To be competitive in today’s international marketplace, NC programs need to be not only fast but efficient. In the case of machining, the most efficient NC program is the one that removes the largest volume of material in the least amount of time.

But it does not typically do it using the fastest possible feedrates.
To create the most efficient machining processes possible, optimization software can determine the best feedrates to use for each cut. Achieving the best feedrates for each cut in an NC program is certainly a desirable goal, but it is practically impossible to do manually, especially on a large mold machining path. Trying to visualize the cutter contact and cutting conditions for each cut in a large NC program, then calculating the best feedrates for the cuts, and finally manually inserting hundreds or thousands of different feedrates for each changing condition is not practical. An incorrect feedrate estimate or editing error can break the cutting tool, damage the fixture or scrap the part.

Without software to optimize the feedrates, the moldmaker or NC program-mer is forced to choose a single feedrate for an entire machining sequence. Because of this restriction he must choose his feedrate and machining strategy to ensure the cutter will not be overloaded and break. Thus he must choose either a slow conservative feedrate with heavier cuts, or use a machining strategy with very light cuts at a higher feedrate. Unfortunately, in his attempt to avoid overloading or breaking the cutter, either choice results in very inefficient machining, and usually premature cutter wear. The resulting wasted time and increased costs are not tolerated in today’s internationally competitive environment.

The most common choice today is to select a machining strategy that allows running the machine at or near its maximum feedrate while using a small enough axial depth-of-cut so there is never an excessive removal rate that could break the cutter. While this high-speed machining technique is attractive because the machine is moving as fast as it can, it is not cutting very efficiently.

Cutting at or near a machine’s maximum feedrate, with very light cuts and a small step-down can actually create many inefficient passes and can defeat the goal of reducing time. Also, this method often results in premature cutter wear due to the light chip load (see Figure 1, page 41). Achieving the shortest cutting time is unrelated to feedrate, but rather is directly the result of achieving the highest volume removal rate. High-efficiency machining—cutting a part in the least amount of time—is the real goal. Cutting at a greater depth than is typically recommended by most high-speed strategies is often much more efficient, but the danger is the cutter may encounter an overloaded condition—causing breakage or exceeding the horsepower on the machine. The key to achieving high-efficiency machining is to vary the feedrates to achieve the highest volume removal rate possible, while still protecting the cutter from overloading or breaking.

High-efficiency machining is only possible with software that will adjust NC program cutting speeds to make the machining process faster, more efficient and of higher quality. This is a knowledge-based machining approach that uses a combination of software to detect conditions and adjust feedrates according to settings entered by each shop’s local machining expert, essentially adding intelligence to each cutter in the shop. During the simulation, the software knows the exact depth, width contact area and direction of each cut because the software also knows the exact shape of the in-process material at every instant of the machining sequence. And, it knows exactly how much material is removed by each cut segment and the exact shape of the cutter contact with the material.
The precise cutter/stock geometry information can be used, for example, to calculate the maximum chip thickness for a given cut. Chip thickness is more than simply feed per tooth or the amount of advance into material for each tooth of the cutter. It is a complex 3-D modeling of the cutter and the material volume, requiring determination of the maximum engagement of the tooth into material. This calculation requires an accurate 3-D model of the instantaneous in-process material.

With this unique knowledge set, optimization software determines the best feedrate for each cutting condition encountered—taking into account the volume of material removed, chip load, and machine acceleration and deceleration requirements. If desired, the software also can divide cuts into smaller segments and vary the feedrates as needed in order to maintain a consistent chip load or volume removal rate. It then creates a new NC program with the same trajectory as the original, but with improved feedrates.

Maintaining a constant chip thickness is especially important in high-speed finishing operations. As much as 40 to 60 percent of mold and die manufacturing costs are associated with the finish machining process. Reducing the time required during this phase is a significant benefit that can dramatically improve the way manufacturers do business. And, using optimization software is a way to combat the chip thinning problem, as recommended by the cutting tool manufacturers to significantly increase tool life.