Making High-End Parts
Swiss turn, screw machines, and rotary transfer
By Robert B. Aronson
The optimal equipment to use when making smaller parts in high volume, as with most manufacturing processes, depends on the product to be made. Key factors are part volume, precision, material, and complexity.
With the present state of offshore competition, the idea commonly expressed by many in this industry in the US is: The easy stuff will leave, so to survive you will need high-end equipment to make the complex, precise parts. It will also help if you have a clear understanding of immediate market needs, and the intelligence or good luck to know what the market will want in the future.
"Nothing's getting bigger" is a common statement from manufacturers in the volume-part industry. This may be a bit of an exaggeration, but generally many of the products now driving many manufacturing operations, led by the medical, communications, and automotive industries, are getting smaller.
Three of the most common machine tools used for volume production of small parts are the Swiss lathe or "automatic," rotary transfer, and multispindle screw-type machines.
Tornos Technologies (Brookfield, CT) offers both Swiss-type single-spindle and multispindle machines. Plus they still offer multispindle cam machines, which are dedicated to long runs. Today, the company is concentrating their production on CNC-controlled units. The main reasons are that the Swiss and multispindle CNCs offer flexibility and the capability to quickly set up for a part. They can use economical, standard tooling. Even the new multis can economically handle runs as small as 50 parts with no top limit to volume.
According to company president Tom Dierks, "With CNC multispindle machines high volume is not as important as it formerly was. Lot sizes commonly run in the 500 - 2000-part range, sometimes smaller if a customer is producing a family of parts.
"Work that was done on Swiss-style cam machines is also migrating to CNC Swiss units. As CNC evolved, users found they could make very complex parts with length-to-diameter ratios of 8 or 10:1, as well as short parts requiring milling, drilling, and boring, in addition to turning, often in one setup. These parts were more complex than a conventional CNC lathe can make.
"Multispindle cam machines still supply a lot of parts in a hurry with one or two-second cycle times. But these are less complex parts. With five to eight spindles, each providing a separate operation, there is a greater chance for error. With a CNC you can eliminate some inaccuracy through electronic compensation. Another limitation of the cam-driven machines is that they can only cut with a form tool, because they can only make a plunge cut. A CNC machine uses standard insert tooling, allowing single-point turning and profiling.
"Part volume and setup time are the deciding factors between Swiss and multispindle machines. For example, a company might have counted on a 50,000-part order and fulfilled it on a multi. But now, because of just-in-time delivery, the parts must to be delivered in batches of 5000 with inherent storage and inventory problems. So, it is to the user's advantage to go to a multispindle that can easily accommodate a part change. The multi is favored for orders when making a family of related parts, chiefly because of quick changeover.
"A multi is also often the best choice when the volumes are sufficiently high, as their shorter cycle times means fewer dollars left on the table.
"A contract shop has to make what's coming in the door and be able to react quickly. If it's a 100,000-piece order, they should put it on a CNC or cam multi. If it's a 5000-part job, it might work better on a single-spindle Swiss," says Dierks.
Another major market segment is high-volume, long-run, precision parts with complex features. These jobs are usually automotive such as fuel injectors and electronic connectors. Often these parts are the result of redesigning so that several parts are combined into one.
"Vanamatic [Delphos, OH] is a shop that works almost exclusively with multispindle automatics, chiefly cam-operated," says Jeff Wiltsie, Vanamatic president of manufacturing. "Parts we run typically have a cycle time of 4 - 12 sec. Process improvements have made us competitive with coldheading operations on high-volume parts and CNC machines on low-volume parts. We beat CNC machines in quoting chiefly because of low setup time and faster manufacturing operations. We can handle tolerances down to about 0.0002" [0.005 mm], but more frequently our specs are around 0.005" [0.013-mm]. More complicated parts range from about 0.5 to 3.0" [13 - 76-mm] diam. Set up of our screw machines averages as low as one hour.
