Think friction welding and the image that most likely comes to mind is a rotary process for butt-joining cylindrical or tubular materials under an axial load. For many years, the impression of friction welding was that of an expensive, custom process suitable only for certain high-volume applications. The process is well-known in automotive, truck, and heavy-equipment circles, where almost every axle or driveshaft has some friction-welded component. Yet practitioners such as Joel Donohue, general manager of American Friction Welding (AFW), a full-service job shop in Brookfield, WI, describes the process as greatly underutilized by industry. Shops such as his can efficiently provide five-piece orders if that's what the customer requires

With patents going back to the late 19th Century, rotary friction welding remains the most common commercial friction welding process. Modern friction welding in the US began moving forward in the early 1960s, mainly at Caterpillar, Rockwell International, and AMF. According to Dietmar Spindler, general manager of Manufacturing Technology Inc. (MTI, South Bend, IN), the first two modern friction-welding machines sold were by AMF for steering worm shafts and by Caterpillar for turbochargers. Both companies used flywheels as a capacitor to store all or part of the energy for welding. Caterpillar called their system inertia friction welding, while AMF called theirs flywheel friction welding. Rockwell built four machines for its own use to weld spindles to truck differential housings, and never sold friction welding equipment to outside customers. In 1975, MTI purchased the patents, rights, and know-how from Caterpillar and AMF for the flywheel-based inertia friction welding process. Ten years later, MTI also purchased the rights to the New Britain line of friction welding equipment

Friction welding basically works by producing heat by rubbing two components together under load. Once they reach the required temperature and material deformation, the action stops and the load is maintained or increased to produce a solid-phase bond. According to Dave Nicholas of Cambridge, UK-based The Welding Institute (TWI), friction is ideal for welding dissimilar metals with very different melting temperatures and physical properties, such as copper to aluminum, titanium to copper, and nickel alloys to other steels. As a rule of thumb, forgeable metallic engineering materials are friction-weldable, as well as castings and powdered metals. MTI's Spindler adds that most metals can be welded and post heat-treated to 100% base metal strength. And every weld should be tested in a setup that mimics its intended purpose rather than relying on standard mechanical test results. For example, a hardened SAE 10B35 or 1045 weld would be the equal of any forging in a drive shaft used to transmit torque, but can be as much as 30% weaker in a push-pull operation

Every friction weld should be tested in a setup that mimics its intended purpose

As a job shop specializing in friction welding, Joel Donohue describes AFW as fairly unique. "We have very few competitors, and we cover many industries across North America," he says. "We weld pump shafts, electric motor shafts, printing press rollers, and a lot of automotive and construction equipment applications such as tie rods and axles." In conjunction with their full-service machine shop next door, American Friction Welding supplies turnkey projects as well as welding-only services for customers. The company uses direct-drive rotary friction welders, where motors rather than flywheels or inertia-type momentum provide the welding energy. "There's a little different heat-affected zone [HAZ] and grain flow, but basically the results are similar between the two," he says Friction welding is both a conventional and growing process for Donohue's company. "We get some offshoots of axle or driveshaft work on a regular basis," he says, "small production runs the customer doesn't want to tool up for. On the other hand, I get at least one phone call a day form people wanting more information on the process, looking for a part sample, or asking if they can pay us a visit. I would say unequivocally that friction welding is under-utilized in manufacturing today

How does Donohue evaluate whether a customer's request is right for friction welding? "The first thing is to get a visual on what type of parts need welding and their configuration," he answers. "If the part's not cylindrical or tubular, which is the most common case for rotary friction welding, you have to tool up for it. Basically the parts need to spin and butt-up at the weld joint. If there's not a natural axis of symmetry, the tooling needs to create it

The next consideration is materials. As mentioned earlier, friction welding is ideal for welding dissimilar metals with different melting temperatures and physical properties, but that's not to say the process is ideal for all metals. "There are a few materials we need to stay away from, namely free-machining and resulfurized steels," Donohue says. "That extra lead or sulfur in such steels creates nonmetallic inclusions for breaking chips. These materials, particularly at the weld joint, tend to counteract what we're trying to do, which is create friction and heat

