Fighting Pressure With Pressure

High-pressure coolant systems are providing improved part quality, increased tool life and other benefits for this precision turning shop.

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Today's shops are feeling pressure from many different directions—local competitors, overseas suppliers, customers who demand better parts in less time at lower prices, the current fall-off in orders . . . the list is formidable. However, Swiss screw machine shop Starro Precision Products, Inc. (Elgin, Illinois) is fighting back by applying a little pressure of its own—coolant pressure, that is. Specifically, the firm has replaced the conventional coolant delivery systems on some of its CNC turn-mill machines and CNC Swiss screw machines with high-pressure coolant delivery systems that, by more effectively cooling the cutting zone, are providing faster machining rates, better parts, greatly improved tool life and other benefits that are enhancing the company's competitiveness and bringing in more business.

Starro Precision views itself as a precision turning firm that is competing against the world. The firm does not simply "rent time on its machines," Starro vice president Lee Dwyer insists. Instead, it prides itself on offering customers "engineered solutions" for the production of demanding parts, that is, it determines the most efficient way to produce challenging parts at competitive prices.

The solution usually begins with producing the part complete in one setup on one of the shop's CNC turn-mill machines or Swiss CNC screw machines.

Starro has invested about $3 million in such machine tools over the last 3 years to achieve a level of cutting-edge technology that would make it more attractive to customers and more difficult for competitors to match.

A System Approach

The solution involves more than just the machines, however. Like many leading-edge metalworking firms, Starro knows that successful machining requires a system approach, so it takes into consideration the type of coolant used, the quality of the raw material, the cutting tools used and other aspects of the process. Yet, although the firm was paying attention to the details, something still seemed to be missing.

"We were realizing only a fraction of the potential that modern technology seemed to offer," notes Mr. Dwyer. "We were having trouble getting rid of the heat generated in the machining process. It limited the speeds at which we could machine and affected cutting tool life. We were stopping constantly to index or replace inserts, which cut into our machine uptime."

Mr. Dwyer wanted a more effective coolant delivery system. He was aware that precisely directed, high-pressure coolant systems used in such processes as gun drilling and creep feed grinding permitted high metal removal rates while keeping the work zone cool, and he wanted similar cooling capability for his turning machines. He researched companies that offered such systems and decided that ChipBlaster, Inc., Meadville, Pennsylvania, offered the equipment he needed for his Mori Seiki CNC four-axis lathes and Star CNC Swiss screw machines. He was sold not only on the breadth of ChipBlaster's product line, but also on the amount of engineering data available from the firm and its assurances of all of the cooperation that would be needed to make the installations successful. In Starro's case, the cooperation included working closely with the suppliers of the CNC machines to interface the coolant systems to the machines and designing coolant tools for them.

Starro began by installing a ChipBlaster EV 2000 high-pressure coolant system on one of its Mori Seiki Model ZL-200 four-axis CNC lathes. The compact (56-inch wide by 48 ½-inch high by 31-inch deep) freestanding unit continuously extracts coolant from the sump of the lathe, filters it and returns it to the machine tool under very high (300 to 2,000 psi) pressure and at flow rates up to 13 gallons per minute. The coolant is distributed through the lathe's existing passages to the individual coolant tools and toolholders, each of which directs a powerful, very focused jet of coolant to the exact spot where the tool meets the workpiece.

By contrast, low-pressure coolant systems typically use adjustable plastic or metal tubing that the operator manually adjusts to direct the coolant stream. The coolant tubes are frequently nudged aside, however, when the operator is changing an insert, clearing chips or doing other chores in the tooling area, and it sometimes takes awhile before the operator notices the problem and redirects the coolant flow to the workpiece. It is not unusual for machining to proceed without benefit of coolant for long intervals; one coolant producer estimates that 40 percent of the time, the coolant hits neither the tool nor the part.

ChipBlaster adds that even when the coolant from a low-pressure coolant system does hit the tool, lack of proper directional control greatly reduces its effectiveness. Also, so much heat is produced that the coolant boils away before it can reach the tool-workpiece interface. The superheated steam creates a barrier that the low-pressure coolant cannot penetrate. Effective cooling does not occur, and little real lubrication is provided. Also, because the low-pressure coolant stream is frequently not powerful enough to flush the chips from the cut, they can clog the cutting zone and create problems.

A Real Blast

By contrast, the high-pressure system delivers the right combination of coolant pressure and volume to prevent the vapor barrier from forming. As a result, the coolant impacts the cutting zone with such force that the chips and the heat generated in cutting them are flushed away, leaving the workpiece much cooler than would be the case with a conventional coolant system.

A high-pressure coolant system keeps the temperature low, changing the way metal is cut. Studies conducted by coolant system suppliers indicate that tools last longer, chips cannot weld to the tool and each other, and surface speeds can be increased by a minimum of 30 percent. High pressure coolant also provides lubricity by blasting lubricating fluid between the chip and the inserts at hundreds of miles per hour. Combined with the much lower temperature, the increased lubricity often results in greatly improved surface finishes.

