FLOWFORM® SCREWS FOR ALUMINUM PANELS
A NEW, INNOVATIVE METHOD FOR JOINING ALUMINUM SHEET METALS!
The Flowform® screw is an innovative alternative to conventional methods of joining sheet metals together. The fastener is based on flow-drilling, a process for producing threaded or unthreaded holes in metal parts. The sharp pointed screw is spun at high RPMs. As the spinning tip comes into contact with the metal, heat is generated, making the material soft and malleable enough to be formed and perforated. The heated metal forms a deep, solid threaded bond around the screw.
- Heat treated 10.9
- 6 Point Torx head
- Polygonal body with an unthreaded sharp concial point
- Based on Toptite 2000® - technology known a flow-drilling
- Polygonal body increases holding strength while reducing materila displacement
- Can be installed with or without a punched or drilled hole
- Easier to install
- More economical than spot-welding
- Does not require a c-frame or counter nut
Areas of Application
- When the assembler/technician has access to one side of the component only
- 5mm only: to replace existing 5mm Flowform® screws or when replacing panels with existing un·stripped threaded holes that hove been created from existing Flowform® screws
- 6mm only: aluminum panels with existing threaded holes created by a Flowform® screw but is now stripped, or the original Flowform® screw has been pulled through/lost
- Ford F-150 - 2015 models
- GM Omega - Fall 2015
- Jaguar XK and X 150
- Lotus - aluminum chassis in the Proton
- Volkswagen - for the cover of the entry step on the Cross Touran
- ThyssenKrupp, for body panels on the Lamborghini
- Fehrer Automotive and Alcoa - for body panels
|Dia Thd Pitch||Min/Max Hd Dia||Min/Max Hght||Min/Max Lgth||Min/Max Thrd Lgth||Min/Max Tip Dia|
|M5 x 0.8mm||12.57mm-13.00mm||3.8mm - 4.2mm||22.00mm - 22.65mm||9.35mm - 10.25mm||-|
The market trend is to constantly reduce the thickness of sheet metal components while at the same time requiring their joints to be stronger. Particularly with regard to joining methods, there is a demand for cost-optimised high-tech solutions.
The screw heats the sheet metal by a combination of axial force and relatively high driver speed. After the screw penetrates the material, the trilobal shape causes a metric thread to be formed in the host material, which can accept a standard metric screw if repairs are necessary. After controlled-torque assembly, the formed hole adapts precisely to the shape of the screw. This assembly process eliminates the necessity for additional securing components, as well as the need for various component preparation steps such as drilling or punching. Fabrication options with or without prefabricated holes in the clamping plate are also possible. The decisive factor here is the thickness of the component to be clamped.
Compared with previously used standard self-tapping screws, Flowform® screws have considerably higher pullout and preload forces, as well as higher overtorque values. The special triangular cross-section of the fastener results in considerably better performance than conventional systems with a circular cross-section. Compared with them, the triangular Flowform® screws achieve a significantly larger delta between the thread-forming torque and the tightening torque (Figure 2).
This results in considerably higher process reliability for threaded fastener joints because drivers can be operated with a larger cut-off window. In addition, users benefit from a wider range of driver options and significant simplification of assembly process monitoring.
Flowform screws also offer considerable advantages over alternative sheet metal joining methods, such as gluing, riveting, clinching or press-in components. The tremendous advantages of this new product are especially apparent with regard to the criteria of ease of loosening, accessibility, positioning, reassembly with threaded fasteners, freedom from fabrication residues, and the possibility of eliminating preparatory processing operations.
Several years ago, Arnold Umformtechnik developed its Flowform® fastening solution and established it on the market to meet the needs of an increasing number of metal joining applications. Now the company is introducing the next step in its development.
Flowform®, the flow hole and threadforming screw, facilitates single-side access and fully automated joints. With its polygonal tip geometry, the Flowform screw forms a flow hole and taps a thread. This thread is able to accept a metric screw if it ever needs repair.
In Flowform® Plus – an enhancement to the original Flowform product – the company is now reacting to customers’ increasing specification requirements.
