Screw Machining: Advances in Automatic Bar Machining Technology

Screw Machining

By Mark Shortt
Editorial Director, Design-2-Part Magazine

While reducing setup times and secondary operations, CNC Swiss Automatics produce parts that are superior with respect to both dimensional tolerances and surface finish.

Manufacturers who are looking to find a screw-machining source shouldn't have to look very far. In the U.S. today, screw machining is reported to be a $2 billion industry that delivers precision parts for everything from watches, heart implants, fiber optics connectors, and solenoids, to aerospace fasteners, photo imaging devices, and automotive drive shafts, sensors, and brake systems. Employing technology ranging from single-spindle, cam-driven machines to multiple-spindle, CNC Swiss Automatics, more than 6,500 job shops regularly turn out precision-machined miniature screws, connector pins, and other small-diameter parts, as well as valves, pinions, bushings, and tubular parts with internal threads.

It's a good thing, too, because manufacturers are being driven more and more to outsource screw machine work by factors related to corporate business strategy, economic necessity, and technical expertise. While many are realizing the need to concentrate more on core competencies, such as product development or marketing, they are also finding it increasingly difficult to hire skilled, competent machinists to operate increasingly complex and expensive equipment. Others are inclined to outsource complex parts, or jobs requiring materials that are difficult to machine, such as high-temperature alloys.

"It is usually more efficient for manufacturers to have vendors compete for their work, and then require them to deliver excellent quality work on time," says Ron Grob, president of Ron Grob Company, a screw machine shop that employs 25 people at its 16,000-sq-ft facility in Loveland, Colorado. Keith Trent, Director of Operations at Precision Swiss Products, Inc., San Jose, California, echoes Grob's assertion. He says that the high quality of precision Swiss screw machine parts, combined with quick turnaround times and competitive pricing by job shops, have strongly encouraged product manufacturers to outsource screw machining operations.

"Screw machining" is a general term for a process used to manufacture a wide assortment of pins, connectors, shafts, bushings, nuts, bolts, and valves, as well as various screws and other intricate threaded parts. It encompasses turning operations by a number of different specialized machines, including automatic and semiautomatic types. Cam-operated, single- and multi-spindle machines, CNC single-spindle machines, and automatic lathes, or chuckers, are among the different types of screw machines widely employed by job shops.

Automatic screw machines, also known as automatic bar machines, use an assortment of tools attached to a special turret. With the tool attachments, a number of different operations such as milling, broaching and cross- drilling, tapping, and slotting can be performed on the screw machine. Automatic screw machines can turn workpieces from 1/16 inch to 6 inches in diameter, and are capable of high rates of production. Parts can be machined from 12-ft-long bars using automatic bar feeders, and thousands of parts can be made with very little variation in size, notes Grob. Single-spindle machines usually have one spindle with five tools, while multiple-spindle machines have anywhere from four to eight spindles configured in a circular arrangement, with each holding a separate workpiece.

Screw Machining Swiss-type automatic screw machines are prized within the industry for their ability to provide high precision, holding tight tolerances with great repeatability. CNC Swiss Automatics, programmed to handle both primary and secondary operations on one machine, can produce parts that are superior with respect to both dimensional tolerances and surface finish "for really no extra cost," according to Greg Cdebaca, president of Pacific Swiss & Mfg. Inc., Clackamas, Oregon.

"The best advantage of using CNC Swiss screw machines is their ability to produce complex parts extremely accurately, usually to 0.0005 inch over several inches, or longer," says Cdebaca. "By using CNC Swiss screw machines, we find that with the technology that the machine tool builders are putting into the machines and the after-market accessories that are available, we are only limited, in what we can make, by our own creativity.

"Many of our customers come to us trying to reduce their part costs," Cdebaca continues. "Having 7-axis machining axis on our CNC Swiss machines, we are able to machine very complex parts in one operation, whereas other shops need multiple setups on a variety of machines to obtain the same part. Therefore, we are able to give a higher-quality part in less time at a lower cost."

Screw Machining
"The CNC Swiss machines are the most appropriate equipment for today's fast turnaround, precision machining," says Jack Graeber, president of Northwest Swiss-Matic, Inc., a QS-9000-certified shop in Minneapolis, Minnesota. "Ideally, we want to do the close tolerance turning and milling work on the CNC Swiss machines. The large form tools on the multi-spindle machines can remove large amounts of material faster than the small, single point cutter on the Swiss machines. But we get tighter tolerances, better concentricity, and nicer finishes from the CNC Swiss machines. We can hold 0.0002 inch on the Swiss machines, depending on the type of material, part configuration, and length of the part."

Advances in Screw Machining Technology

By expanding opportunities for cutting operations, the multi-axis, CNC Swiss turning machines reduce the number of separate secondary operations required and decrease overall cycle times. They also require shorter setup times. From a quality perspective, optical vision inspection arrays work with computer measuring systems to permit complete inspection of critical dimensions in real time. As a result, nearly instantaneous corrections can be made to the turning process.