"While we do run high-volume work, our current move is towards making short-run work look like long-run work through the use of setup reduction, quick-change tooling, and throughput engineering.
"Recently, we've seen a trend towards purchased technology, [automation, high-tech expensive equipment, and lights-out operation], designed to improve quality and reduce or eliminate people from the equation. While this seems like a good idea on the surface, we are simplifying our processes and becoming more flexible by reducing lead and setup time and maximizing throughput."
The Swiss turn or Swiss-style machine has long been recognized as the most suitable lathe for making high-precision, complex parts. The unique feature of this machine is that the stock passes through and is positioned by a bushing, and has one or more cutting tools that cut the stock close to the bushing. Cutting in this way virtually eliminates part deflection during the cut permitting very small features, and high precision.
"Swiss turn is designed for more precise parts where the turning length exceeds three times the part's diam," explains Paul Huber, president Comex Machine Tools (Bridgeport, CT). "Turning is at the point of support because of the bushing which means no part deflection, which is a key point. But, he cautions, "There are some parts that can't be made on a CNC. Speed is the issue."
According to Miles Free, director, technical services, Precision Machined Products Association (PMPA, Brecksville, OH) there are two general types of Swiss machines, those with fixed headstocks and those with sliding headstocks. "The sliding version is for longer parts. Usually with a Swiss machine you have a lot of value added to the part. With the Swiss machines, parts range from nearly invisible to about 3" [76-mm] long. Maximum diameter I've seen is around 0.75" [19 mm].
"These machines can be CNC or automatic, or cam-controlled. Generally, for part volumes over 5000 parts automatics are more economical, because they are faster. CNCs take longer to do all of the operations. But CNC machines can be set up faster because it's a matter of programming, while setup for the automatics can be longer because it involves the replacement of cams and all tools. In some cases, manufacturing new cams will add weeks to lead time.
"One potential drawback with the Swiss turn machines is that Swiss machines require ground bar stock, which is more expensive. This is necessary to get both the precise tolerances and the concentricity and straightness needed.
"Presently the greatest time-consuming part of the manufacturing process is gaging, particularly if it is done manually. Because of the accuracies required, sophisticated gaging is often required.
"Two areas where machine makers are working are dry machining and part handling. With little or no lubrication, tool wear and getting the heat out of the part being manufactured are problems.
"Removing the part is often a manual task on most types of machines. But robots are now being used, such as with the Tornos machines. There is also a trend toward using the Swiss machines and multispindle automatics as the basis for a manufacturing cell.
"Another problem in the industry is insufficient process control. Machine operators can tell you their size reading, but they often don't get actual process data on horsepower, speed, tool wear, and temperatures. So, while the process works, often they can't identify the critical process parameters that were involved. They could if they had sufficient process control," Free states.
Cams versus CNC is a major consideration. Cams are still useful. Instead of servomotors, cams do all the pushing and pulling needed to position and actuate tools. Despite the publicity given CNC and multiaxis machine tools, cam control is still a viable manufacturing technique. However, the decline in the number of qualified operators and the lack of younger workers entering the field has put more emphasis on CNC machines. CNC has the advantages of requiring less machining talent to operate, wider availability of computer-literate workers, single setup operation, and greater accuracy when working with some specifications.
The key is volume, "If you are making a lot of the same thing, then cams, not CNC may be the best way to go," says PMPA's Free.
Davenport, a machine long known in US industry, is a prime example of the cam-machine's success. "Our new Davenports are a marriage between the proved and proven," explains Robert Brinkman, president of The New Davenport (Rochester, NY). "They are cam-actuated machines that combine traditional cam actuation with modern controls. A typical cycle time is 2 sec. The original basic model was cam-driven and used pulleys and gears to change feeds and speeds. The newer version is servodriven and provides more rapid changeover. This new design can hold less than 0.0001" [0.003 mm] with form tools. There can be three cutting actions: form cutting, shaving, then thread rolling.