While needing to consistently reduce CO2 emissions, the automotive industry is also striving to reduce the weight of its vehicles. Increasingly, thinner and stronger steels or aluminium are being used, and the use of mixed materials is also on the rise. Even with these changed parameters, the screws must also be able to guarantee a reliable fastening.
One customer approached the Forchtenberg-based fastener experts with such a request. In short, it needed to improve and enhance Flowform to meet these new requirements.
Lightweight structures require positive weight reductions
“With the original Flowform, the limits on joining steels depended on the thickness of the steel at a maximum of 600 MPa, while with Flowform Plus, the limit is 1,000 Mpa,” said Heiko Miller, development project manager for Flowform Plus at Arnold Umformtechnik.
With Flowform Plus the Arnold development team changed the dimensions of the fastener from 5mm diameter with a length of 20mm, to 4mm diameter and a length of 20mm. Depending on the head size and length the original Flowform 5.0 weighs around 4g, while the new Flowform Plus 4.0 weighs around 3g. That represents a 25% weight saving. When you extrapolate this to the total number of screws used in the car body it adds up to quite a lot of weight saved. The weight of 500 original Flowform fasteners per vehicle is around 2kg, while 500 Flowform Plus 4.0 fasteners in a vehicle weigh 1.5kg. That alone saves 500g.
Moreover, the Flowform tip geometry has been improved and the heat treatment process adapted, so that fastening performances are almost the same.
Fastening point validation secures the future
So when is the Flowform Plus the appropriate fastener? “Customers are usually pretty specific about what they require for a particular fastening point. Our role is to investigate whether the better solution for the specific application is the Flowform or the Flowform Plus. So, for example, in our metal joining laboratory we carry out feasibility tests, using original materials.”
“This is where we determine the strength values of the compound structure, and of course the failure point limits. The joining point investigations go into somewhat more detail. In this case customers have already defined the materials pairing they wish to use. Customers also need us to make a recommendation with specific strength values achieved for the joint,” explained Nadine Schmetzer. Since 2017 Nadine has occupied the position of research and development for metal joining technology, which includes Flowform Plus. Over the years Arnold Umformtechnik has built-up a wealth of knowledge and expertise of such investigations, and customers are delighted to avail themselves of it. The new metal joining laboratory at Dörzbach, part of the research and development section, contains a variety of different test rigs, as well as all the standard plant technologies, including a robot cell for close-to-series trials.
Typical applications for Flowform® Plus
In principle the Flowform Plus has been designed for higher strength steels and thicker metal combinations. “With the use of aluminium components, some joints are now thicker, consisting of several layers. Previously, a three layer fastening meant that the top and centre layers had to be pre-drilled. Now, depending on the joined materials, using Flowform Plus means that thicker material combinations of up to 7.5mm can be joined without pre-drilling,” explained Heiko.
This is because the reduced diameter causes less material penetration. So there is a smaller gap between the metals and with the smaller dimension, there is less friction surface. So the tapping torque is lower than for the standard Flowform 5. Since the torque levels generally comes out lower, the fastening can also be tightened to a lower tightening torque. According to Arnold Umformtechnik, the fastening strength is not quite as good as with the Flowform 5, but it is good enough for the application.
As far as the fastener itself is concerned, a higher strength can be achieved by selecting a suitable material, matched with the appropriate heat treatment process. With the smaller diameter of the Flowform Plus, and therefore its smaller head diameter, the screw is also suitable for use on narrower flanges.
Arnold’s developers gave an example to demonstrate what is possible with Flowform Plus. “We joined a three layer metal join consisting of 2.5mm thick aluminium sheets using the standard and also the new Flowform fastener. The result clearly showed less gap formation with the Flowform Plus. Moreover, the Flowform Plus join was made without pre-drilling. For applications using adhesive, which is the normally the case, by omitting pre-drilling, the adhesive is contained, so cannot escape into the hole,” continued Heiko. “Making a join without pre-drilling is much better for process
cost-efficiency. Users do not need to install expensive optical technology to check that the screw is centred on the hole during the joining process,” continued Schmetzer.
