Speed Up, Slow Down For High Productivity

May 9, 2006

These two, 6 mm ball end mills were used to cut identical molds. The ball end mill on the right was controlled by NC optimization software and shows less wear. It also cut parts in less than half the time of the identical tool controlled by a standard NC program.

Software optimization increases feedrate when a small amount of material is to be removed, and slows it when the tool encounters more material.

HIGH-EFFICIENCY MACHINING — cutting a part in the least amount of time — is the goal of shops looking to improve productivity. Many manufacturers choose to run machines at or near their maximum feedrates with light cuts and small step-downs. This high-speed technique is attractive because the machine is moving as fast as it can. However, this strategy can create many inefficient passes and defeat the goal of reducing machining time. Although doing this means that there is never an excessive metal-removal rate that could break the cutter, this machining technique is not cutting efficiently.

Achieving the shortest cutting time is not related to feedrate, but rather is a direct result of achieving the highest volume removal rate. Cutting at a greater depth than is recommended by most high-speed strategies often is more efficient, but the danger is that the cutter may encounter an overloaded condition. That could cause breakage or could require power that exceeds the horsepower of the machine. The key to achieving high-efficiency machining is to vary the feedrates to achieve the highest volume removal rate possible, while protecting the cutter from breaking or overloading.

Software optimization
HIGH-EFFICIENCY MACHINING IS possible only with software that adjusts NC program cutting speeds for any condition encountered. One such knowledge-based machining system is Vericut from CG Tech, Irvine, Calif. ( Through a simulation process, the software learns the exact depth, width and angle of each cut. And, it knows how much material is removed in each cut segment and the exact shape of the cutter contact with the material.

The program's optimization module, called OptiPath, reads the NC tool path file and divides motion into a number of smaller segments. When it is necessary, based on the amount of material removed in each segment, the software assigns the best feedrate for each cutting condition encountered. It then develops a new tool path, identical to the original but with an improved feedrate. In areas of light material removal OptiPath increases feedrates. It decreases feedrates as more material is removed. This prevents cutters from breaking and keeps the machine from exceeding its horsepower. Similar high feedrates are maintained when it is possible, but the software provides greater cutting efficiency and requires less time than when feedrates are stepped down slightly for each pass. Without changing the cutting tool trajectory, the updated information is applied to the new tool path.

To determine the optimum feedrate for each segment of a cut, OptiPath combines operator-specified feedrates for a number of predetermined machining conditions with factors such as machine tool capacity (including horsepower, spindle type, rapid traverse speed and coolant); fixture and clamp rigidity, and cutting tool type (material, design, number of teeth and length). The program also considers factors that are dependent on the nature of the toolpath such as cut depth, width and angle, volume removal rate, entry feedrate and cutter wear.

This solution is automatic and determines the best feedrates before the program is loaded on the machine. The software also uses the expertise of the NC programmer and machinist to determine the best feedrates for specific cutting conditions.

Software vs. reactive machining
ADAPTIVE CONTROL TECHNOLOGY, WHICH SENSES CUTting conditions and adjusts feed rates in real time, seems to be a viable alternative to software optimization, but there are a number of issues to consider. The first is set-up and maintenance expense. Each CNC machine must be outfitted with its own adaptive control technology, which can cost thousands of dollars per machine. Then, each unit must be individually installed and configured. Active control technology can behave differently on different machines and with different controls, even if made by the same manufacturer. Once the active control technology is set up and operating correctly, there are also adjustment, reliability and maintenance procedures to follow as with any electro-mechanical system.

Active Control technology is reactive. That means that it adjusts feedrates based on the feedback received from the spindle drive motor 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 feedrates for diverse cutting conditions. For example, ramp cuts do not always significantly increase spindle loads. They increase the loads on axis motors as it becomes harder to push cutters through material, but it doesn't become equally difficult to turn the spindles. By the time loads increase to the point that spindles become difficult to turn, breakage is imminent.

Active control systems can also lead to problems when machining with high-tech carbide insert milling cutters. These cutters, which are designed to cut at an optimum chip thickness, cut very freely and don't require much horsepower for high volume removal rates. But there is a point at which the chip thickness becomes too great, causing premature cutting-edge breakdown. Ultimately, that leads to early tool failure. So, in this example, spindle load is a poor basis on which to base the maximum federate, because the increased load on the spindle is negligible, even if the feedrate is too high. By the time the active control adjusts the feedrate, it's too late.

The bottom line is that active control technology is limited to adjustments based on when spindle loads cross a preset threshold. This technology has no 'knowledge' of what the cutting conditions really are during the machining process, so it cannot accurately determine the ideal feedrate for every cut.

Instead of striving for constant spindle load, software optimization maintains a constant cutter load. When ramping, maintaining a constant cutter load produces safer feedrates. Also, maintaining a constant cutter load prolongs tool life when machining with high-tech milling cutters.

The software method of feedrate optimization is cost effective because a small number of software licenses can provide optimization capability for dozens of CNC machines of all types, with all kinds of controls.