The high-pressure coolant system also encourages the formation of short, more uniform chips, which flow from the cutting zone more readily than long, stringy chips that can wrap around the cutting tool, toolholder, workpiece and so forth, to create problems. One of the reasons for the formation of the shorter chips is said to be that the coolant reaches the chip-tool interface so effectively that it produces a shorter shear zone and correspondingly smaller chips. Because smaller, more uniform chips are produced, cratering, plastic deformation, spalling and other damage commonly encountered with cutting tools is greatly reduced. Premature failure of tools is avoided, and wear becomes the (more predictable) basis for tool changes.

The First Unit

One of the reasons that Starro purchased its first high-pressure coolant system was a job involving a 2 ½-inch diameter by 0.9-inch long steel, compressor piston with a demanding set of specifications. All ODs and IDs must be held to tenths, and surface finish is critical. The part is produced from 2 ½-inch-diameter bar on one of the shop's Mori Seiki Model ZL-200 four-axis CNC lathes.

"We had been meeting most of our targets for the piston with the conventional coolant system," Mr. Dwyer recalls. "However, we were going through drills and inserts at a horrendous rate, and the need to frequently change tools was preventing us from improving production of the part. We needed to reduce our insert consumption, run faster to cut our cycle time for the part and increase our uptime on the machine.

"We noticed a dramatic improvement as soon as we installed the high-pressure coolant system," Mr. Dwyer continues. "Before, the parts were hot as they came off the machine. Now, they were cool. Part finish improved dramatically. And even though we increased our cutting speeds 30 to 40 percent, tool life doubled and in some cases tripled. The job changed from being one that we had to watch constantly to one that became so predictable that we could let it run unattended at times. We were so impressed by the improvements in the machining of the piston gained by switching to the high-pressure coolant system that we immediately began identifying other demanding jobs in the house that would similarly benefit from the process."

Months after Starro installed its first high-pressure coolant system, it installed a second unit, a Model J4-2000, this time on one of the shop's Star SA-12 CNC Swiss screw machines (photo on page 37). The system has a flow rate of 5 gallons per minute at pressures up to 1,000 psi. The coolant supply line from the free-standing unit branches into as many as eight separate lines that connect to individual tools on the screw machine's gang tool post and four-spindle attachment (photo at left). Star and ChipBlaster collaborated on an interface that permits coolant flow and pressure levels to be individually adjusted for each coolant tool.

One of the first jobs to run on the high-pressure-coolant-system-equipped Star CNC Swiss screw machine was a precision shaft for an automotive component. The 4 ½-inch long part is machined from 1-inch diameter, 303 stainless steel bar. Main features of the part are the demanding dimensional tolerance and surface finish requirements for its several ODs—and a 0.120-inch diameter hole drilled through its center to a depth of 4 inches.

Normally, such a part would be rough turned and ground in separate operations to satisfy the stringent roundness and surface finish requirements. Starro had to find a way to make the part better and faster. The scrap rate for the process had to be kept so low that the customer would not be able to cost justify finish-grinding the parts.

Starro planned the job to be produced in a single setup on the Star machine equipped with the high-pressure coolant system. A solid carbide coolant drill is used to drill the 4-inch long, 0.120-inch diameter hole while the ODs are turned simultaneously using multiple tools in the cut. "We could have gun drilled the center hole," Mr. Dwyer explains, "but we would not have achieved the feed rates that the carbide drill achieves with the high-pressure coolant system—a factor that helped us reduce the cycle time for the part."

Hard Jobs Only

Starro tends to reserve its CNC machines equipped with high-pressure coolant systems for more demanding jobs, such as an aircraft valve component that the company machines from 1-inch diameter, 303 stainless steel bar. The part has a number of OD features, including grooves that must be held to tenths tolerances. As is the case with the automotive shaft previously described, the valve component is made on a Star CNC Swiss screw machine equipped with a high-pressure coolant system.

"With the high-pressure coolant system, we're running the job 20 to 30 percent faster than before," Mr. Dwyer reports. "At the same time, tool life for the job doubled, resulting in a savings of about $1,200 per month. Without the savings made possible by the high-pressure coolant system, we wouldn't have brought anything to the table that other vendors couldn't offer."

Starro purchased its first high-pressure coolant system about 17 months ago. Today the firm has four units, two on Mori Seiki four-axis CNC lathes and two on Star CNC Swiss screw machines, and it has ordered another two units. Mr. Dwyer expects that about 30 percent of the shop's machines will eventually be equipped with such systems to handle the more difficult jobs characterized by tenths tolerances, demanding surface finish requirements, challenging geometry, tough materials, extra-deep holes, special need for chip control, and so on.

Mr. Dwyer views the high-pressure coolant system as an investment that is helping his shop reach a higher level of competence. "Shops buy expensive machines and expensive cutting tools but tend to ignore the proper use of coolant," he observes. "Conventional coolant systems may wash the chips away, but they don't effectively cool the tool. You're dealing with 1,500° F temperatures at the tool tip, which can lead to cratering and chip welding, which lead to premature tool failure. With high-pressure coolant systems, the tool stays cool, and I can machine much faster."

High-pressure coolant systems enable Starro to handle an increasing percentage of jobs that are beyond the competence of many shops, and these jobs are having a positive effect on the bottom line despite the current business downturn. "There is a direct correlation between the new technology that we implement and new sales," Mr. Dwyer notes. "At the same time, the older technology is experiencing declining sales because most of that work is going overseas. Today, to produce demanding parts, you need predictable, controllable processes. To compete globally, you must have the best process. For us, that includes high-pressure coolant delivery."

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