Another phase in evaluating if a part is right for friction welding is asking how it would be joined otherwise. "Many people realize they can mate dissimilar materials, but that doesn't mean they've fully evaluated the alternatives," adds Donohue. "We often find early on that we can replace castings or forgings, which can be expensive to tool. For example, we can use friction welding and pre-machined bar stock to create a cast-like blank. This helps keep material costs and machining time down, for relatively low up-front tooling cost. And friction weldng is the preferred process in such cases, as conventional MIG or TIG arc welding would represent a weak point in the part

Post-weld testing also is an integral part of the process. "We test for many conditions depending on the part, material, and customer requirements," Donohue explains. "Typical test results might be for torsional or bend moments, hardness evaluation of the heat-affected zone, and a tremendous amount of ultrasonic inspection to evaluate the weld

The last item in evaluating a part for friction welding is volume. According to Donohue, high volumes are a no-brainer, as the process is speedy and can turn out hundreds of parts per hour depending on the application. "But given the right attention, prototype and short-run parts can be cost-effective as well," he says. "A customer came to us for a 2.5" [63.5 mm] Hastalloy shaft for a highly corrosive pumping environment. After looking at the part, it turns out two-thirds of the shaft stays in an enclosed gear case. We were able to replace two-thirds of the original Hastalloy shaft with stainless steel by friction welding. Not only did we save about 75% in material costs per shaft, total machining time per part went from eight hours to two hours

Friction welding is ideal for joining similar materials, but there are some metals unsuitable for the process.

While rotary welding remains the most common friction welding process so far, others exist and represent alternative friction welding solutions for particular applications. One is friction stir welding. A friction stir weld is formed by plunging a rotating shouldered pin tool with a pin length slightly less than the required weld depth into the faces of the materials to be joined until the tool shoulder is in contact with the work surface. The rotating pin within the workpiece causes friction, heating the metal and producing a plasticized tubular shaft of metal around the pin. As the pin moves in the welding direction, the leading face of the pin, assisted by a special profile, forces plasticized material to the back of the pin while applying a substantial forging force to consolidate the weld

According to TWI, further developments within friction stir welding demonstrate that plate thicknesses to 75 mm in 6082 aluminum alloy can be welded in two passes. Other metals including copper, lead, magnesium, and titanium have been successfully stir-welded, but more studies are needed

Friction welding's benefits extend beyond joining dissimilar materials in that the process is environmentally friendly, producing little smoke or slag and requiring no flux, gases, or filler material. Friction welding is also energy-efficient, using between 25 to 100 watts per square centimeter of weld area under normal operating conditions

The fact that friction welding is a simple, machine-controllable process helps. According to MTI's Dietmar Spindler, many manufacturers rely on in-process monitoring for weld quality control

Standard friction-welding process controls monitor and feed back pressure and slide control in a closed loop to improve accuracy and cycle time. A standard operator interface would show part number and weld cycle, last weld report, and a real-time view of speed, position, weld time, and dwell time. Control options include the ability to select and save weld parameters by part number and weld options, such as one or two-stage welds

"In the last two years, we have re-controlled most of our machines to better monitor pressure, time, rpm, and other weld parameters," says Joel Donohue of American Friction Welding. "We do work to ISO and QS9000 requirements for customers, so the process is certifiable. Last year, having better control of the process was prompted by the customer, this year it's by us

While manufacturing engineers continually seek ways to reduce cycle times and increase production, tapping holes typically remains an operation of incremental changes and chronic problems. But with the latest tapping equipment, manufacturers are capable of tapping holes faster and more safely, with enhanced thread accuracy

Bilz Tool Co.'s HFP Tool Control system helps control tapping operations reliably and economically even in high-production, multispindle applications. The tapholder's transmitters detect dull taps, tool breakage, and missing threads, eliminating the need for probing stationsImprovements in rigid, or synchronized tapping, along with advances in taps and tapholding equipment, are now giving holemakers the ability to achieve optimal thread quality, lower costs per hole, and extend tap tool life, while also providing high production rates through lowered cycle times. Recent tooling enhancements leading to better tapping include self-reversing taps, broken-tap detection, coolant-through taps, shrink-fit toolholding, and quick-change tooling, to name a few

