Keeping accuracy within reach: part quality is at risk if a machining center cannot hold tolerances at the farthest reaches of its work envelope. This makes volumetric accuracy a key indicator of a machine's performance. One machine tool builder discusses the implications

Modern Machine Shop, March, 2005 by Russ Olexa

Machine shop owners and managers face many challenges. But one that can be the most troublesome is a machining center that fails to produce accurate parts.

One cause of bad parts could be problems with the machine's volumetric accuracy. One example is when a shop gets in a large part that occupies the maximum of the machine's work envelope. The machine is able to hold tolerances within a specific area of the work envelope, but when the machine is cutting at the outer reaches of this area, features in this vicinity on the large part are not produced to tolerance. The same thing happens when small parts are located on corresponding portions of a tombstone fixture that takes up most or all of the work envelope. These parts are machined less accurately than those clamped elsewhere on the tombstone. What is going on here?

The culprit could be deficiencies within the machine tool's volumetric accuracy that go unnoticed until a part or parts are placed in the farthest reaches of the machine's work envelope. Each machine tool has a work envelope where a part can be placed and effectively cut. In a three-axis machine, the work envelope is made up of the total area that the cutting tool can reach with movement of the three axes.

Another way to understand volumetric accuracy is to picture two points of space within a 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 the 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, its volumetric positioning would be perfect, and theoretically, any point the machine can reach should have a true position in the work envelope.

"However, the reality is that if you start at the X, Y, Z zero position, and a tool is driven to a point at this defined position, it doesn't exactly go there, because of positional error in ballscrews or linear motors or whatever device is driving it there," says Scott Walker, president of Mitsui Seiki (USA) Inc. (Franklin Lakes, New Jersey). This happens, he explains, because there are geometrical inaccuracies inherent in any assembly that prevent it from getting there. Machine geometry is a function of moving a certain amount of mass along the machine's way system. This mass needs to move in a straight direction without pitch, yaw and roll, because any variation is amplified as the mass moves further and further along the machine's way system.

Mr. Walker adds, "You want to make sure that the machine's dynamic volumetric capability is at least 50 to 80 percent tighter than the part tolerance. In high-precision machining, most people use an 80 percent capability. For instance, if I have to produce a part to thousandths of an inch, such as an aircraft gearbox, the machine tool must be capable of true positioning to 0.0002 inch. That way I have 0.0008 [inch] to play with for all the other things, such as heat distortion, that are affecting the quality of the part."

Built-In Precision

Mr. Walker says that volumetric accuracy has to be built into a machine before it's purchased. In fact, volumetric accuracy is dependent on the part application and must be tailored accordingly, as in the case of exceptionally heavy workpieces (see "From The Ground Up").

To reduce pitch, yaw and roll that affect the machine's volumetric accuracy, Mr. Walker believes that the machine has to be hand-scraped along its ways. To check hand scraping for accuracy, an optical device called an autocollimator is used to analyze the pitch and yaw. By looking through this telescope-like device, one can detect misalignment by the position of the cross hairs in the autocollimator's sight. Movement of the cross hairs up and down indicates pitch error, whereas movement left or right indicates yaw error in the way surface. Roll error is measured with a precise spirit level. Typically, the autocollimeter is moved 100 mm along the way surface to measure pitch and yaw error at each interval. Then the spirit level will be moved along the way system to measure roll error at the same interval. This surface must be hand-scraped to reduce those values to acceptable tolerances.

Also, when heavily massed machine tool components are placed together during assembly--for example, when a four-ton column is placed on the machine's bed--the column compresses and bends the bed. For the column to track straight, a slight bow is produced in the way system to compensate for this distortion. Then, when the column is added to the machine tool, the column's weight compresses and allows the axis to run flat or straight.

If the X, Y, and Z axes can have reduced pitch, yaw and roll; their weight and their influence on each other can be compensated for: and the axes now track straight, then the straightness can be measured in arcseconds. A very high-accuracy machine would have straightness arcsecond tracking of less than 2 arcseconds, which would be considered excellent.