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It’s Volumetric!

Robb Hudson
By Robb Hudson CEO, Mitsui Seiki USA Inc.

For aerospace part production, machine tool volumetric accuracy is vitally important. To define it, 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 variation of these three movements is amplified as you get further and further away from the machine tool’s way system.

Without precise volumetric accuracy, the worst scenario is the machine tool will produce unacceptable parts. More commonly, the operator will chase tolerances, which increases scrapped parts.

Another way to understand volumetric accuracy is to picture two points of space within the machining cube. In theory, these two points have a perfect position in relationship to each other. They could be exactly 1,000 mm apart in the X-axis, 500 mm apart in Y-axis and 300 mm apart in the Z-axis. The first position is 0, 0, 0 in X, Y and Z, and the second position is defined by the values given above. If the machine can move to the second point with absolutely no deviation in relationship to the first point without control compensation, its volumetric accuracy positioning would be perfect.

To achieve a high volumetric accuracy, the machine needs to be designed to provide stable thermal characteristics. 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.

The process of scraping actually is not what many people think. We’re not trying to scrape a particular linear axis perfectly flat. What we’re trying to do is get the mating parts to fit better together and compensate for pitch, roll and yaw of a given linear axis.

Every machine tool that’s made is scraped to a particular specification for that machine tool based upon its kinematic design and the weight that those particular components are going to have to bear.

Other key elements are spindle thermal control, ball screw “drunkenness” control – ensuring that the thread was put on in a very concentric manner, and using very large, rigid castings with high nickel content for casting thermal stability.

To check the volumetric accuracy on a machine tool, the taper cone-cutting test is one method and this is something we do on every 5-axis machine produced.

At customer locations, many are using coupons capable of 5 axes point-measuring that are certified to their coordinate measuring machines. They use these coupons in the machine to measure the machine’s accuracy with machine probing. This is very effective for assuring the machine is accurate and nothing has happened to the machine tool to degrade the machine’s accuracy.

A question may surface about CNC control compensation. Certainly, even in extremely high-precision machine building such as ours, we rely on a certain amount of control compensation for positioning accuracy.

A CNC control can make compensation for linear position errors, but not for pitch, roll and/or yaw errors. The fundamental way to achieve long-term high volumetric accuracy is in the construction and assembly characteristics and methodologies.

The most difficult, critical aerospace components require high-precision dimensional accuracy, and that accuracy has to be reliable and stable over the course of production, which could be days for one part.

And then of course, some of these new aerospace programs have a very long life cycle—sometimes as many as 20 years. Many of these applications require a machine tool volumetric accuracy of 20 – 25 microns and some even less than that.

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