When little things mean a lot: measurement technology plays a significant role on machining centers used at high speeds

Modern Machine Shop, March, 2004 by Peter Zelinski

But here is another case where measurement technology combined with algorithms in the CNC can reduce the amount of lost time. In the case of thermal expansion, the algorithms draw on data captured from temperature sensors throughout the spindle. The thermal expansion, like the centrifugal expansion, is a repeatable effect; however, it is not a linear effect. Modeling the thermal behaviors of a particular spindle design accelerating to a particular speed yields a curve shaped like the one in Figure 2. As an alternative to waiting for the spindle to warm up, says Mr. Sudhakar, the CNC can allow for thermal expansion during the warm-up period by continually adjusting the tool offset in accordance with such a curve. The CNC can control the accuracy of the cut to within 20 microns in this way, without allowing for any warm-up delay.

The CNC can hold an even tighter accuracy (10 microns) if the machine is allowed to retain just 3 minutes of the warm-up delay. Why the difference for this short wait? Because there are components of the process for which no temperature readings are available. Consider the toolholder, for example, in most cases, a tool change will consist of inserting a "cold" toolholder into a "hot" spindle. The size of this toolholder therefore will change much faster than the size of the spindle, as the toolholder's temperature races to catch up. A small wait time can allow the heat and thermal expansion of the tooling to settle into parity with the expansion of the rest of the system.

Vibration

One more effect that becomes pronounced at higher values of rpm is vibration. It may be a product of the interaction between the tool and workpiece that produces chatter, or it may come from centrifugal force acting on some unbalanced mass in the tool-toolholder assembly. The first of these effects tends to become more pronounced as speed increases, while the second of these effects can only become more pronounced as speed increases, because of the relationship between centrifugal force and speed. In either case, controlling vibration is essential not only to the accuracy and productivity of the process, but also to the lives of the spindle and the tool.

Mikron's system for controlling vibration involves accelerometer readings of the spindle's front bearing (see Figure 3). These measurements provide the data for a system the company refers to as its "Advance Process System," or APS. The system recognizes that the ear of the operator or machinist is no longer an appropriate means of managing vibration when a 40,000-rpm machining center is involved.

For processes attended by an operator, APS presents a multicolor display on the CNC interface. The color of the vibration reading--green, yellow or red--gives the operator an immediately recognizable clue as to the severity of vibration the machine is seeing. Vibrations below 3G (three times gravity) fall in the "green" range, vibrations below 6G are "yellow" and anything beyond that is red.

Diagnostic work is still necessary to find the source of the problem, but these readings make it possible to narrow down the potential culprits. Vibration severity that changes as the depth of cut changes is more likely to involve chatter, while vibration that continue s both inside and outside the cut is likely to be the result of an unbalanced weight.