Examining chip by chip

Features - Machining

Paying attention to chip formation enables machinists to better control the outcome and success of operations.

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Good chip formation
Photo credit: Allied Machine & Engineering

Whether examining how tool selection is impacting chips, how coolant plays a role in chip evacuation, or looking at the size and shape of chips, chip formation says a lot about the application being run. Knowing what different chip details indicate allows machinists to better manage chip formation, make adjustments, and prevent tool failure. Better chip formation means a more successful application.

Chip shape, size

A key indicator of a good chip is the shape. The preferred outcome for any application is chips shaped as sixes and nines or a single conical shape. These small, manageable chips are essential for efficient, predictable drilling, but it’s important to know what other shapes and sizes can indicate. For example, a straight, flat chip is a result of elasticity. If the chip is a continuous ribbon, there are likely many adjustments needed to achieve ideal chip formation.

Chip size impacts evacuation and there are two factors impacting the size of chips in drilling tools: chip breakers, also known as chip splitters, and lip geometry. Chip breakers thin chip width for easier evacuation – the wider the chip, the easier it rolls onto itself and breaks. Lip geometry acts as a mechanical chip breaker, fracturing a chip by curling it on top of itself or impacting chip forming with the backside of the lip radius. Although harder materials will curl a chip on top of itself creating chip fracture, gummier materials often skip over the lip radius, fracturing only after impacting the back of it. Still, the purpose of the combined chip breakers and positive lip geometry is to break off the chip so it’s narrow enough to easily evacuate.

Chip fracturing can also occur naturally from the velocity differential between the outside and inside of a chip, creating a cone-shaped chip that curls on itself and fractures. Larger diameter inserts have a higher velocity differential than smaller diameter inserts, so it’s easier to fracture chips – larger chip breaker spacing results in more chip fracturing. 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.

Chip thickness

Heavier feed rates form thicker chips while lighter feed rates form thinner chips. Chip thickness decides how the chip will fracture but this also depends on the material being machined. At the same time, changing speed impacts chip thickness – higher tool speed generates more heat in the cut, making the material more elastic. So, balancing speeds and feeds is necessary. With many materials, a thicker chip means there’s a greater chance of exceeding the elastic limit of the materials, increasing the likelihood of chip fracture while thinner chips are more elastic and farther away from the elastic limit needed to fracture the chip.

Soft, gummy materials such as soft carbon steels, 300 series stainless steel, or pure titanium have such a high elastic limit that increasing chip thickness has a negative effect on chip formation. These materials require specific lip geometries to potentially create an acceptable chip. However, to better understand chip thickness it’s important to look at the chip deformation ratio of materials – the ratio of deformed chip thickness over the undeformed chip thickness (feed rate). For most steels, this ratio is typically 2:1 to 3:1; however, it can be as high as 5:1 to 10:1 for those soft, gummy materials. This measurement is an indicator of chip form and elasticity in the material being cut, and chip formation is more difficult with higher deformation.

 

Pay attention to any changes in chip formation, it could be because of a built-up edge on the tool (BUE).
Photo credit: Allied Machine & Engineering

Coolant use

When it comes to coolant, through-tool coolant paired with the right drill geometry is critical for the best chip formation and evacuation. Additionally, changing coolant type, pressure, and volume influences thermal shocking of chips, which can change chip properties, making them more or less likely to break into manageable segments. For example, coolants can decrease material elasticity due to strain hardening that occurs as coolant quickly cools hot, elastic chips, embrittling them to the point of fracture by a reduced elastic limit.

For chip evacuation, coolant pressure and volume are important. To evacuate a volume of chips, a set amount of kinetic energy is provided by coolant volume. Drilling can occur uninterrupted from the top of the hole to the bottom if enough coolant volume is available, which will be evident during the application with a steady load meter reading while drilling. An unsteady load meter reading will be detected when drilling into the hole with insufficient coolant volume. This doesn’t mean drilling with insufficient coolant isn’t possible, it demonstrates the drill must be altered to fit the environment.

In contrast, pressure is the force behind the coolant, providing a fixed volume through a given diameter. As pressure increases through a fixed orifice diameter, coolant volume will increase. When drilling small diameters, high pressure is needed for sufficient 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; it provides an upward force on the chip to aid in flushing chips through drill flutes and out the hole. Although flood coolant can be used alternatively to through-tool in short drilling applications under 2x diameter, it doesn’t promote good heat transfer in deeper holes and can push chips back into the hole, causing chip packing.

Through-tool coolant is also important when factoring in heat because it provides coolant at the cutting edge where it’s needed to cool the tool. When machining, 60% of heat generated in plastic deformation of material remains with the chip formed while 40% remains with the tool and workpiece. To have sufficient tool life, the heat staying with the tool must be removed by coolant. When more coolant pressure and volume goes through the tool, the cooler it will run, resulting in greater tool life and the tool can potentially be run faster.

Tool selection

Chip formation can also indicate if the best tool is being used; if it’s not meeting standards, a tool geometry change may be needed. The geometry of a cutting tool has a significant impact on the chip formed. Increases in rake angles can improve chip formation, yet this comes 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 – the angle formed by the pure plastic deformation of the workpiece material. Here, material starts deforming or chip forming in front of the cutting edge. For both material properties and running parameters, the angle varies; however, the goal should always be to make the shear plane angle more vertical – steeper shear planes result in better chip formation.

Chip thickness comes into play 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, forming a thicker chip. More rake angle means more shear angle, which means better chips, but balance is key as well. Really sharp cutting edges 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’s aggressive but not overly so.

Changes in chip formation

Pay attention to any changes in chip formation. If it’s 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 such as coolant or material. In new applications, it may be best to drill shallow test holes and look at the chips to make sure they’re small and segmented. Being conservative in the beginning with speeds and feeds could aid in better understanding chip formation and what adjustments are needed.

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 or wrap around the drill body, damaging the drill or causing drill failure. Lastly, poor chip formation impacts hole quality. If chips are dragging or packing in the flutes, there’ll be poor hole finish. Noticing any changes in chip formation is important for tool life, hole quality, and 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’s necessary to examine chip size, shape, and thickness, it’s just as important to know how coolant, tool selection, and changes in chip formation tie into the application. Look at the chips created and break it down chip by chip – both proper chip formation and chip evacuation are required for successful high-production drilling.

Allied Machine & Engineering