Speeding up tapping has always been a challenge for tapping manufacturers and toolmakers. Although tapping is a popular, economical way to handle threading operations, it can be plagued by the chronic problem of broken taps, and tapping speed is hampered by the fact that the process requires two spindle movements

Although machinists want ever-faster tapping speeds, in some ways the technology's tapped out, so to speak, in terms of speed limits. Self-reversing heads have helped somewhat in speeding some small tapping applications to a maximum of about 6000 rpm, but some experts say most tapping operations can't be done at that speed

"Simply because of the physical limitations of twisting a thread into the part, a reversing tapping head has helped the customer decrease the cycle time, but the actual time required to thread in and out of a part has been pretty constant over the years," says Bilz Tool Co. (Melrose Park, IL) Vice President Tom Gibbings. "Materials like solid carbide taps and coated taps have helped somewhat, but the process is that you must go a certain depth into a part and come back out. Unless you have a reversing tapping head, you have to stop the machine spindle in the bottom of the cycle, put it into reverse, and reverse back out, which is the way most tapping is done

Bilz' rugged, heavy-duty GNCK 12 BC/25-ESX-16 tapholders are designed for automated tapping operations on transfer lines, machining centers, and flexible manufacturing lines. (Left) Bilz' ThermoGrip tool-clamping system affords accuracy and reliability for tapping with its heat-shrink toolholding technology

High-volume tapping operations, like the heavy-duty tapping done in the automotive industry, can be too demanding for some reversing tapping heads. "It's coming into its own," adds Gibbings, "and some companies have had a lot of success in the job-shop arena, where the shop owner has gotten an order for 100 or 200 parts at a time. But where production tapping is an issue, most tapping heads, except for our heavy-duty GNCK tapping head, haven't held up very long because of the constant reversing that takes place inside the tool itself

"Tapping usually is on a complicated part, like an automotive part, and it's almost always the slowest single operation in the sequence of operations," Gibbings points out. "Increasing the speeds would be very helpful, but our highest speed with a reversing tap head is 4000 rpm, and frankly, that's running quite fast for tapping, and we've had occasional problems with misapplication of tap design or machine programming

Tapping tactics can differ according to spindle speeds, feed rates, and numerous other machining application variables. "Tapping is often misunderstood," says Mark Johnson, president of Tapmatic Corp. (Post Falls, ID). "With other machining operations, an operator can change feeds and speeds independently, because the spindle is turning in one direction. With tapping, however, two spindle reversals are needed for each tapped hole

Feed rate and spindle speed must work in tandem to tap threads accurately. "Change one and you've got to change the other," Johnson adds. "Some people overlook that when they try to run faster." Boosting spindle speed means operators must change the feed rate accordingly. As an example, Johnson notes that running a .25-20 tap at 5000 rpm would mean a feed rate of nearly 250 ipm (6.4 m/min). "While it depends on the geometry and size of the tap, it's not easy to simply increase the cutting specs," he says

Speed variation complicates things further. A spindle-speed setting of 3000 rpm may fluctuate from 2990­3010 rpm, Johnson notes, and to compensate for these minor fluctuations in spindle speed, some manufacturing engineers use length-compensating toolholders with tension and compression. Tension is the tendency of a tap to pull out of the drive spindle, while compression is the tendency to push back

"When a feed rate is too slow going in or too fast coming out of a hole, tension acts to compensate for the difference between the desired and actual feed rates," he says. "When a feed rate is too fast going into a hole, compression acts to compensate for the difference." Understanding the role of tension and compression is important, Johnson explains, since it has an impact on tapping performance that should be considered as users compare different tapping devices and methods

Rigid tapping, which keeps the feed rate fully synchronized with the rotational speed of the spindle using a feedback mechanism, does not require moving a tap in tension and compression--at least, theoretically. Tension is good on CNC machines and most tapping attachments, says Johnson. "If your spindle speed fluctuates on a CNC machine without rigid or synchronized tapping, for example, you have to control your feed rate based on the pitch of the tap. Otherwise, you can shave threads going in and out. Using tension and underfeeding at about 95­98% allows the tap to pull ahead of itself-acting as its own leadscrew-pulling away from the tapholder and compensating for rpm fluctuations

