As parts and assembly tolerances continue to tighten, fab shops look for new ways to adapt and change their methods to achieve greater accuracy and improved production flow. Additional set-ups and transfers result in inaccuracies, downtime and loose tolerances. Abrasive waterjet machining solutions can help minimize the amount of additional processing to a single operation - as was the case for Petersen Inc. of Ogden, Utah, a worldwide provider of fabrication, machining, assembly and engineering services.
Large-scale fabrication operations are not immune to global manufacturing trends toward higher precision and greater production efficiency. To keep pace with these demands, Petersen constantly upgrades its capabilities to meet the exacting requirements of customers in the nuclear, aerospace, petrochemical, mining and general industrial segments.
Petersen's offerings have grown dramatically since it was founded 55 years ago to provide basic welding, machining and fabrication. Today, the company operates more than 100 machines, including abrasive waterjet equipment, multi-axis CNC turning and milling, grinding, EDM and secondary processes, as well as advanced welding and metalforming technologies. The shop's large-part machining capabilities allow it to turn workpieces measuring up to 15' in diameter and mill features within an envelope up to 12' vertically and 66' horizontally.
Petersen has 450 employees at three locations, including the 600,000-sq.-ft. primary manufacturing facility in Ogden and a 72,000-sq.-ft shop in Pocatello, Idaho. A 900,000-sq-ft warehousing and distribution center in Ogden houses the company's logistics and inventory services.
"We have a very clear picture of who we are," said Bart Anderson, manufacturing engineer at Petersen. "We are a job shop. Most projects are low to medium volume, and every job is different in size and quantity. We do many one-off projects where we bid on a single item, large or small, and build it. Other jobs are low-volume production, with 10 to 100 units in a month. We also produce small parts for customers who have a production line and need a parts supplier."
Part sizes vary widely, from several inches long all the way up to mining industry components that measure 60' to 80'. Individual workpieces typically weigh from a few hundred pounds to over 5,000, and the shop routinely works on weldments and assemblies that weigh 5, 10 or as much as 50 tons. To move such behemoths, the shop has cranes with up to 75-ton capacities that work together to maneuver the parts. But on occasion, when assemblies weigh in at more than 130 tons, the shop has to bring in mobile cranes to transfer them to transport vehicles. But regardless of part size or weight, tolerances continue to tighten.
"Parts and assemblies that used to have wide open tolerances now have tighter and tighter requirements," said Anderson. "The accuracy of the individual parts that make up a weldment has a huge effect on the finished assembly. The better accuracy that you start with, the better accuracy you finish with. And the methods we use to achieve that accuracy have had to adapt and change. We are always looking for the best possible ways to process material."
A key contributor to greater accuracy and improved production flow at Petersen has been abrasive waterjet machining technology. The company uses waterjet cutting as a bridge between traditional plate cutting methods and precision machining operations.
"We use waterjets in all the industries we service to minimize the amount of additional processing," explained Anderson. "Where parts cut on other equipment often require further processing, such as grinding or machining, to reach final tolerance, waterjet often enables us to achieve that tolerance in a single operation."
Waterjet machining capability came to Petersen via happenstance. When the company took over the machining operations for a large automotive supplier, it acquired a preowned OMAX 55100 JetMachining® Center with a 4' 7" by 8' 4" cutting envelope. Prior to that, the shop conducted plate and shape cutting via a CNC burn table with oxyacetylene torch and plasma cutting capabilities. As Petersen began using the OMAX machine, it gained an understanding of how much the technology could accomplish and has since added an OMAX 120X JetMachining Center and most recently an OMAX 160X JetMachining Center.
Initially, the OMAX 55100 was used exclusively for smaller parts, but that changed with a job that involved fabricating a storage cask for spent nuclear fuel. The workpiece material was high-strength ASTM A537 carbon steel, and the finished plate was 14' long, 0.312" thick and weighed 150 lbs.
