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Specifically, increases in rake angles can improve chip formation, yet this does come at a cost because the greater the rake angle the weaker the cutting edge.
Rake angle also highly influences the value of the shear plane angle, which is the angle formed by the pure plastic deformation of the workpiece material. Here, the material starts deforming or chip forming in front of the cutting edge. For both material properties and running parameters, the angle varies; however, it should always be a goal to make the shear plane angle more vertical because the steeper the shear plane the better chip formation.
Chip thickness comes into play here as well. The more elastic a material is the steeper the shear plane angle will be, resulting in a thinner chip. Conversely, the harder the material is the flatter the shear plane angle will be, which means a thicker chip is formed. All in all, more rake angle means more shear angle, which means better chips, but balance is key here as well. A really sharp cutting edge will make great chips but will fail and break due to a smaller cutting edge cross section and weaker cutting edge, so find balance in the rake angle—one that is aggressive but not overly so.
CHANGES IN CHIP FORMATION
A final thing to look for when examining chips is any changes in chip formation. If chip formation is altered during an application, it could be caused by a myriad of elements: wear on the tool, built up edge on the tool (BUE) or changes in the environment like coolant or material changes. In new applications, it may be best to drill shallow
test holes and take a look at the chips to make sure they are small and segmented. Being conservative in the beginning with speeds and feeds could also aid in better understanding chip formation and what adjustments need to be made.
Awareness of any changes in chip formation is key, though. Poor chip formation can cause major problems in drilling applications. Long, continuous chips are difficult to evacuate and can become packed in the drill flutes, damaging the drill or even causing drill failure. These long chips could also become wrapped around the drill body and again cause tool failure. Lastly, poor chip formation impacts the hole quality. If chips are dragging or packing in the flutes, there will be poor hole finish. Noticing any changes in chip formation is important not only for tool life and hole quality but also for the overall success of the application.
Knowing more about the chips formed in any metal cutting application enables machinists to better control the outcome and success of drilling operations. While it is necessary to examine chip size, shape and thickness, it is just as important to know how coolant, tool selection and changes in chip formation tie into the application as well. So take a look at the chips being created and break it down chip by chip because both proper chip formation and chip evacuation are required for successful high-production drilling.
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CNC WESTJune/July 2022
Built up edge (BUE) on inserts.