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

Once the three axes on a typical three-axis machine track straight, then the last adjustment necessary is to get good perpendicularity of those axes. If the machine has precise perpendicularity and straightness, then when the cutting tool is tracking through space, it will have excellent true positioning in a defined area. In short, the machine tool's volumetric accuracy will be high.

Positioning repeatability and volumetric accuracy are related, says Mr. Walker. Repeatability, however, describes a machine's ability to return to a single point in space along one of the machine's axes, whereas machining accuracy involves many points tied together. "Repeatability was a big issue in the early days of building machine tools, when ballscrews were not preloaded as they commonly are today," notes Mr. Walker. Thus, he concludes, repeatability no longer holds the relevance it once did for machine tool buyers.

Precision Performance Standards

In addition to applying standards set by national institutions to help end users define what a specific machine tool characteristic or capability should be, Mr. Walker says that builders also apply their own standards for measuring a machine tool's accuracy. For example, certain builders have a perpendicularity standard that uses a cylindrical square to measure the machine's perpendicularity. Likewise, they use a ceramic square placed on the table to check for the flatness of a pallet as it rotates in relationship to the spindle.

Builders will measure a group of tolerances that they specify in their production plan, allowing them to give the machine's true positioning in some number. For example, they may state that the machine is capable of positioning 20 microns or better in a meter cube, or that the machine is capable of positioning 75 microns or better in a 1,000-mm cube. "True position is very difficult to measure, because it states that anywhere I position a point off the spindle, such as the tip of a drill, this position will not deviate in relation to a X, Y, Z location specified somewhere in space by any more than 20 microns," explains Mr. Walker.

According to Mr. Walker, machine tools are often designed and manufactured to have a "sweet spot" in their work envelope where they have the greatest accuracy. "You can't have perfect perpendicularity and precise roundness capabilities because of the way the machine is designed for one or many applications," he says. If critical tolerances are confined to certain portions of the machine's work envelope, special attention must be devoted to the machine's precision in that portion of its work envelope.

The situation becomes more complex when discussing a five-axis machine, Mr. Walker says. For example, a number of five-axis trunnion-type machines are on the market. However, some of these machines have five-axis capability to within 10 arcseconds, whereas others are within 0.5-arcsecond accuracy. "The problem with five-axis machines is that you have issues of axis orbit, which is the axis rotation orbiting between two rotating axes," he explains. "Hysteresis is also a concern in motor reversal, because you have to put a lot of energy into the motor to drive it, and then when the motor has to slow down and reverse direction, you have to take all that energy out, which takes time. This creates servo lag and will affect the true dynamic positioning capability in the machine's work envelope."