A major advancement in CNC Swiss automatics has been the development of the rotary guide bushing, Grob said. The advance led to the incorporation of various types of insert tooling, which enabled higher surface footage while holding close tolerances and concentricities. Also, the development of multiple axes with live tooling also greatly enhanced the capabilities of Swiss automatics, Grob added. "Now, a sub-spindle with full C-axis can be used to pick off a part and perform several complex operations while another part is being started on the main spindle, resulting in a faster cycle time," he noted.

But according to Grob, the greatest strength of a Swiss automatic screw machine is also one of its greatest weaknesses. The length of the guide bushing (about 3/4 inch on larger machines) limits the length for making multiple passes on a turn, Grob explained. Although long, slender parts can be turned in one pass, the removal of large amounts of material (1 1/4-inch down to 1/4-inch diameter) is a slow process, he said.

Sometimes, manufacturing advancements come in the form of process efficiencies rather than technology. One company that can attest to this is HK Screw Machine Products, a job shop in Oceanside, California that developed a proprietary cut-off process to save time on production runs. On a job that averaged about 5 million parts per year, the firm was employing a cut-off process that typically lasted "around 6 seconds," according to Gerry Krippner, president. But by designing and developing a proprietary cut-off process, HK Screw Machine was able decrease the operation by 3 seconds, or 4,170 hours per year, Krippner said. "That was all accomplished because we have people who are willing to ask 'how can this be done better?' and a management team who is willing to support those efforts," he revealed.

"Simplicity drives our innovation," Krippner continues. "In our case, we are talking about single-spindle, cam-driven machines. We do best in volumes between 10,000 and a million per run per machine. Below that, CNC is better from a set-up point of view. Above that, we can compete by using multiple machines. We compare favorably with multi-spindles on harder materials where one operation might dominate process efficiencies."

While shops generally employ the latest technology that they can afford, a significant portion of their success is often due to their emphasis on the human side of the equation, a fact that is not lost on Ron Grob. "In order to do the best possible job at the lowest cost in the shortest possible time, we encourage free customer interaction between our customers and employees at the lowest possible level," explains Grob. "Our customers' engineers frequently work directly with our manufacturing engineers or machinists to work out any production issues. Our screw machining team not only has many combined years of experience, but their creativity and technical expertise is constantly challenged and kept fresh by the variety of materials and part designs coming through their department."

Solving Problems for Manufacturers

Screw MachiningRon Grob Company performs screw machining for manufacturers in many industries, including electronics, medical devices, energy production, fiber optics, hydraulics, semiconductors, and after-market automotive parts. Customer products range from aortic punches, to pool cues, printed circuit board in-circuit test fixtures, mass flow valves for computer chip manufacturing, and fruit peeling equipment. Because the company has in-house thread rolling capabilities, it makes many parts from thread-rolled rods, which it custom rolls to its customers' thread and material specifications.

"Similarly, we centerless grind rods and make parts from them," says Grob. "We also have in-house OD/ID Grinding, thread grinding, CNC milling, and straightening machines, so we routinely make parts involving those operations. We do a considerable amount of shaft work on our Swiss automatics. Lengths over 24 inches are fairly common. Many of the lead screws we make on the Swiss machines start out as thread rolled rods. Because of our ability to make almost any kind of part except sheet metal, we often manufacture complete assemblies."

Diameter tolerances of 0.0001 inch with a surface finish of 8 rms are not uncommon, and a total tolerance of 0.0005 inch is held on a daily basis, Grob says. "We hold a length tolerance of 0.005 inch on parts up to 100 inches long," he asserts. "When we can't hold diameter and finish tolerances on the automatics, we usually can do it on the grinders."

One part was a 1.250-inch-diameter x 82.400-inch-long tube with a 0.005-inch length tolerance. The challenge with the tube was the length tolerance, which Grob anticipated would be very difficult to maintain in production. "We knew that the material needed to be very straight and round before screw machining if we were to have a chance at holding the length tolerance," Grob said. "We straightened the tubing (which is notorious for being bent as it is received) in our Meeco bar straightener to within 0.010 inch over the entire length of 82.400 inches, before centerless grinding it. We developed an attachment for our IEMCA bar feeder for the Swiss automatic that allowed us to use just one incremental Z-axis move in the Swiss program to have complete control over the length."

Another part was an aortic probe, which had a 0.118-inch-diameter nose with a 0.025-inch-diameter stem 3.625 inches long. The challenge was that the surface finish over the 3.625-inch length was required to be 8 rms. The inch-diameter bull nose needed to be the first portion machined. Beyond the nose, the part blends down to 0.025-inch diameter and tapers up to 0.039 inch over the 3.625-inch length.

