Coolant boosts productivity on CNC turning centers. (Coolants/Lubricants/Filters).
Quick to realize the benefits of two new universal turning centers largely dedicated to produce hydraulic line fittings, Iberia Machine, Inc. (Iberia, OH) also recognized that the high-caliber machine tools would not be the only new ingredient in its quest for quality, volume, and productivity.
For Iberia Machine, a general machine job shop with the tools and experience to handle most types of metal cutting fabrication, producing fittings and connections used in hydraulic lines on trucks, farm machinery, and other heavy equipment had developed into an important aspect of the company's overall operations. Two new fully-automatic, dual-spindle Eurotech Elite machines figured prominently in the company's plans to meet growing demand from customers in the U.S. and abroad.
The parts quality and delivery responsiveness achieved with the flexible and powerful machines pleased customers. Seeking to fully capitalize on the machines' potential, Iberia Machine turned its attention to the cutting tools. Following an evaluation aimed at arriving the ideal solutions for the 12 cutting stations on each machine producing the fittings, the company settled on high-end carbide tools from companies like Sandvik Coromant, Manchester, and Carboloy.
Wanted: Higher Productivity
But productivity seemed weak. "We were bothered by not being able to run at the recommended levels for feeds and speeds," says Rodney Whited, president.
Most operations were set 10% to 15% below conventional machining guidelines in order to achieve part specifications and tool lives compatible with realistic operating costs. And even at these settings, tool wear, scrap rates, and other factors limited the opportunities for highly-desirable fully automatic operation of the machines.
Operators speculated that the inherent nature of the applications somehow restricted cutting performance, given the high quality of the machinery and the cutting tools. But they were also concerned about erratic coolant behavior.
Performance limitations all stemmed from tool wear, notes Whited. If the coolant frequently breaks down, tool wear increases, finishes suffer, uptime is reduced, and yield --the number of parts a tool produces--is definitely lowered. You can still make good parts, but during the time spent changing tools you are not producing them.
"We wanted to be more productive," Whited says. "And we wanted the kinds of margins on surfaces and tolerances that would allow a higher level of unattended operation. If the coolant had something to do with this, we needed to deal with it."
In collaboration with Blaser Swisslube (Goshen, NY), an international supplier of oils and coolants for all metal removal operations, and factory-trained fluid specialists from its distributor, Prexta Tool, Inc. (Akron, OH), Iberia Machine initiated what turned into a two-week coolant evaluation program and dedicated one of the two CNCs to the project.
The project team immediately made note of the impact of emulsion stability on cutting performance, but also the significance of a coolant's inherent level of lubricity. Tool wear is a consequence of excessive heat generation from friction. Given the nature of the parts material and the different CNCs cutting operations--notably the operations involving the more expensive tools, such as drilling, threading, and cutting--Blaser Swisslube recommended a coolant providing strong lubricating qualities. The new product would allow higher rates of metal removal.
The choice, Blaser's Vasco l000 [R], represented a strategic change in fluid type for Iberia. It was a change from a chemical synthetic product to a water miscible oil emulsion. And, instead of a formulation relying on a conventional mineral-oil-base, the new coolant would use vegetable oil as the base carrier for all other ingredients; a type of oil that is commonly used as a polar additive in mineral-oil-based coolants to enhance their lubricity.
Vegetable oil, because of its chemical structure, is exceptionally lubricious in the lower machining temperature range. The molecular structure and the molecules' great affinity for metal surfaces produce a lubricating film with the capacity to absorb very high pressures between the cutting tool and the workpiece.
The nature of the emulsifier used also enhances coolant attributes such as washing action, stability ,and tramp oil rejection. The stability of the emulsion is not affected by water hardness and displays no tendency to foam in soft and hard water.
Just as during normal production, different types and sizes of hydraulic fittings were produced during the evaluation. The settings of individual operations were increased from their original levels while the impact on the surfaces, dimensional stability, and tool life was monitored.
Automatic parts counters, integral to the CNCs machining management system, provided accurate records on tool life. Coolant properties, such as pH and emulsion concentration, were checked twice daily. The sump was continuously monitored for signs of chemical, physical, or biological instability. Make-up charges to compensate for drag-out on chips and parts were monitored.
Turning, Drilling Improve
The various models of hydraulic line fittings measure 1.5" to 6.0' in length, with outside diameters ranging from 3/8" to 1.25'. The ends are machined with either internal or external thread, or with external barb ridges on the models intended for crimp fit connections to hydraulic hoses.
All fittings are produced in 300-series stainless steel, for the most part #316, a tough material to machine, especially in the smaller part sizes. Work specifications typically call for 100-micro-inch surface finishes and dimensional tolerances of 0.0002".
The CNCs two spindles are set up to complete the machining operations required at either end of the part. After feeding bar stock at its first station, the primary spindle performs rough and finish turning at separate stations, followed by grooving, threading and, lastly, lateral drilling operations. The secondary spindle first cuts off the half-finished workpiece, and then duplicates the operations of the primary spindle on the second half of the part.
