March 2010 / Volume 62 / Issue 3|
By Alan Richter
Courtesy of Heidenhain Applications, accuracy issues and infrastructure requirements for large-part machine tools.
The trend in precision metal part production may well be toward miniaturization, but the demand for massive components perseveres.
With big machines, there is always business even during downturns because there are so few companies that can invest in this kind of equipment, according to a spokesperson for Fairbanks Morse Engine. The Beloit, Wis., company machines engine blocks, including ones for Navy ships, using its Ingersoll, Puma and Cincinnati Gilbert machine tools. Fairbanks Morse also produces engine blocks for stationary applications.
The spokesperson noted that a big engine block, which is typically made of GGG40 cast iron, weighs up to 17 tons, measures about 100 " tall × 6 ' wide × 33 ' and takes 13 to 18 months to complete from fabrication to machining, assembly and testing.Tight Tolerances
Just because the parts are massive doesn’t mean the tolerance requirements are loose. Large-engine crankshafts are often from 25 ' to 30 ' long and have tolerances of 0.0018 " to 0.0019 ", according to Fairbanks Morse. When evaluating a large machine for purchase, the builder is supplied with a tolerance specification and a blueprint, and the builder must be capable of making that block. Otherwise, an end user won’t buy its equipment.
Votaw Precision Technologies is another large-part manufacturer that focuses on maintaining accuracy. The company targets tolerances of 0.0004 " and tighter when producing parts even on its biggest machines, according to Richard Roy, maintenance manager. Those include a 5-axis Cincinnati and a 5-axis SNK, which has travels of 700 " in the X-axis, 16 ' in Y and 6 ' in Z, as well as a rotary table for A- and B-axis movements. “This knucklehead will go to a horizontal position and rotate a full 360°,” he said.
The Santa Fe Springs, Calif., job shop produces an array of aerospace parts, such as ones for Raptor fighter planes, the Mars Land Rover, Aries 1 rockets and various satellites. Workpiece materials include aluminum, titanium, steel and magnesium. The starting weight of typical workpieces for the SNK is 1,000 to 20,000 lbs., of which up to 70 percent is turned into chips. “We have overhead cranes in our shop to handle parts that weigh up to 30 tons,” Roy said, adding that cycle time for the larger work ranges from 1 to 2 days to 2 weeks.
Roy noted that the lead time for obtaining a machine can be long but the company won’t sacrifice size or accuracy to obtain equipment quicker. “Our company’s priority is to obtain equipment that is able to perform to the highest standard in size, speed and accuracy the aerospace industry requires,” he said.
Kevin Nelson, vice president of operations, Mid-State Machine Products, Winslow, Maine, concurred that it takes patience to wait for a large-envelope machine tool, with 1 year being a common lead time. “For the extremely large units, lead times can extend outward from a year and a half to 2 years,” he said.
Mid-State Machine produces large components that weigh up to 10 tons for both the industrial gas turbine and wind energy markets. To manufacture these large-envelope components, MSM uses 2-axis vertical turning centers from O-M Ltd. and 4-axis, 30-hp Toshiba horizontal boring mills, with the largest boring mill having a 140 " X-axis travel, 90 " of Y travel, 70 " of Z travel and a 360,000-position rotary table. The shop also has a host of 5-axis Makino machining centers for its smaller-envelope workload.
“The volumetric tolerance on a machine the size of a Toshiba BP130R.22 can be as close as a couple of thousandths, which is sufficient for most applications,” Nelson said. “However, as conditions such as ambient temperature and part weight change, the skill of the machinist is needed to produce extremely tight tolerances that are outside of the standard volumetric tolerances of large-envelope machines.”
To maintain a high level of machine accuracy, Votaw checks its machines’ travels using laser calibration every 12 months, according to Roy. “If it’s off, we compensate to make it more accurate.”Upgrading Equipment
Although Votaw focuses on obtaining new high-accuracy machines, it did purchase a used Viper 5-axis vertical machining center from a company that was going out of business based on the machine’s capabilities and cost savings. “It depends on the market when you buy used machines, but you can save at least 50 percent,” Roy said.
After purchasing the VMC, which has a 15 ' X-axis travel, 8 ' Y-axis, 3 ' Z-axis and tilting A and B axes, Votaw realized it needed to tighten its tolerances to meet its customers’ requirements. “We ran this used machine on many projects and saw that it wasn’t as accurate as we expected and just wasn’t holding tolerances,” Roy said. “We had to repeatedly do time-intensive adjustments.”
Votaw did some testing on the VMC—its main measurement feedback system consisted of rotary encoders on the ballscrews—and found one axis was experiencing a cyclic mechanical error the company believed originated in the ballscrew and rotary encoders. That error was then transferred to one of the axes. “That was unacceptable,” he said. “Even our adjustments didn’t meet expectations.”
One of Votaw’s vendors indicated there was nothing that could be done and advised the job shop against doing a linear scale upgrade because the vendor thought it wouldn’t be effective. “And they sold linear scales!” Roy said.