"Since producing the 8-rms finish was the problem, it was necessary to initiate a search for suitable insert tooling," Grob explained. "The material is 316 stainless, and even though good finishes can be maintained, chipping up turned out to be a major problem due to the relatively large amount of material that needed to be removed. The chips wanted to wrap themselves around the part, causing scouring over the length of the shaft. Even though the scouring was minute, the finish could not be held. Also, since the bull nose was relatively heavy in proportion to the rest of the part, it wanted to whip around and bend, or cause the part to break off.

"The insert problem was solved using Kennametal S10P-22R back turning inserts with a 0.004-inch nose radius. Their geometry worked perfectly to keep the required surface finish. The chip problem was overcome by intermittently putting a slight incremental move of minus 0.001 inch every 0.400 inch (before the chip hit the bottom of the chip pan). Much to our surprise, it left no scouring on the shaft. By using a simple Delrin support programmed to maintain support on the bull nose portion of the part, the whipping problem was solved."

Precision Swiss Products, Inc. serves customers who outsource "very complex partswhat we call 'a very busy part,'" says Keith Trent, Director of Operations. The firm's customers include manufacturers of heart implants and catheters, microwave equipment, and components used in data retrieval and transmission, as well as aerospace photo imaging products. Precision Swiss holds tolerances as tight as 0.0001 inch on its Swiss screw machines, which give the company the ability to offer high quality with quick turnarounds and no secondary operations, Trent says.

As an example of the company's capabilities, Trent cited a recent job in which the firm easily met a deadline for manufacturing a special threaded and slotted screw, with cross-drilled holes, in a quantity exceeding 20,000. The company made use of tooling manufactured in house, as well as its employees' expertise in programming, to manufacture the parts in 35% less time than expected.

Pacific Swiss & Mfg. Inc. employs 11 people at its 5000-sq-ft facility and has plans to expand in the near future, according to Greg Cdebaca. At peak time, the firm runs its six-machine CNC Swiss department approximately 125 hours per week.

"Our customers are in the connector, electronics, high-tech, medical, dental, telecommunications, hardware, and automotive industries," says Cdebaca. "They produce anything from miniature connectors to medical/surgical equipment. Parts are generally of complex shapes or difficult-to-machine materials, such as nickel, Haynes alloy, and Hastelloy, among other alloys. Our customers primarily outsource their complex parts, or hard-to-machine materials like the high-temperature alloys that other shops do not have the capability to make or do not want to run.

"We offer a full range of capabilities with CNC Swiss Automatic screw machines. All are state-of-the-art, from Citizen, from the 2-axis B-12 through the 7-axis L-20 and L-25. By utilizing right angle heads and NSK air spindles on both the front and back workstations, we can be very creative on how we manufacture complex parts."

Northwest Swiss-Matic, Inc. manufactures precision screw machine parts for customers in the automotive, medical, electronics, and military industries, among others. Quantities are often large, with orders ranging up to 100,000 parts per day. Critical-tolerance metal spools and pistons, manufactured for leading companies in the automotive, hydraulics and pneumatics, and aerospace industries, are examples of precision components produced by Northwest Swiss-Matic. The firm produces individual parts ranging in size from 0.010 inch to 2 3/4 inches, with lengths to 3 feet. Some parts require accuracy to 0.0002 inch.

Northwest Swiss-Matic is certified to MIL-I-45028, MIL-I-45622, and the intent of MIL-Q-9858, in addition to QS-9000.

HK Screw Machine Products, Oceanside, California, operates "just under 70" automatic screw machinesall single-spindle, cam-driven Brown & Sharpes, according to Gerry Krippner, president. The company, which devotes about two-thirds of its 40,000 square feet of manufacturing space to screw machining operations, works in conjunction with its sister company, HK Plastics Engineering, Inc., which offers plastic injection molding and tool making services. In addition to making a variety of inserts, pins, shafts, and bushings for plastics injection molding applications, HK Screw Machine produces parts that are used in the manufacture of solenoids for the electronics industry; valves and sprinklers for irrigation systems, and miscellaneous cleaning products for pool cleaning systems.

HK Screw Machine offers the expertise of a "highly efficient, Browne & Sharpe only" shop, according to Krippner. While taking full advantage of the high productivity and cost effectiveness of the older cam-driven machines, the company also uses CNC machining to make cams, and wire EDM to make form tooling, Krippner says.

Price Manufacturing Company Inc., an ISO 9002- and QS-9000-certified screw machine shop in Corona, California, takes on "hard-to-machine jobs that are problematic in an OEM environment," according to Ively Schiffmacher, co-owner. Sensors, airbags, and brake systems for the automotive industry, fasteners for aerospace products, and electronic connectors are some of the products manufactured by the company's customers. By developing tools designed to eliminate the creation of burrs, the company had success on a recent job that required the machining of threads, slots, and grooves without using costly deburring processes. Price was also able to eliminate chipmarks on the outside diameters.

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