Customer requirements called for the plate to be bent 90 degrees after having slots cut in it, but the plasma process created a heat-affected zone along the edge of the cut line, leading to problems. The company considered machining the slots after the bending operation, but this would require additional set-ups and transfers of the large, cumbersome part. Petersen's experience with smaller-scale abrasive waterjet machining suggested that the technology might be a way to cut the material without creating heat-affected areas.
"We did testing with a supplier in Salt Lake City that had a larger waterjet machine, and it was highly successful," said Anderson. "We improved our results drastically, almost an order of magnitude for cutting accuracy. The tolerances we achieved on the cask component ranged between 0.010" and 0.020". With the waterjet, we were holding close to a normal machining tolerance on a fabricated 14' -long part."
Such increased accuracy enabled the shop to eliminate post-cutting operations and permitted it to cut features in the flat pattern that plasma cutting was unable to accurately produce. While cutting time increased slightly, the waterjet eliminated the necessity for an entire other setup in a different part of the shop, as well as material handling time, part staging and storage and the machining operations themselves.
"With the number of parts we had to cut, there was enough justification to buy a machine large enough to handle the cask parts," said Anderson. "So five years ago we purchased the OMAX 120X with a 10' by 26' cutting window." To multiply the machine's benefits, Petersen ordered it with two cutting heads. Both heads are mounted on the gantry machine's bridge. This allows Petersen to cut two parts simultaneously for some jobs, a feature that the company takes advantage of on 40 percent or more of the work processed with the machine.
All plate cutting for the storage cask job moved from the plasma burn table to the waterjet. Waterjet cutting also has replaced the plasma process on approximately 25 percent of the shop's other projects.
"As our waterjet capabilities increase, we are able to pull a lot of work from plasma cutting and move it to the waterjet," Anderson said. "We cut it to the size that we want and, with little or no further processing, put it right into a weldment."
Petersen keeps its OMAX machines busy. The machines run 80 hours of shifts per week, typically with multiple jobs going across them each day. Other times, a machine may be dedicated to a specific higher production job for a week or two. On average, the company estimates that 15 to 20 jobs are cut on the waterjets per week.
Petersen's big OMAX 160X provides a cutting window of 13' x 26'. And like the shop's 120X, the new machine is also equipped with dual cutting heads. But one of the cutting heads on the new OMAX 160X is an OMAX A-Jet head that can cut complex 3D shapes as well as easily cut beveled edges, angled sides and countersinks.
"The principle reason we bought the newest machine was for the A-Jet capability that lets us create weld bevels and edge chamfers," said Anderson. "We cut these features directly on the waterjet table instead of having to move the parts to separate machining or manual grinding operations."
The new machine's larger table size also provides greater productive capacity. For example, it enables the shop to simultaneously cut identical 65"-diameter steel disks from two 0.500"-thick stock plates. Previously, this job was quickly done one at a time on the OMAX 120X machine, so the shop has been able to double its production while maintaining the high accuracy and tight tolerances of the more than three dozen cut-out features on these disks.
Before acquiring the OMAX 120X machine, Petersen did evaluate other brands of waterjet equipment and came to the conclusion that OMAX continued to be the shop's best performance option.
"The capabilities of the OMAX machines are strong, and the results we get are great," said Anderson. "The software is simple to learn yet extremely robust performance wise. It and the well-built machines allow us to hold accuracies in the range of 0.003" to 0.005" over long cut distances. In fact, we currently maintain cutting tolerances well under 0.010" over an entire 26' span, and we are able to repeat that, day in and day out."
Petersen's diversification through a wide range of industries helps cushion the effects of economic volatility. With the recent slowdown in the petroleum and mining industries, the most active area for Petersen currently is the cleanup side of nuclear power. The company works with both the Department of Energy under the Nuclear Regulatory Commission and with utilities themselves.
"Over the last 8 to 10 years, a number of fabrication shops have closed, and it is really sad to watch that capability become extinct throughout the country," said Anderson. "Because of our capabilities, one of which being abrasive waterjet cutting, and our diversification in a number of key industries, we have been able to not just stay in business, but continue to grow as well."