Dynamic milling is becoming more popular due to its ability to improve material removal rates while maintaining process security. Incorporating two different machining strategies creates the dynamic milling concept and allows for advantages not previously realized.
Radial chip thinning is the first strategy exploited in dynamic milling. This is the phenomenon of creating a smaller chip by engaging a small percentage of the tool diameter, less than half (Figure 1). When less than half the tool diameter is engaged in the cut, average chip thickness is less than the advance per tooth. Ten to 20 percent radial engagement of the tool diameter results in a significant reduction in average chip thickness.
When using radial chip thinning, average chip thickness should still meet the tool manufacturer’s recommendation for chip load. If it does not, the tool’s capability is not being maximized. It also can result in the chip not being physically large enough to hold the heat produced by the cutting operation. A small chip has less capability to hold heat, which produces the undesirable result of either the tool or the workpiece absorbing this latent heat. Feeding the tool faster prevents excessive radial chip thinning.
If machining applications were only in a straight line, light radial depths of cut would be all that a machinist would need to monitor. However, parts come in all shapes and sizes, with some being very complex. Machining in a straight line is normally not an option, since pockets, internal corners and curved surfaces need to be created. So, controlling radial engagement is not enough.
A better parameter to monitor is the tool engagement angle (TEA). This is the angle drawn from the point where the tool enters the material, through the center of the tool, and back to the point where the tool exits the cut (Figure 2).
By holding this engagement angle constant, the tool is always in a controlled situation. In this environment, there is always the same cutting condition. One can confidently rely on radial chip thinning, which allows elevated sfm, depth of cut, and feed rates while using tools with a higher number of teeth and longer cutting-edge lengths.
Incorporating dynamic milling on the shop floor requires resources from three specific areas. First, a machine tool controller with sufficient processing capability is necessary. Second, a dynamic milling program will have a lot of lines of code which must be read efficiently and quickly. Finally, a CAM system capable of writing dynamic milling code is also required. The software needs to calculate how to create toolpaths so that the desired part is created yet does not exceed the programmed TEA. Finally, a cutting tool with the correct characteristics and capability is required.
Cutting tools for dynamic milling are typically solid carbide. Long lengths of cut and a high number of teeth allow the dynamic milling process to maximize material removal rates. Through-tool coolant is not required since milling is never confined in slots or pocket corners. Chips are easily evacuated without coolant, or, if the material requires coolant, external coolant can be easily applied to the cutting zone.
Typically, chips are very short in the radial direction. They are, however, very long in the axial direction (Figure 3). This chip configuration does tend to cling together and create problems with chip evacuation in deep pocketing applications. Chip splitters along the cutting edge can break the chip in the axial direction, eliminating this problem.
Walter USA has focused on support for customers who utilize the dynamic milling strategy in their shops. For example, the company has created a line of solid-carbide milling tools—MD133—which incorporate all the necessary tool characteristics required for this application.
Standard MD133 tools are offered in both 3xDc and 5xDc flute lengths with either five or six flutes. This well-balanced number of flutes allows high feed rates while maintaining enough chip space to avoid chip packing. Necessary chip splitters are also incorporated in a staggered configuration so that the chips are broken in the axial direction, yet do not affect surface quality of the finished part.
To further increase the application range of MD133 tools (Figure 4), they are offered in two different grades: WJ30RD for ISO P materials (secondary use in ISO K) and WJ30RA for ISO M materials (secondary use in ISO N and S). Multiple grades allow for an exact design for the specific materials being machined.
Dynamic milling offers an efficient way to machine parts with better process security. When the process has been correctly implemented, production rates will increase while cycle times and production costs decrease.