Uses for the new Flowform include aluminium-steel mix car body frames, intensive closed section profiles, single-sided access applications, hybrid fastenings, and battery packs.
Many savings over the entire process
With the smaller head diameter of Flowform Plus it is possible to work with the customer at an early stage in order to adapt the component’s engineering design. For example, flanges can be designed to be narrower, creating a corresponding weight saving.
With its lower material penetration, less axial force is required for similar material combinations compared with the standard Flowform. This means that the process work load can be reduced. So robots can be correspondingly smaller. With statically stable robots there is less load on the screw during the joining process. “A further advantage is that users can continue to use existing equipment for the screwdriving process. There’s no need for major conversions,” said Nadine.
It is important to the Arnold developers that they create the optimum fastening solution for their customer’s component. Which is why customer service is just as important as liaising with the equipment suppliers. “Early communication with all the parties to the process is ultimately the key to implementing the best and most cost-efficient processes,” explained Heiko.
Flow-Drilling Screws Help Carmakers Shed Weight
At first glance, the Ford F-150 and the Chevrolet Corvette would appear to have nothing in common, beyond the fact that they’re both gas-powered vehicles. After all, you wouldn’t tow a bass boat with a Corvette any more than you would zip a 5,000-pound pickup around a test track.
However, the same small component plays a key role in the design and manufacture of both vehicles: the friction-drilling screw.
That’s because the bodies of both vehicles are largely made from aluminum, which helped reduce their weight significantly. For example, the aluminum frame of the 2014 Corvette is 99 pounds lighter and 57 percent stiffer than the previous-generation steel frame. The 2015 F-150 is 700 pounds lighter than its predecessor.
Faced with the need to join aluminum to aluminum and aluminum to steel, GM and Ford have been forced to find alternatives to the tried-and-true spot welding technology they had been using for decades to join all-steel assemblies. Friction-drilling screws are one such alternative.
Although friction-drilling screws have been around since the late 1990s, the fasteners are increasing in popularity due to the continued emphasis on “lightweighting” in the auto industry; a greater familiarity with the technology; and advances in the equipment for installing them.
“The early adopters in this country are now on their second or third programs [with friction-drilling screws],” says Jim Graham, president of Weber Screwdriving Systems Inc. “In Europe, the technology has been on solid footing with automotive OEMs since 2002.”
Besides the Corvette and the F-150, friction-drilling screws are being used in many other vehicles, including the Cadillac CT6; Acura NSX; Mercedes-Benz SLS; Audi TT, A4, A6 and A8; Porsche 911 and Boxter; Lotus Evora; Jaguar XK; Ferrari California; and Lamborghini Gallardo. Auto parts suppliers are also taking advantage of the technology. For example, Hella is using friction-drilling screws to assemble its Bi Xenon headlamp modules.
The fasteners are also seeing interest from manufacturers of white goods, aircraft, fabricated metal products, and buses and heavy trucks.
Going With the Flow
A friction-drilling, or flow-drilling, screw is a self-piercing and extruding fastener for joining layers of sheet metal. Combining the properties of friction drilling and thread forming, the screw acts as both a fastener and a drilling-and-tapping tool. It penetrates the layers, extrudes a short boss, forms its own threads, and applies clamping force between the sheets.
The fastener has a wide, flat head; a relatively thick shank; and a pointed tip. The head can be designed for external or internal drive systems (including hex head, TORX, TORX plus and cross recesses), and the bottom surface of the head can be undercut. The shank is divided into three zones: a pointed, unthreaded tip (for drilling); a short partially threaded midsection (for thread-forming); and a fully threaded upper section (for applying clamp load).
The installation process has six distinct steps: heating, penetration, extrusion forming, thread forming, screwdriving and tightening.
“It takes 2 to 3 seconds to drive one of these fasteners,” says Boris Baeumler, applications engineer at DEPRAG Inc. “In that time, we change the driving parameters four times.”
In the heating step, the tip of the fastener is pushed against the material with high force and rotated at high speed. Friction between the screw and the material heats the surface to 150 to 250 C, depending on the materials and how thick they are.
