Proactive vs. Reactive Machining

On the surface, adaptive control (AC) technology seems to be a viable alternative to software optimization. After all, AC senses cutting conditions and adjusts feed rates in real time. It can be directly connected to a CNC machine tool.

But there are a number of issues to consider if you’re thinking of investing in AC technology. The first is set-up and maintenance expense. Each CNC machine must be outfitted with its own AC – which can cost thousands of dollars per machine. Each must then be individually installed and configured, and ACs behave differently on different machines and controls. Once the AC is setup and finally operating correctly, as with any electro-mechanical system, there are also adjustment, reliability, and maintenance considerations.

Next, AC technology is a ‘reactive’ system. ACs adjust feed rates based on feedback they receive from the spindle drive motor – that is they adjust feed rates to maintain a constant load on the spindle drive. This type of optimization is appropriate for certain types of very rigid cutters that can take a heavy load, such as face mills or large end mills.

But, spindle load optimization cannot always provide the best feed rates for diverse cutting conditions. For example, a ramp cut does not always significantly increase spindle load. It increases the load on the axis motors as it becomes harder to push the cutter through material, but it doesn’t become equally difficult to turn the spindle (Figure 1). By the time it becomes difficult to turn the spindle, you better be hiding behind a scatter shield!

Figure 1: Tangential force (spindle load) is not greatly affected by end cutting. Because AC uses spindle load to control feed rates, it doesn’t detect the poor cutting conditions and slow the feed rate accordingly.

 

Another example is machining with today’s high-tech carbide insert milling cutters. They are designed to cut very freely (don’t require much horsepower for high volume removal rates). The goal for these cutters is to cut at an optimum chip thickness. But there is a point where the chip thickness becomes too great, causing the cutting edge to breakdown pre-maturely. This ultimately leads to early tool failure. Spindle load is a poor indicator for the maximum feed rate to use, since the increased load on the spindle is negligible – even if the feed rate is too high. By the time the AC adjusts the feed rate, it’s too late.

The bottom line is that AC technology is limited to adjustments based on when the spindle load crosses a pre-set threshold. It has no ‘knowledge’ of what the actual cutting conditions really are during the machining process, so it cannot accurately determine the ideal feed rate for every cut (Figure 2).

Figure 2: AC technology attempts to choose the ideal feed rates based on pre-determined spindle load tolerances.

 

OptiPath, on the other hand, automatically adjusts feed rates based on the specific cutting conditions for each segment of the tool path. It is the only product available that optimizes feed rates based on solids verification technology. Rather than react to feedback from the spindle drive motor, OptiPath assigns the best feed rate based on the current cutting conditions (volume of material being removed, depth, width, and angle of cut).

Instead of striving for constant spindle load, OptiPath maintains a constant cutter load. In the ramping example, maintaining a constant cutter load produces safer feed rates. For high-tech milling cutters, maintaining a constant cutter load prolongs tool life. Sometimes it is desirable to maintain a constant chip thickness while cutting – a simple task for OptiPath, but something AC cannot do.

OptiPath is also a more cost effective method of feed rate optimization. A small number of software licenses can provide optimization capability for dozens of CNC machines – of all types, driven by all kinds of controls. An AC is limited to a single machine.

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Optimization and High Speed Machining
Software Optimization vs. Adaptive Controls