Turning the tough ones
The turning of stainless steels, superalloys and other “difficult” materials grows less difficult all the time, as a result of equipment improvements and refinements in High Pressure Coolant (HPC) machining practices. In many such applications with flood or low pressure coolant and advanced tooling, throughput gains of 20% or more are reported, together with a doubling of edge life. In addition, when you turn the pressure up to the 70-300 bar range, you increase potential gains by another big step. In fact, with proper HPC practices and tooling, you can routinely expect order-of magnitude gains in edge life and/or hourly output.
First let’s take a brief look at some recent key improvements for turning the exotic superalloys, stainless steels and titanium materials.
A drop-in in retooling of a rough turning operation on 316L stainless steel extended tool life from 4 to nearly 10 parts per edge, despite running the part 20% faster. The insert which provided the advantage was coated with the new ISCAR 6015 grade. The job ran with conventional flood coolant.
Likewise, turning of hard superalloys has been improved with whisker-reinforced ceramic inserts (such as the new ISCAR IW7) that performs well even in roughing applications. Indeed, these inserts have enabled eight- to ten-fold improvements in roughing and semi-finishing removal rates compared with carbide inserts. The new ceramic insert has withstood cutting speeds high enough to heat and soften hard materials such as Stellite, and performed reliably enough to permit unattended turning of superalloys.
Controlling Heat, Managing Chips
Gains stem mainly from better control of heat and chip formation in these gummy, long-chipping nickel- and cobalt-based alloys, made possible through advanced insert design. Even without HPC, the driving strategy in insert design for these materials is to provide a very sharp edge, slippery, heat resistant coatings and aggressive chipbreakers. That combination quickly breaks up the chip and propels it away from the insert and cutting zone before it can adhere to the cutting edge, overheat the insert or clutter up the tool-workpiece interface with chips that create recutting conditions.
Remember, chips can be much harder and more brittle than the base metal, compounding the damage caused by recutting. With HPC in the picture, coolant (boiling point 350C.) remains in the liquid phase, thereby maintaining its lubricity, cooling power and chip-flushing capacity. Moreover, the flow rate under true HPC conditions is high enough to create a “hydraulic wedge” in the cutting zone, significantly reducing friction and all its consequences.
HPC
While we have all heard about the promise of high pressure coolant (HPC) machining, we may have shied away from it because of the added equipment cost and uncertainties of an “untried” technology. Let’s take a look at these issues:
Untried? The technique is well proven among the aerospace, powergen and turbomachinery industries. On one hand, experienced practitioners report two- and three-fold gains in machining rate with no loss in edge life. Others mainly concerned about edge life, report up to sevenfold improvements at equal removal rates. These are actual results on ID and OD work on titanium and Inconel turbomachinery parts, titanium airframe parts and a variety of stainless steel components.
Added equipment cost and availability? This was true in the early years, but not now. When first introduced in the ‘50s, there were no spindles fast enough or coolant pumps powerful enough to make the process workable on the shop floor. However, today most machine tool providers routinely offer optional high speed spindles and high pressure pumps. High Speed Machining is more the norm than the exception in industries that must contend with stainless steel and superalloys.
HPC-Ready Tooling
Now “HPC-ready” tooling has become more widely available. While providing the geometries and physical properties tailored to particular difficult-to-machine materials, true HPC tooling (such as the ISCAR JETHP line) also features the means to deliver coolant through the tool and discharges it in a tight, laserlike stream, aimed directly into the cutting and secondary shear zones. This is critical. There it cools, lubricates, creates the hydraulic wedge effect and quenches the chips so they break up into compact, manageable curls. On its passage from reservoir to cutting zone, the coolant also lowers the temperature of tool and insert. True HPC tooling is specifically designed for 70-300 bar pressures.
HPC tools are indispensable to truly optimal performance; standard through tool coolant systems simply are not adequate. The main differences are twofold: (1) where and how precisely they pinpoint the stream as it leaves the tool and (2) orifice diameter to deliver the correct pressure at the exit point. It is like the difference between true power washing and spraying with a garden hose.
Benefiting from Experience
Experienced HPC practitioners have learned enough about the process to provide tips for newcomers. Here are a few to help you get started on the right foot:
Use carbide tooling. Ceramic and CBN tools do not deliver the same degree of improvement in the HPC realm.
Direct the coolant through the tool. Don’t take a “flood coolant approach” with a HPC coolant stream.You’ll just make a mess, create an employee hazard and miss out on the main benefits of the practice in the first place.
If you need more cooling power (i.e. the coolant is vaporizing or chips aren’t flushing well enough), turn up the flow rate, not the pressure. This way is far more cost effective.
How to estimate coolant requirements? A good rule of thumb is 0.5 gpm/horsepower. For example, a cut requiring 10 hp will need 5 gpm to achieve the high pressure effect.
If you are still having difficulty turning those materials, look around. Better answers, including today’s HPC machining, are more readily available for the asking. Your competitor may have found them already! CM
Article courtesy of ISCAR Canada.
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