“At the beginning of the cycle, you want a lot of down-force to help generate friction,” says Baeumler. “With aluminum, you need less force, say, 1,500 to 2,000 newtons. With steel, it might be 1,800 to 2,500 newtons.
“Driver speed is also important. Aluminum tends to dissipate heat quickly, so we run at a higher speed, say, 6,000 rpm. With steel, we tend to run at slightly lower speed, perhaps 4,000 rpm.”
As the materials heat up and get soft, the fastener starts to penetrate the stack and create a hole. The material extrudes up and down along the points of the screw, forming a boss.
“As soon as the fastener penetrates the material, we reduce the down-force,” says Baeumler.
The fastener continues to penetrate the material stack until the tip penetrates the bottom of the stack. The conical geometry of the fastener helps to extrude a short boss on the bottom side of the stack.
Next, female threads are created in the extrusion by the thread-forming zone of the fastener. This step is performed at a lower speed—approximately 2,000 rpm. After the threads are created, the screw is threaded into the newly created nut member until its head seats against the top sheet. This is performed at 200 rpm, to avoid damaging the newly created threads.
Finally, the fastener is tightened to a preset value.
“We monitor all the variables—torque, speed, thrust and fastener depth—all the way through the process, and we feed that data back to the motion controller,” says Graham.
Compared with spot welding, clinching, self-piercing rivets and conventional threaded fasteners, friction-drilling screws have many advantages. For starters, they are installed from one side of the assembly. Access to the opposite side of the assembly is not necessary. That’s a real advantage, since the C-frame tooling for clinching or riveting can be difficult to maneuver with a robot.
In addition, friction-drilling screws create strong joints. Tests on high-strength sheet steel indicate that friction-drilling screws produce joints with greater peel strength than spot welding, clinching, self-piercing rivets. Tests of sheer strength show that friction-drilling screws performed at least as well as clinching and self-piercing rivets and only slightly less than spot welding.
Because the fastener drills its own hole, there’s usually no need to punch or drill holes in the materials or to align the holes prior to assembly. There is a caveat, though. As the fastener is forming a boss, most of the material flows out, toward the bottom of the stack, but a slight amount also flows up, toward the driver. A recess under the head of the fastener is often enough to capture this material. However, if the layers are particularly thick, if three or more layers are being fastened, or if one of the materials is incompatible with the process, a “clearance hole” may be needed in the top layer. This will prevent the formed boss from creating gaps between layers.
“You need to provide a place for the material to flow,” says Baeumler.
Another advantage of friction-drilling screws is their higher drive-to-strip ratio compared with thread-cutting screws, especially with thinner sheet metal. “With thin sheet metal, especially steel, it’s common to strip out self-drilling screws,” Baeumler points out. “The difference in torque between driving a screw and stripping it is extremely small. Friction-drilling screws form out the material, so there’s more material for the screw to engage, and the torque window is a lot bigger.”
And, like conventional threaded fasteners, friction-drilling screws can be removed and replaced for serviceability. There are no issues regarding thread tolerances, because the fastener creates and engages its own threads at standard pitches. But, unlike thread-cutting screws or drilling-and-tapping operations, friction-drilling screws do not create waste. No chips are generated during installation.
Friction-drilling screws cannot be installed with just any screwdriver. Because there are additional variables beyond torque and angle to measure and control—driver speed, axial force and fastener depth—friction-drilling screws require sophisticated driving technology and are typically installed with fully automatic equipment. The driver can be mounted to a linear actuator or, more typically, a six-axis robot.
Installing friction-drilling screws “is not like traditional screwdriving applications, where the fastener is a commodity item,” says Baeumler. “When you specify these fasteners for a project, you need to consider the equipment at the same time, even at the design stage.”
Parameter settings, such as driver speed and axial force, vary depending on the thickness of each sheet, the number of layers, the properties of each material, surface treatments and overall joint requirements. Thorough testing on coupon assemblies is imperative.
Another difference between standard automatic screwdrivers and equipment for friction-drilling screws lies with how the fasteners are fed. Despite their relative bulk, friction-drilling screws require gentler handling than ordinary screws. That’s because the tip, which initiates the friction, is critical for the application. Dumping the screws into a vibratory feeder is not an option.