"Generally, parts run on these five-spindle automatics are 1" or less in diameter with a maximum length of 4" [102 mm]. Common products are small screws, tire-valve stems, electrical contacts and connectors, and machine components," says Brinkman.
Finding qualified personnel can be a problem. One issue is that some don't recognize the difference between programming and machining. "There are many skilled programmers, but being a skilled programmer doesn't guarantee full knowledge of the machining aspects of the job," says PMPA's Free. "The guys who learned the business on cam machines have a body of knowledge that the CNC people might not have. One example might be material behavior: how will the part or chip behave under certain process conditions? The folks with experience and knowledge in both areas are like gold--valuable and rare.
"In selecting equipment it's important to understand what the tolerance class is. If you are making commodity-type parts for a cheap bicycle there are different standards than when making aerospace parts. With the tighter tolerances, we are to a point where temperature and even the coefficient of friction between the part and part holder matters. If you have a spec requiring tenths, and the part moves, you are in trouble.
"Part volume and end-use market are other determining factors. If you are making 50,000 parts, a small fraction of a cent is critical because of the multiplier effect. On the other hand, for a smaller batch size for items like medical bone screws, unit cost is not so much of an issue. Parts can vary several dollars in price because the price of screws is such a small part of a very costly operation.
"Labor costs are always a key issue. There is a tremendous savings if your production equipment allows you to have one person running two or more machines. Often a machine's high initial cost can be balanced by a reduction in labor or lights-out [untended] operation capability. And if a machine gives you a finished part complete in that one setup, you avoid transferring parts to a secondary operation with its attendant costs," Free concludes.
"Screw machine" is a generic term for a lot of things, but the part is often under 0.5" [12.7 mm] in diameter. And, multispindle screw machines provide several operations in a single chucking. This means no stackup of tolerances.
The earliest automatic screw machines were designed to cut fastener screw threads. Before these machines were developed, operators had to actuate the tools on these manual machines. With automatics, cams controlled the cutting action. The next step was the multiple spindle, cam-actuated machine such as the Davenport. "High-tech parts will stay in the US, and the easily made parts will leave," is the opinion of Olaf Tessarzyk, CEO, Index Corp. (Noblesville, IN). "About 50% of the nation's machines are now going 24/7. The other US shops, with older, lower-tech machines, particularly those that have a multistep process, may have trouble holding tight tolerances. Unfortunately, many companies have a lot invested in older equipment, and are reluctant to pay for necessary upgrades.
"Recently we have found that smaller job shops who never talked to us are seriously considering our products. Buyers are going after more difficult work, a trend that has been common in Europe for several years.
"Index machines have multiple spindles with CNC technology and are designed for easy changeover. Our machines have up to six spindles, each independently driven in three axes. This, along with front-open access and standard tooling make it more practical to run smaller job sizes.
"Tight tolerance, concentricity, and surface finish are critical in most operations. If you have to rechuck, you may have problems meeting those specs. Uptime is also critical. That means you will want both short setup time and overall machine availability.
"With cam machines you can have a problem with lack of versatility," says Tessarzyk. "They have their place with high-volume runs, but when you have to change feeds and speeds, or modify cams, you may have setup times of weeks. Plus, cam systems primarily use contour tools, which take a long time to produce, while with our multispindles we work with standard inserts."
Rotary transfer machines work best when making a single part in high volume or part families where you only have to change certain features. In operation, a bar is fed into the first of as many as 16-stations grouped around a rotary table and a blank is cut off. The part is indexed from station-to-station with specific machining functions performed at each stop. A rotary may use either horizontal or vertical spindles to handle complex part requirements.
"Most of our jobs are parts between 2 and 50-mm diam bar stock and within a 4" [102-mm)] cube for cold-formed components, forgings or small castings. We can also accommodate lengths up to 8.5" [216 mm], and we have the ability to do longer shaft applications up to 22" [560 mm] on more specialized Hydromat Machine models," explaines Bruno Schmitter, president/CEO, Hydromat Inc. (St. Louis).
