Page 51 - 2022cnc6-7

spacing the more chip fracturing that will occur. Smaller diameter inserts are limited to the velocity differential available due to the restriction on the chip width required to easily evacuate chips through the holder gullet.
Example of good chip formation.
THICKNESS
The thickness of the chip varies with the feed rate; heavier feed rates form thicker chips while lighter feed rates form thinner chips. The thickness of the chip formed decides how the chip will fracture, but this is also dependent on the material being machined. At the same time, changing the speed impacts the chip thickness; the higher the speed of the tool, the more heat generated in the cut, which makes the material more elastic. So a balance between speeds and feeds is necessary. With many materials, a thicker chip means there is a greater chance of exceeding the elastic limit of the materials, which increases the likelihood of chip fracture; on the other hand, thinner chips are more elastic and, thus, farther away from the elastic limit necessary to fracture the chip.
Soft, gummy materials like soft carbon steels, 300 series stainless steel or pure titanium have a high elastic limit— so much so that increasing chip thickness has a negative effect on chip formation. Materials like these require specific lip geometries to potentially create an acceptable chip. Nevertheless, it is key to look at the chip deformation ratio of materials to better understand chip thickness. Chip deformation ratio can be defined as the ratio of deformed chip thickness over the undeformed chip thickness (feed rate). For most steels, this ratio is typically 2-3:1; however, it can be as high as 5-10:1 for those soft, gummy materials. Ultimately, though, this measurement is an indicator of chip form and elasticity in the material being cut, and the higher the deformation, the more difficult chip formation will be.
COOLANT
When it comes to coolant, through-tool coolant when paired with the right drill geometry is critical for the best chip formation and evacuation. Additionally, changing coolant type, pressure and volume influence the thermal shocking of chips. This can change the properties of the
chips and make them more or less likely to break into manageable segments. For example, coolants can decrease material elasticity due to the strain hardening that occurs as coolant quickly cools hot, elastic chips. The cooling of elastic, continuous chip formation embrittles chips to the point of fracture by reducing their elastic limit.
For chip evacuation, coolant pressure and volume are important. To evacuate a set volume of chips, a set amount of kinetic energy is provided by the coolant volume. Drilling can occur uninterrupted from the top of the hole to the bottom as long as enough coolant volume is available, which will be evident during the application with a steady load meter reading while drilling. With an insufficient coolant volume, an unsteady load meter will be detected when drilling into the hole. Although this does not mean that drilling with insufficient coolant is not possible, it does demonstrate that the drill must be altered to fit the environment.
Pressure on the other hand is the force behind the coolant that provides a fixed volume of coolant through a given diameter. As coolant pressure is increased through a fixed coolant orifice diameter, the coolant volume will increase. When drilling small diameters, high coolant pressure is needed in order to provide sufficient coolant volume, but as drill diameters increase, high coolant volume becomes more necessary than high coolant pressure. In high-production drilling—especially deep hole drilling— through the tool coolant is critical because it provides an upward force on the chip to aid in flushing the chips through the drill flutes and out of the hole. Although flood coolant can be used alternatively to through-tool coolant in short drilling applications under two times diameter, in deeper holes flood does not promote good heat transfer and can also push chips back into the hole, which can cause chippacking.
Through-tool coolant is also important when factoring in heat because it provides coolant right to the cutting edge where it is needed to cool the tool. When machining, 60% of the heat generated in the plastic deformation of the material remains with the chip formed while the other 40% remains with the tool and workpiece. This portion that stays with the tool must be evacuated by coolant in order to have sufficient tool life. Clearly, when more coolant pressure and volume can go through the tool, the cooler the tool will run. This then means that there will be greater tool life and that the tool can potentially be run faster.
TOOL SELECTION
Chip formation can also indicate whether the best tool is being used. If the chip formation is not meeting the standard, a change to tool geometry may be needed in order to improve the situation. Clearly, the geometry of a cutting tool has a significant impact on the chip formed.
CNC WEST June/july 2022 www.CNC-West.com 49