Tapmatic's RSR VMC self-reversing, right-angle tapping head machines an aluminum workpiece. The tap's constant speed feature allows cutting at the tap manufacturer's recommended speed for a maximum tap life

Compression works the opposite way, but there's a problem. "As the tap starts to dull or wear, it begins to enter the hole at a different point each time. Why? Because the holder is going into various stages of compression before the tap can start. You usually don't want compression on a CNC machine, because you want the tap to start at the same point each time to control the thread. You can never get in trouble using a hard start--meaning there's no compression--the tap is solid, it starts at the same point each time. The only thing that compression can do is allow you to enter a previously tapped hole." Tapmatic manufactures a variety of CNC, self-reversing, tapping attachments that use tension only, which is consistent with Johnson's advice

Machine-tool builders are selling rigid tapping, Johnson says, and using that approach makes sense if you're only tapping six or seven holes an hour. "If the customer is tapping 100­200 holes an hour instead, he just can't make production using rigid tapping, because rigid tapping is slow and maxes out at a certain rpm," he states. "With rigid tapping, you might go in at 2000 rpm. As the tap gets to the bottom of the hole, your spindle has to decelerate until it gets to zero, then it has to reverse itself and accelerate until it gets back to 2000 rpm

With self-reversing heads, you're not fighting the acceleration and deceleration; you save time and reduce wear on the spindle. "You can, more than likely, cut your time in half--without changing feeds and speeds--just by putting a self-reversing tapping head in there," Johnson claims. If it's taking 4 sec per hole, you'll probably do it in about 2 sec, because you've eliminated the acceleration and deceleration." Eventually, rigid tapping "maxes out" because acceleration and deceleration planes come to a point where they just can't keep up, he adds, and anything above the peak of that curve represents additional time savings

To improve its tools for heavier-production environments, Tapmatic introduced a new generation of self-reversing tapping attachments, the RDT line, specifically aimed at high production on CNC machining centers. "We went into a two-piece housing with the RDT so we could put a large rubber diaphragm in between the housing and the offset piece, the arm," says Johnson. "That diaphragm allows us to equalize the pressure on the inside of the head and vent air out--reducing the tendency to suck coolant inside. When you get into some heavy, flood-coolant situations, over a period of time the reciprocating and rotating spindle acts like a gigantic vacuum and sucks the coolant in. Equalizing pressure prevents that

"We went to one-to-one gears. We put needle bearings in the front--instead of running on a bushing. We added a ball-sleeve upper bushing that the back of the spindle can ride on--it actually rotates with the spindle. We designed a completely new alignment collar, which has a lip that goes over the housing to prevent coolant from entering through the back

Tapmatic also added its RSR-VMC self-reversing, right-angle tapping head for vertical CNC machining centers, a model that Johnson says was strictly customer-driven. "While there aren't too many people that need one of these devices, it's a boon to those that do. It took time and persistence, but Tapmatic came up with a design that made customers happy because it not only saved a lot of time, it saved them a whole other operation, from having to take the workpiece off one machine and do it on another

Dull-tap detection promises to circumvent machining problems with broken or dull taps, warning operators of incorrectly tapped threads. In particular, high-speed, high-production multispindle tapping operations stand to gain the most from this type of technology, according to Bilz' Gibbings. With its High Frequency Detection HFP Tool Control System, Bilz' rugged, heavy-duty, multispindle HFP line of tapping heads is capable of doing hundreds of thousands of cycles, Gibbings says, while it monitors any tool problems occurring in automated tapping operations on transfer lines, machining centers, and flexible manufacturing lines

"On multispindle lines, it's difficult for manufacturers to do tapping, particularly in vehicle manufacturing with engines and transmissions," Gibbings says. "On an automated transfer line, a multispindle head will drill a gang of holes, then the part will be checked with probes to see if the holes are actually there, because if a drill breaks in that hole, you don't want to come in with your reamer or tap and break it off. So, it's drill, probe, and then many times, it will require reaming, probing, then tapping and probing