To solve that problem, Weber uses a step-feeder to deliver the fasteners, while DEPRAG uses its patented sword feeder. DEPRAG takes the additional precaution of blowing the fasteners head-first through the feed tube, instead of point first. A revolving mechanism in the drive head flips the fastener for installation. As a side benefit, the extra revolving step ensures that there’s always a screw ready in the drive head for the next cycle.
A high-volume automotive body line can consume a lot of fasteners, so reliable feeding is critical. Screwdriver suppliers have solved that challenge a couple of ways.
Weber developed a dual-tube feeding system. Two feeding tubes run to the drive head. If one tube gets jammed, the driver automatically swaps to the backup tube.
DEPRAG has developed a different method. Rather than blow the fasteners to the driver—and add one more tube to the robot’s dress-out package—DEPRAG can supply the drive head with a swappable, reloadable magazine. Each magazine holds 30 screws. While the robot is installing fasteners from one magazine, a standalone feeding station is reloading another.
Yet one more difference between standard automatic screwdrivers and equipment for friction-drilling screws is in the jaws on the drive head. Screwdrivers for friction-drilling screws have “active jaws” rather than spring-loaded jaws, explains Graham. The powered jaws hold onto the screw until the tip has correctly engaged the material. In the past, the screw might have fallen over.
Early this year, both Weber and DEPRAG will release new versions of their friction-drilling screwdriving equipment.
Graham says the third generation of Weber’s equipment for installing friction-drilling screws is “a quantum leap” from previous iterations, offering faster driver speeds (as much as 11,000 rpm), higher axial thrust, and greater control over the installation process.
“Now, we can accommodate different screw geometries,” says Graham. “We can fasten steel alloy sandwiches, steel-carbon sandwiches, or any combination of materials.”
If a six-axis robot will be used to install the fasteners, engineers are well-advised to choose a model that can hold up to the forces involved.
“Once the screw is in position, the robot must provide a very solid backstop so the driver can generate the necessary downward thrust,” says Graham. “The last thing you need is to start pushing on the assembly and have the robot move away.”
To that end, robot suppliers are stepping up to the plate. For example, FANUC America Corp. recently introduced its new M-900iB/280 robot at the Fabtech show last November in Chicago. The robot was shown simulating the installation of friction-drilling screws on an automotive door panel assembly.
The robot’s casting shape has been optimized to provide enhanced arm rigidity compared to previous models. The robot has a 2.65-meter reach and a compact wrist with heavy payload capacity. FANUC engineers enhanced the allowable load inertia at the wrist so the robot could handle heavy objects with stability.
As with any automatic screwdriving application, engineers need to provide room for the tooling to access the fastening location. A flat, open area with at least 10 to 12 millimeters of clearance is good. Room should also be provided on the back side of the assembly to accommodate the formed boss and the projecting screw point. Friction-drilling screws cannot be installed in a solid block of metal.
If a six-axis robot will be used to install the fasteners, engineers need to provide enough room for the robot to access the driving locations. Engineers should allow clearance for the driver and tooling, as well as for the cylinders that apply axial force during fastening.
“You can’t drive these screws in spots where you have close vertical restrictions,” says Baeumler.
The part itself—and any fixturing—should be designed to accommodate the axial force applied during fastening.
“With thinner materials, we can reduce the down-force [to avoid crushing the part],” adds Baeumler. “However, there’s a trade-off. Friction will take longer to heat the part, so cycle time will increase.”
Finally, it’s also worth noting that friction-drilling screws are not the sole answer to joining structural aluminum components. Indeed, multiple technologies are used to assemble the F-150’s body-in-white, including self-piercing rivets, structural adhesives, clinch joints, spot welds, laser welds, friction welds, and good old-fashioned nuts and bolts.
The hair is raised and slicked back like a horse's mane. Not surprising since he has business with Alex, Stick chuckled. Do you think there will be problems.FlowForm Screw Demo - Aluminum Repair // Démo des vis FlowForm - Réparation de l'aluminium
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