Rough and tough - high-efficiency roughing versus high-speed roughing

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To prove out these roughing theories, data was logged from a sensory toolholder with a solid carbide end mill cutting 1018 steel on a vertical machining center. A sensory toolholder is a wireless force sensor used for tool monitoring. During machining, it generates force and torque directly at the toolholder. The data can then be transmitted to a receiver and used to optimise machining processes. First, a traditional roughing approach was used to run the part. Not only could one hear the variation in the cuts, but it could clearly be seen in the logged data. Through graphs, the toolholder sensors show a tale of cutter loads with considerable variation and very sharp spikes, especially in torsion (green) and bending moment (red) (see Figure 1). The spikes indicate that the machine operators need to decrease their feed rates for the worst-case scenario, but the many low areas in the graph mean that for the remaining operations, the program will not be running at optimum material-removal rates.

Adaptive clearing quickly reaches the maximum efficient machining rate

However, high school pre-calculus tells us that the total area under the graph is what is actually important. The more total area under the graph, the more material being removed and the more efficiently the machining operation is running. The graphs in Figure 1 display several low and flat areas, indicating inefficiency with the occasional spike of higher material removal. Adaptive clearing quickly reaches the maximum efficient machining rate and maintains that rate throughout the cut, then repositions for the next cut. This allows milling at an efficient rate during all cuts with constant cutter forces throughout. Constant forces mean fewer vibrations within the tooling and less shock to the cutting edges caused by those vibrations. This extends tool life and reduces tooling costs.

Henrik Jørgensen at the Husky machine in the test centre. (Source: Hasco)

Figure 2 shows consistency between when the tool is cutting, in all cuts, and when the tool is not cutting. One can quickly see that the tool spends more time at a maximum-efficiency cut, which is indicated by more area under the graph. In real numbers, this means that the traditional roughing pass took 8:09 minutes to complete the roughing operation, while the adaptive clearing technique took only 2:01 minutes.

While saving time is a huge plus, the benefits are deeper than that, including reducing tool wear and breakage, while also having more predictable cutting conditions. For example, a larger axial depth of cut allows the use of more of the cutter flute length without overusing the bottom of the cutter. Additionally, the generated heat is spread out along the whole cutter flute length, rather than concentrated at the bottom of the tool.

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