Coolant Usage in Holemaking

For optimized performance of solid-carbide drills, coolant must flow to the tool’s cutting edge

January 24, 2012

Proper cooling techniques are important in all holemaking operations.

If not properly cooled, drills heat up rapidly. This is especially true with solid-carbide tools, which are able to reach higher speeds than even high-speed-steel (HSS) drills, generating more heat.

The first problem that likely will occur without proper tool cooling is increased wear at the outer corners of the cutting edge, where the highest cutting speed and highest concentration of heat occur. This affects tool life negatively and creates an environment in which chips become larger and more unmanageable. This can then lead to a need for more torque and horsepower to drill the hole and eventually to a catastrophic failure of the tool.

The ideal cooling method in most solid-carbide drilling operations is through-tool coolant delivery. However, not every shop can afford such machine tools with the capability to deliver coolant in this method.

According to Walter Tools Product Manager Pat Nehls, although these tools may be more expensive, they produce a more secure application by delivering coolant to the cutting edge better, cooling the area and aiding in chip evacuation.

"Coolant delivery to the cutting edge is extremely important when drilling with solid-carbide drills," said Nehls. "When using carbide tools you can typically run at higher cutting speeds, creating more friction. This means more heat is generated, and more challenges arise at the cutting edge/workpiece interface that can negatively affect tool life and hole quality. It's important to minimize the heat at this location."

Although many shops don't have the ability to use coolant-through tooling because of the machine's limits, peck cycles, proper coolant direction, and tool coatings can all help.

Directing coolant toward the flutes ensures that as much fluid gets to the cutting edge as possible without impeding chip flow out of the hole. When a drill is used without internal coolant flow, at least one coolant jet should be positioned as parallel as possible to the drill to achieve the best results. Poor chip evacuation can result if the cutting fluid nozzle is not properly directed onto the tool's flute. This can lead to undersized holes, faster tool wear, and even tool breakage.

"If you look at a vertical application, the action of the spiral-flute tool acts as an auger to pull the chips out of the hole, and the coolant often pushes them back into the hole," said Nehls. "You run into a dilemma about how deep you can go. If you are in a horizontal application, it may be easier to get the chips out, but it's more difficult to get the coolant into the hole."

Typically, when hole depths move past 3xD, solid-carbide tools with an internal coolant supply are used. However, recently released solid tools can perform 5xD and 8xD drilling without the need for internal coolant.

A good rule of thumb, however, is that when you are drilling deeper than 6xD, a peck drilling cycle should be implemented.

Peck Drilling

Usually drilling a hole can be done in a single operation. But if deep holes are drilled (more than 6xD) using an external fluid supply, a peck drilling cycle might be the only option. Peck drilling is a multistage drilling operation that is designed to clear the chips and get coolant down to the bottom of the hole, where it is needed most.

"Our recommendation is to go to 6xD in the first drilling operation and then peck drill every 1xD until the bottom of the hole is reached," said Nehls. "The main issue is to get the chips out of the hole, so that is why you are retracting the drill between pecks. You also get coolant to the bottom of the hole and cool the tool. As the holes become deeper, we need to peck more often."

Tool coatings such as aluminum oxide protect the tool from heat. Long tool life can be achieved, even at a higher temperature, because of this protection. Coatings also add lubricity to the process rather than relying on the coolant to be the single source of lubricity.

Under Pressure

Using the proper pressure and volume is important in drilling operations. High-pressure external coolant can even have a negative effect on the overall process. The drill can be pushed sideways, reducing the drill flutes' ability to evacuate the chips from the hole because of the volume and pressure of coolant going into the hole.

"In cases like this, we recommend using a higher volume of coolant, but a lower pressure," said Nehls.

High-pressure coolant, either through the tool or through an external line, should be considered to increase tool life and production.

Also, small-diameter drills often need more coolant flow than larger drills, because the fluid volume that is able to travel down the small flutes of these tools is lower. Cutting fluid pressure and volume are both critical for smooth chip evacuation – particularly at high speeds. And, to run at the same cutting speed as larger drills, small drills require much higher RPMs.

"When you have high RPM and high feed per tooth, your inches per minute rate is much faster," said Nehls. "Unfortunately, at the same time, when a problem occurs, it happens in the blink of an eye."

Coolant Maintenance

Coolant volume, pressure, and direction are important in drilling operations, but if the coolant is not properly maintained, the full benefits cannot be realized.

"Work with your coolant supplier to optimize the concentration level and additives in the coolant," said Nehls. "Tramp oils need to be reduced as well."

Proper coolant maintenance is more than simply topping up the levels with water, a common mistake that reduces the concentration level of the coolant, reducing its lubricity and heat-reducing properties.

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