5 Questions with Robb Hudson

1) What is volumetric accuracy in a machine tool?

RH: Machine geometry is a function of moving a certain amount of mass along the way system. This movement of mass needs to move straight without pitch, yaw, and roll, because any movement variation is amplified as you get farther from the machine tool’s way system. Without precise volumetric accuracy, the worst scenario is the machine tool will produce unacceptable parts. More common, the operator will chase tolerances, increasing scrapped parts.

Picture two points of space within the machining cube. In theory, these two points have a perfect position in relationship to each other, for example, exactly 1,000mm apart in the X-axis, 500mm apart in Y, and 300mm apart in Z. The first position is 0, 0, 0 in X, Y, and Z, and the second position is defined by the above values. If the machine can move to the second point with no deviation in relation to the first point without control compensation, it’s volumetric accuracy positioning would be perfect.

2) Why is high volumetric accuracy important to aerospace manufacturing?

RH: Aerospace components require high-precision dimensional accuracy, and that accuracy must be reliable and stable throughout production. Some of these aerospace programs have a very long life cycle – sometimes up to 20 years, and many of these applications require a machine tool volumetric accuracy of 20µm to 25µm; some even less than that.

3) How can you check volumetric accuracy on a machine tool?

RH: Mitsui Seiki was the first company to introduce the taper cone-cutting test to check 5-axis geometric accuracy to ensure all five axes of the machine tool are properly coordinated and working together as designed. For every 5-axis machine produced, we take what looks like a spiral double gear blank, mount it on a 45° fixture on the table, and flank mill the entire OD of that taper cone. We then take the taper cone into the metrology room and use a roundness measuring instrumentation to check the high, middle, and the low round-out locations on the taper cone, typically less than 5µm.

4) How do you build a machine that achieves high volumetric accuracy?

RH: Volumetric accuracy is first dictated by inherent mechanical accuracy of the machine tool. To achieve the highest level of mechanical accuracy, the machine’s way and mating surfaces need to be hand-scraped. We’re not trying to scrape a linear axis perfectly flat, we’re getting the mating parts to fit better together and compensate for pitch, roll, and yaw of a given linear axis. A CNC control can make compensation for linear position errors, but not for pitch, roll, and/or yaw errors. Every machine tool is scraped to a specification for that machine tool based upon its kinematic design and the weight particular components will bear.

Other key elements are spindle thermal control, ball screw drunkenness control – ensuring that the thread was put on concentrically – using large, rigid castings with high nickel content for thermal stability.

The machines constructed in our highly controlled assembly environment allow Mitsui Seiki to offer products with a 5-year accuracy guarantee.

5) Can compensation routines in the control system achieve volumetric accuracy?

RH: The short answer is no. Even in extremely high precision machine building, we rely on a certain amount of control compensation for positioning accuracy. But the fundamental way to achieve long-term high volumetric accuracy is in the construction and assembly characteristics and methodologies.