"The probe can tell if you broke a drill or a reamer, but it can't tell you if you actually put threads inside of the hole," Gibbings adds, "so Bilz came up with what we call a dull-tap detection system, which incorporates the use of a tap holder and a radio-transmitter system that actually tells you, first, if you tapped to depth, and second, if the threads are actually in the part

Bilz' HFP Dull-Tap Detection System features a radio transmitter implanted in the shank of the holder, which tells the operator if the part has any faults, he says. "We had a previous, less-sophisticated system that we had for about 15 years, but this is an upgrade with sophisticated capabilities

Coolant-through tapping equipment also helps prevent problems with tapped holes. "Coolant-through helps tapping a great deal in blind holes," notes Gibbings, "which are difficult to do anyhow. The coolant blows the chips out, and lubricates the tap threads

The coolant-through DuraFloat line of floating tap and reamer holders recently introduced by T.M. Smith Tool International Corp. (Mt. Clemens, MI) features only two moving parts and uses an antifriction parallel floating drive that improves tool reliability and extends tool life. The design also provides coolant-through the holder at high pressure. The unit's centering toolholder mechanism design allows the inlet flow to be controlled, reducing pressure on the mechanism and allowing use of higher coolant pressures

Lee Flick, T.M. Smith Tool International's director of manufacturing and engineering, says the company applied for patents related to the centering mechanism on the DuraFloat floating tap and reamer holder, which can shoot high-pressure coolant-through up to 1000 psi (6.9 MPa)

"We took a cross-section of the market, took the best of all worlds and made a tool from it," Flick notes. "It's more rugged than our traditional design, so it's suitable for the heavier reaming applications, yet it's also adjustable pressure, allowing you to adjust it downward for light tapping or small threadmaking applications

Quick-change tooling systems, like those offered by Lyndex Corp. (Northbrook, IL), are popular with customers needing speed and flexibility on automotive transfer lines. "We have a quick-change system that is compatible with the Bilz-type tapping system, where you have a tap adapter that you put the tap into by depressing a plunger," says Lyndex's Tim Fara. "The tap squeezes in there and then you can quick-change it out of the tap holder. A lot of customers still like that because they need to do the quick-change, getting a tap in and out, and then be up and running again. We offer that product in the CAT-V flange as well as the automotive shank, with the automotive shank being primarily for transfer lines

Lyndex also makes another tapping system that, instead of interchanging the collet that holds the tap, interchanges the entire nosepiece, Fara adds. "It's a quicker and more accurate way of doing it, and it's their version of how they want to do the quick-change mechanism," he says. "The difference is that it will interface with more spindles easier than the automotive style will--and there's a big benefit there

In addition, Lyndex makes rigid tapping systems, and offers the ER collet system, which Fara says is the most accurate toolholder for tapping on the market with tolerances guaranteed to be within 0.0003" (0.008 mm) at the collet's nose

"What we see in tapping is that customers are trying to streamline their machine efficiency, and that's why we have gone with the ER tapping collet, to allow them to use current tooling with a minimum amount of changeover to get into synchronized tapping," Fara says. "And our ER collets are, in the worst-case scenario, 0.0005 TIR. In most cases, 0.0002­0.0003 would be our standard TIR; the industry standard is 0.0006­0.0008. That's important, because TIR is directly proportional to tool life. The better your TIR, the higher your tool life will be

Other key technologies for automated tapping operations Lyndex offers are automatic depth control and automatic-reversing tapholders, Fara says. "When you're tapping, one of the hardest things today on a machining center--be it horizontal or vertical--is the constant clockwise, counterclockwise, clockwise, counterclockwise rotation of the spindle to go in and out," Fara notes. "We are trying a lot of different things because tapping is one of the slowest processes in the machine-center-working environment--that's why you see a lot of shops still have tapping done as a secondary operation

Tapping suppliers need to continue working with their customers to find better methods to improve cycle times with faster thread-turning, Fara notes. "One of the biggest issues is the quality of the tap as well as the quality of the holder that's holding it," he says, "because the thread consistency, thread quality, is so much determined by the TIR, the runout of the tool. It's so critical that it's the biggest thing we need to focus on