A key objective for the coolant evaluation was to have all cutting stations operate at machining guidelines, while at least maintaining parts quality and tool life. The higher settings were achieved almost immediately, and invariably accompanied by improvements in other areas. The improved cutting performance and coolant behavior remained stable, and, after a two-week period, the second machine was converted to the new coolant. In the ensuing year, the experience on both machines--still operating with their original coolant charges--has been consistent with the results seen during the evaluation period.
The two turning operations, for example; are typical of the progress seen not only in tool life since cutting rates were increased, but also in parts quality and consistency.
Overheating and rapid tool wear previously made it difficult to hold dimensions and surface finish, even with feedstock closely sized to machined dimensions. The change of coolant increased tool life by 40% in the rough turning operations, which removes 0.0100"--0.0125' in one pass. In the finish turn, set to remove just enough stock to hold size and provide for a good surface--0.0025"--0.0030" in one pass--tool life increased by 30%, along with a 20% improvement in the surface finish. And in both operations, the consistency in the parts quality improved appreciably as well. "It makes sense," notes Whited. "When the tools stay sharp, you will be more consistent with finishes and dimensional tolerances.
Operating consistency has been particularly noteworthy on the grooving stations. This is where the crimp fitting barbs are created in the exterior surfaces of the ends of some fittings. Tools would break frequently, seemingly for no apparent reason, or quickly wear out. A break might suddenly occur after just 300 parts, or the tool would stop holding finishes after 500. The lack of consistency, and the short runs, made it virtually impossible to operate the grooving stations unattended. Now, the machine operators count on at least 1100 pieces per corner of the cutting tool; on balance, a 30% to 40% improvement in tool life.
In the threading operations, the direct cost of the tools consumed had always been a concern. Machinists know that number 316 stainless steel--the material Iberia uses for most of the fittings produced--is a work hardening material. The more it is heated, the harder it becomes, and, as a result, tougher to machine; a behavior that exacerbates the threading operation's natural tendency to generate large amount of heat.
True to form, the internal and external threading operations had always performed erratically. Tools were burning up, resulting in excessive wear and short, unpredictable tool life. Now, after the feed and speed settings on this operation were raised by 20%, the tool life is consistently 40% to 45% higher than before the coolant change. Tool life in the cut-off operation is up 30% to 40%, a valuable contribution considering the high cost per corner of these single-corner tools.
The most impressive productivity improvement, however, materialized in an area where none had been anticipated: drilling. Holes with a depth of five to six times the drill bit diameter are drilled from each end of the part to create a continuous cavity, using drill bits ranging in size from 1/8" to 1.25" diameter.
The operation is demanding, considering depth, drill bit sizes, and the material. Still, this is the one operation that all along had been performed at recommended speeds and feeds. It had consistently yielded a tool life of approximately 1000 parts between tool changes, which was compatible with the other, slower, operations, and considered acceptable.
"The change was astonishing," notes Whited. Tool life of the smallest-diameter drill bit, for example, increased from around 1100 parts to over 7000 between drill bit changes--more than a 500% increase.
On balance, taking into account the performance of all work stations in machining all of the types of hydraulic connectors produced by Iberia Machine, the coolant change improved the productivity of the CNC turning centers in several important ways:
* Now, with all tools operating at the guidelines for feeds and speeds, the average cycle time to produce complete parts is 10% shorter than before.
* Tool lives at the higher production rates are 40% longer.
* Scrap rates are down significantly, and parts quality--always the paramount concern--improved; again most notably in the drilling operation. A 70 micro inch finish is consistently achieved (part specs call for 100) and, in some sizes, a 63 micro inch surface specification is met without subsequent reaming to polish the drilled finish.
* The two machines are now operated with an uptime ratio of 98%, up from 90%, an increase attributed to factors such as fewer tool changes and more consistent cutting performance.
Coolant properties remain stable
The two CNC turning centers are operated with individual 60-gallon sump systems, replete with continuously running sump separators, pumping coolant at 400 psi. The lube oil for the machines is compatible with the coolant; it does not mix with the coolant, but quickly gravitates to the top of the sump where it readily is skimmed off.
Operator skin disorders have cleared up, and rust staining of unwashed parts is no longer an issue. Foaming, especially when drilling long holes in the larger parts, used to be persistent and restrict unattended operation. No such tendency has been seen. Coolant consumption is down about 50%.
Satisfied that the two principal objectives of the coolant evaluation have been met--achieving higher productivity and lower tool cost--Iberia Machine has come to value another outcome even more: predictability.
"We have a consistent operation now," Whited comments. "We produce parts that consistently exceed requirements on surfaces and tolerances by wide margins. And that means more to me than pushing the machines harder." Blaser Swisslube
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|Publication:||Modern Applications News|
|Date:||Apr 1, 2003|
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