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Dynamic Milling on the other hand keeps radial engagement low but increases cutting speeds and feed rates to compensate for any productivity loss. By optimizing the cutting parameters as described above, the resulting cutting forces and cutting temperatures are reduced. As a result, tool life is extended substantially, along with improvements in the cycle time, creating an overall benefit in the cost situation of producing the workpiece. Ideally, dynamic milling will utilize the full length of the cutting edge and incorporate tools with more cutting edges than traditional methods would allow. The longer length of axial engagement and more cutting edges will increase the so-called Axial Contact Points which will stabilize the end mill and minimize vibrations.

Combining Machining Strategies In the early 1920’s, a gentlemen called Solomon, wanted to know what would happen when cutting speed is increased above what was normal at that time. The outcome of his extensive testing developed into what is now called the Solomon-Curve. This states that beyond a certain cutting speed, temperatures are no longer increasing.

As the traditional method typically has only one or two Axial Contact Points there is a risk that one even further reduces the axial engagement to prevent vibrations. This could lead to a situation where an end mill is shifting from being in the cut to not in the cut. While this situation will make the operator think it is more controllable, it is still not better than before. Instead of more axial passes, a much smarter way would be to increase the number of Axial Contact Points by increasing the axial depth and reducing the amount of radial engagement and perform more passes. This preferred method allows for more equal heat distribution along the cutting edge and greatly reduced stress upon the machine and cutting tool all the while providing economic benefit. In the next episode, we will continue with the content of Dynamic and Trochoidal Milling.

Traditional Method and High Velocity Milling Traditional roughing comes along with heavy radial engagement, up to full slot. As a result, the chip thickness is inconsistent and a wide chip shape creates a challenge for chip evacuation. Also, much more heat is introduced into the tools cutting edge along with high and varying cutting forces. To prevent accelerated wear, optimal cutting speeds and feed rates are not able to be achieved. Instead, machining parameters are kept to the lower end of the scale. It is dependent upon the tool’s capability and the machinability of the material on how much axial engagement is possible, typically not much.

Chip Thinning Effect At 50% radial engagement or greater, up to full slot (Ae) the average chip thickness is of course equal to the feed rate (fz). Decreasing the radial engagement below 50% while maintaining a constant feed rate leads to an over proportional decrease of the average chip thickness (hm). To compensate for this decrease and bring the chip thickness back to optimum, the feed rate per tooth must go up – again over proportional. As every material and machining situation has an optimum chip thickness therefore the factor of increase can be very high. At the same time, the cutting tool speed can also be increased, again over proportional, as the tool has less time in cut and less heat is created. This combination of increased feed and higher speeds allows for maximum increases in Metal Removal Rates (MRR).

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As a side note. All of this applies to roughing of course. Finishing does not optimize chip thickness but cusp height to achieve best surface qualities and highest dimensional accuracy. < Chip thinning allows higher feed rates and optimizes same time >

The second part to High-Speed Cutting is High Efficiency Milling (HEM). This Milling strategy targets constant tool engagement and minimization of air cutting or other unproductive moves of the machine and respectively of the tool. Basically, it optimizes the tool path to save time independent from cutting parameters.

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There is a term within Dynamic Milling that takes into consideration the radial chip thinning effect which happens at low radial depths of cut to an over proportional increase in the corresponding feed rate per tooth. This is referred to as High Feed Machining (HFM).

Additionally, and even more interestingly, was the fact that respective cutting forces stayed the same or even slightly decreased! This outcome (actually, a patent at that time) is the basis for all High-Speed Cutting (HSC), which includes High Speed Milling (HSM). This method of machining allows one to go as fast as the machine and the material being cut allows – in theory and in practice.