"Hydromats are very competitive in high-volume production, but can also be used effectively to produce part production lot sizes down to 50,000 pieces for individual parts or 10,000 pieces for parts in families.
"The part itself determines the type of machine and the number of toolspindle units required, which can range from 10 to 24. Process sequencing is laid out first and that is driven by the part itself. Then we determine how many toolspindle units are needed to do the job. The machine can have turning, drilling, machining, and broaching capabilities. In some cases it can do some assembly operations," Schmitter says.
"This design lets the customer match what the machine needs to do to produce the parts," says Schmitter, "so you don't over buy on equipment, such as spindles or support equipment. Modular construction lets the users match the machine's capabilities to the part's size, production, volume, and complexity.
"Many companies are refining their operation and investing in new equipment, but I feel too many are not making the moves needed to stay competitive. To compete in the high-volume market you have to reduce the labor content and work-in-process cost. You also need to run parts at the fastest possible cycle times.
"In most cases, the Hydromat machines are able to complete parts as an alternative to multiple machine cells. It is usually not competitive to go with multiple machines. That means you have more people, more resources, and more work-in-process and all that runs against the bottom line.
"To ensure the necessary flexibility, Hydromat now offers the Epic concept. These are machines with CNC embedded in every station. We can do a complete layout changeover in a few hours. You have to be able to take on both the short-run and high-volume work with the same machine.
"Rotary transfer machine builders have experienced pressures to handle lower volume operations. This demand has required the development of hybrid machines that can handle a variety of assignments. To meet this need Hydromat offers the AT 115-10 CNC System that meshes machining center capability with transfer line technology. There are up to nine machining stations, each with 3 - 5 axes with robot load/unload all under the direction of a common controller. To ensure productivity, there are redundant, common tools, batch sizes are typically 5000 parts and up.
"We will also be introducing the Eclipse Machine model, which is a 12-station rotary transfer machine with rotating, workholding spindles. Each station has a three-axis module. You can build up any module with a rotating toolspindle unit or a fixed turning slide. Even multiple-turret units can be adapted. This machine comes with a Siemens 840D controller with user-friendly programming," Schmitter concludes.
Among the new products:
- From Miyano (Wood Dale, IL) the BX-26S gang tool lathe with two spindles offers complete part machining of complex barwork 1" diameter and under in a single setup. It is a multifunction machine that turns, mills, and drills. Two gang slides and a 3-D linear turret further contribute to the BX26S' precision and faster cycle times.
- The BX-26S' 3-D linear turret and traverse-type identical left and right spindles, both with 5 hp (3.75 kW), ensure stable cutting from end to end, and make the BX-26S well suited for long shaft work. Turning at a maximum speed of 8000 rpm is made possible by built-in spindle motors and high-rigidity linear guides. Eliminating the guide bushing speeds operation time.
- The compact BX-26S (87 X 52" [2210 X 1321 mm]) also offers revolving tools (eight tools max at 4000-rpm max), L-spindle brake, cut-off confirmation (by spindle torque), parts catcher and parts conveyor, high-pressure coolant, and axis traverse of 944 ipm (24 m/min).
- The ANS-3100P, built by Fuji (Vernon Hills, IL), is a modular machine with one spindle, one turret, and a robot for high-volume production of small parts. These machines feature built-in robotic automation and in-process auto gaging. A major market is automotive parts.
- Typical production run is 100,000. This lathe has a four-axis rack-and-pinion swing-arm robot. Its controller features faster and smoother simultaneous multiaxis movement to access the front, back, left, and right of the traverse axis. The unit is available with a 12-pallet work stocker and other auxiliary loading options. It operates at a maximum of 3500 rpm, and has an eight-position turret, and a 20-hp (15-kW) spindle motor. Part size ranges from 0.5" diam X 0.75" long (13 X 19-mm) on up.
This article was first published in the October 2005 edition of Manufacturing Engineering magazine.