of the other spiral. That seals the unit axially. Radially the unit is sealed by the gap between the stationary and orbiting spirals. The smaller the gap, the better the seal and efficiency of pumping. But if they touch, it is trouble.

The efficiency is determined by pumping losses. The closer the outer surface of the inner scroll comes to the inside surface of the outer scroll, the less leakage there will be. This phenomenon is mathematically explained by exponential functions — so microns matter.

A spiral is a curve whose radius is equal to a constant multiplied by an angular displacement from zero. It is a continuous curve — not an arc or a straight line, and not a bunch of little steps like Stepper motors make. Servo motor drives are better in this application because they produce continuous motion. Using G code to interpolate a spiral tool path moving in two linear axes is an approximation. And as the size of the spiral decreases, the amount of approximation increases. Also, when milling machine axes change direction, there is a backlash error. Ball screws have backlash.