As one builder sees it, the key to high speed is a horizontal machining center.
Dowel-shaped roller pin bearings on Toyoda's FA 450II HMC provide rigidity comparable to a boxway design and more surface contact to minimize wear during high-speed machining.
A horizontally oriented spindle creates a natural, gravitational chip-evacuation system for high-speed machining.
Drive technology is actually ahead of the curve, for the moment, providing feedrates faster than can be applied. However, the technology appears to be reaching the limits of speed on ballscrew machines.
Toyoda defines high speed as spindle speeds greater than 10,000 rpm and machine feedrates of more than 300 ipm. All this while maintaining path accuracy. Although both vertical and horizontal-type machines can easily be configured to provide the necessary speeds, feeds, and accuracies for high-speed machining, the basic design of a horizontal machining center often best compliments such operations.
A particular advantage of high-speed machining on a horizontal is the spindle orientation to the work area. The horizontal Z-axis position creates natural, gravitational chip removal.
For example, milling a mold pocket on a horizontal machine lets chips fall out of the cavity. In comparison, the same process performed on a vertical machine fills the cavity with chips during the cut. This means that coolant streams must be powerful enough to wash these chips up and over the side of the mold. And deeper cavities only compound the problem.
Although chip evacuation may not initially seem like a good enough reason to pick a horizontal over a vertical machine, there is a drive, particularly in the automotive industry, to mill aluminum dry. This decision is usually prompted more by regulations on coolant and chip disposal than by the application. Fast and light high-speed cutting requires less coolant because the heat is removed in the chip. Horizontal spindle designs let these chips fall out and away, minimizing reliance on coolant for flushing.
Table configuration of the horizontal machining center is another important design aspect. A rotary fourth axis providing 360° positioning or simultaneous contouring is an integral part of the horizontal structure. Vertical machines, on the other hand, offer this feature as an add-on, which creates a weak link in the design and consumes valuable workspace. The horizon-tal's table design also plays an important role in the cut.
To understand this role, one need look no further than a typical die/mold application involving a ballnose cutter. The weakest cutting point on this tool is dead center where there is no chip evacuation and poor tool geometry. This forces the tool through the material. Vertically positioned, it is nearly impossible to use the tool any other way. But with a fourth axis, horizontals can tilt parts 15°, for instance, and cut with the side of the tool. This dramatically increases the cutting area and improves chip evacuation.
Additionally, the pallet shuttle eature of a horizontal machine lets operators set up the next part while the machine continues uninterrupted cutting. As a protected work area at the front of the machine, this feature provides a clean, self-contained environment for part loading.
Toyoda uses a dowel-shaped roller pin bearing on its 40- taper model FA 450II horizontal machining center. This design provides more surface contact per bearing, as compared to a typical linear ball way system. It also minimizes wear and is nearly as rigid as a boxway.
Although fast, roller bearings cannot achieve the same vibration damping qualities as a boxway design. Therefore, Toyoda does not use roller bearings on its larger 50-taper machines.
What the company does use is a special hydrostatic technology that relies on tighter tolerances, special material for the ways, and lightweight oil lubrication to achieve speeds of 1,700 ipm. While most consider 1,000 ipm the maximum speed for a boxway machine, Toyoda's system is 70% faster than that — with the rigidity and vibration-damping benefits of a boxway design. However, this speed requires a spindle that is not only rigid enough, but can also keep up.
In true high-speed applications, a machine's spindle should never have to stop or slow down. It should run as fast as it can, all day long. Performing in such demanding conditions requires a rigid design and high-quality components that dissipate heat and dampen vibrations. Many lightweight machines can't handle high speed and typically have a duty rating that shows the machine can run wide open only 25% of the time.
Toyoda uses ceramic-coated bearings with high-pressure mist lubrication systems for maximum cooling within the rating limits of the bearings.
The faster a machine moves, the more damaging any vibration is to the spindle and cutting tool. This is when the pin roller bearings are an advantage over the roller balls of other machines.
As with way systems, bearing technology limits today's spindle performance. But there are two cutting-edge designs that may contribute to the redesign of future spindles. These will turn faster than the current 25,000 to 30,000 rpm and do it with a 40-taper configuration.
These designs are hydrostatic bearings, which run on a film of oil, and magnetic levitation bearings that float in a magnetic field. Although both technologies are now available, they are expensive and have reliability problems. They can run at 100,000 rpm, but require further development before being considered practical.
Drive technology is actually ahead of the curve, for the moment, providing feedrates faster than can be applied. But the technology appears to be reaching the limits of speed on ballscrew machines.
It's getting to the point where people talk more about acceleration rates and G forces than inches per minute. Currently, Toyoda is up to a 1 G acceleration on some ballscrew-driven models and believes linear motors are the future. But linear is still expensive and has some problems, including heat dissipation. However, it is hard to ignore an acceleration of up to 6 G and rapid traverse rates of 5,000 to 6,000 ipm.
In addition to a capable way system and spindle, a machine must have a control that manages the large blocks of information involved with high-speed machining. Control processing time (as fast as a millisecond) and look-ahead functions that pre-process these blocks of information are features that prevent data starvation in high-speed applications.
Toyoda's Fanuc CNC, for example, can change between three different drive tuning sets. Operators can run in a standard mode for drilling, tapping, and milling. Then they can shift into a high-speed finishing mode by entering a G-code that tightens up the machine's state-of-tune automatically. Switching back and forth between modes increases production and optimizes finishes without limitation or undue wear on the machine.