Cool groove: Turning Performance

Grooving most any material creates friction, generating heat that directly and negatively impacts tool life. The benefits of properly applying water- or oil-based coolants that reduce friction are easy to comprehend.

Coolant manufacturers engineer their fluids to reduce friction through lubrication and heat dissipation. Less heat when grooving extends tool life, improves surface finishes and increases the potential material-removal rate. Metalworking fluids also help transport chips from the cutting zone and can prevent corrosion in the machine.

Fluid Basics

The selection of a suitable fluid requires a basic understanding of the coolant formulation and the application in which it will be applied.

Oil-based lubricants, with or without additives, are utilized to achieve a fine surface quality on the final component. These petroleum or mineral oil products offer excellent lubricity, help prevent rust and are fairly easy to maintain.

All images courtesy Horn USA

Water-soluble coolants or emulsions, free of mineral oil, are commonly used when cooling takes priority over surface finish. These oil-in-water, “soapy” solutions lubricate and can contain special additives for protection against rust.

Synthetic coolants contain no petroleum or mineral oils and are engineered to efficiently transfer heat. These fluids are considered nonflammable, microbial resistant and contain rust inhibitors.

Going Dry

Cooling methods that do not have lubricating effects include cryogenic gases, compressed air and dry machining. Cryogenic cooling involves the use of liquid gases, such as nitrogen, at temperatures as low as -321° F (-196° C). Special delivery systems, such as insulated tanks, vacuum-insulated hoses and tools specially designed to cope with such harsh temperatures, are needed. Compared to liquid nitrogen, CO2 compressed into liquid form requires fewer resources to maintain and cope with temperatures as low as -108° F (-78° C). Both techniques provide unique advantages when grooving titanium and nickel-based alloys. These materials are heat resistant and the machining process creates high thermal loads at the cutting edge. Without proper coatings or cooling methods for heat transfer, the excessive heat leads to rapid edge wear.

Hard turning with PCBN tools is possible under dry conditions.

Efforts to reduce the cost of machining processes have led to innovations in dry machining methods. Several years ago, the European automotive industry calculated that 14 to 16 percent of machining costs could be attributed to using, maintaining and disposing of cutting fluids. New advances in tool coatings enable manufacturers to utilize the dry cutting of investment castings, as well as other materials. When machining fiber-reinforced plastics, ceramics, graphite and carbide in the green (unsintered) state, compressed air is used to blow dust particles from the cutting area. The dust is then collected by a vacuum system for recycling or disposal.

In contrast, flood cooling directs large volumes of coolant toward the working area. Providing such large volumes of fluid is effective at cooling, lubricating and removing large chips. This is generally sufficient provided there are few obstructions between the workpiece and cutting tool.

Horn says its new S100 insert for grooving and parting-off precisely delivers lubricating and cooling fluids. The “.3V” chipbreaker geometry and wear-resistant TiAlN coating for machining stainless steels is an expansion of the existing S100 system. The coolant port works directly at the cutting zone and creates a focused coolant jet, reducing the chance of built-up edge and assisting in chip formation.

However, using high-pressure coolant provides significant advantages over flooding the workpiece with large volumes of coolant. With as little as 73 psi (5 bar) of pressure, tool wear begins to decrease compared to using flood cooling alone, which increases productive machining time. Built-up edge begins to decrease when the pressure exceeds 290 psi (20 bar), decreasing machine downtime. In addition, breaking chips in long-chipping materials, such as high-strength nickel-base alloys, is possible above 2,175 psi (150 bar).