As a result, Votaw turned to A Tech Authority, Chino, Calif. Votaw requested replacing the measurement systems on the X, Y and Z axes with Heidenhain linear scales, so A Tech Authority mounted two Heidenhain LS 100 series linear scales of varying lengths and one LB 382 scale and ran the necessary cables.
After the linear scale upgrade was completed, laser calibrations showed success. “The cyclic error disappeared,” Roy said, noting that the VMC was able to accurately produce parts for the 787 Dreamliner airplane and the F-35 Lightning II aircraft. Overall, the accuracy improved to ±0.0002 " from ±0.0005 " on the three axes.Thermal Compensation
Part accuracy can also be negatively impacted by ambient temperature variations, which can cause the machine and workpiece to expand and contract. At Votaw, most of the 240,000-sq.-ft. shop is in a controlled environment where the temperature is held at 68° F, ±2° F. “In this controlled environment, our machines are able to perform at their highest accuracy,” Roy said, adding that workpieces are “soaked” in the air to ensure they are at the same temperature before machining.
Because air conditioning the environment where Fairbanks Morse Engine machines its big blocks would be too costly, the company reports that it relies on machine tool builders to install the necessary equipment in the machines to keep components, such as headstocks, ways and hydraulic systems, at the proper temperature so tolerance doesn’t drift. Generally, there isn’t workpiece growth.
Fairbanks Morse also applies flood coolant when machining and runs the coolant through chillers to maintain a temperature from 72° to 74° F. That’s also the coolant temperature targeted in the company’s laboratories, which are air conditioned.
Mid-State Machine Products takes a different approach. There, everything is normalized to 68° F in accordance with industry standards, according to Nelson. The process used to normalize part geometry compares the part temperature to the temperature of the setting master and then makes the appropriate thermal compensation adjustments for the specific material being cut. Many of MSM’s work centers are capable of compensating for thermal expansion within the machine envelope itself, but there is not a method of automatically compensating for temperature variations within a part, he added.
“As a result, we routinely depend on our machinists to perform thermal compensation calculations on all critical features and make the necessary adjustments prior to creating the final part geometry,” Nelson said.Infrastructure Requirements
It’s not enough just to have monster machine tools to produce massive parts. A parts manufacturer also has to have the infrastructure to support the machines, move and maneuver the workpieces and transport the finished parts out of the shop.
One of the most critical elements when installing a large-envelope machine tool is the foundation. Each machine tool builder provides specific foundation requirements that become an integral part of the machine tool geometry and accuracies. The geometry and accuracies of large-envelope machine tools, which may weight 40 tons or more, can be negatively impacted if foundations are not built in accordance with the machine builder’s specification, Mid-State Machine’s Nelson noted.
He added that the end user needs to consider soil conditions under the foundation to determine soil density. “The amount of soil compaction the machine sits upon can significantly alter foundation requirements,” Nelson said. “The foundation design may need to be modified to compensate for poor soil conditions to ensure that proper support will be provided even under extreme loads.”
Meeting the machine builder’s foundation requirements so a machine achieves its advertised accuracy isn’t inexpensive. Foundations for large-envelope machines cost from $600,000 to $1 million, according to Fairbanks Morse. The engine block manufacturer outsources that work to companies that specialize in it.
Companies also need to invest in cranes to move workpieces, which usually aren’t machined complete in one fixturing. Fairbanks Morse has a 250-ton crane and two 85-ton cranes, which also lift fixtures and the machines themselves.
Nelson emphasized the need for worker safety when handling large workpieces. For something odd-shaped that the shop hasn’t handled before, Mid-State Machine will assign a team to brainstorm ideas about how to handle it safely.
Moving completed parts out of a facility also requires infrastructure. For example, Fairbanks Morse ships by rail, which requires a rail system on its campus and the capability to put rail cars inside its buildings to load them underneath the cranes. It sometimes takes two or three railroad cars to transport an engine, which can weigh up to 170 tons, because it has to be disassembled to lighten it, according to the company.
A company machining large parts also must have something that’s not as visible but as essential as the massive machines and infrastructure: a culture that nurtures continuous improvement. “It takes a customer-driven company that wants to constantly improve and expand their capabilities,” said Votaw’s Roy, “with a vision of the future and acquiring equipment that meets or exceeds our customers’ expectations.”
In addition, even highly skilled individuals need to band together to succeed at large-part machining. “There has to be a very open-minded team of people who work well together to understand potential issues that could arise with the work,” Nelson said. “Machinists have to able to set up and run the jobs after our engineering department has released them. We tap into the collective ‘brain trust’ of our skilled workforce to arrive at the best solutions, rather than depending on a single person to have all the answers on any of these jobs, as each one is unique.” CTE
About the Author: Alan Richter is editor of Cutting Tool Engineering, having joined the publication in 2000. Contact him at (847) 714-0175 or email@example.com.
Courtesy of MAG Americas
Multiprocessing system makes short work of long parts
Fairbanks Morse Engine
Mid-State Machine Products
Votaw Precision Technologies
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