Active thread cutting and self-tapping screws2017-02-02

The AST11 controller relies on the “friction torque value program” to accurately calculate the friction torque and apply a constant clamping load when driving tangential or thread forming fasteners. Photo courtesy of DEPRAG Inc.
The Rolok thread rolling screw is characterized by a thread with three asymmetric lobes that form a pair of threads. Image courtesy of Semblex Corp.
The HET handheld electric screwdriver can provide a torque of 0.1 to 30 Newton meters. Image courtesy of Weber Screwdriving Systems
The Polyfast thread forming screw has a 45-degree asymmetrical thread, can provide a narrow shank and a 26-degree tip, simplifying the installation process. Image courtesy of Semblex Corp.
The thread of the FLOWpoint Delta PT thread forming screw has a 30-degree side surface, which can produce high tensile strength, torsion strength and clamping load, but the radial force is small. Image courtesy of EJOT USA
Feeding tangent, thread rolling and forming screws is often a challenge, especially those with profiled heads and shorter lengths. Image courtesy of Design Tool Inc.
Movies and TV shows will continue to improve the abilities of the protagonists, who can complete tasks with little or no help from others. In the field of fastening, self-tapping screws are more suitable for this role than any fasteners because they can form mating threads in metal and non-metal substrates. By eliminating the tapping process, these fasteners (thread cutting, thread rolling and thread forming screws) can help manufacturers save costs and increase production.
Gene Simpson, vice president of quality and engineering at Semblex, explained: “The performance of self-tapping screws was much more important when self-tapping screws were first introduced in the early 20th century.” The design provides manufacturers with high performance in every application. Joints, including those involving mating parts made of engineering plastics and light metals (such as magnesium). ”
Nidec America Corp., one of the world’s largest electric motor manufacturers, has benefited from self-tapping and self-drilling fasteners for more than a decade. At the beginning of this century, Nidec’s engineers faced challenges related to assembly, which involved joining aluminum radiators (stamped or extruded) with heat transfer pads, and such rivets could only be supplied by one company worldwide Quotient obtained.
The workers put the rivets into the mandrel and then installed it in a special rivet gun. However, spray guns require frequent maintenance, and it usually takes several weeks for suppliers to repair and return the spray gun. Another disadvantage is that the cost of each rivet is 11 cents.
In just a few months, Nidec management met with Arnold Industries, an industrial fastener expert, to discuss alternatives. For many reasons, Arnold recommends using #5-40 Taptite II thread rolling screws. Fasteners only cost 4 cents, the cycle time is the same, and the installation requires the use of a standard pneumatic screwdriver. In addition, the large-diameter head of the screw can provide a larger, more uniform clamping load and higher heat transfer efficiency.
Since the early 1990s, as OEMs have increasingly used self-tapping and self-drilling fasteners, thread rolling and thread forming have made considerable progress in a relatively short period of time. One reason for the growth is that fastener suppliers have specifically developed screws for high-performance plastics and metals. It is also important that the assembler can use a variety of equipment to properly install these fasteners. From hand-held screwdrivers to automated systems, they always apply the appropriate torque to form a perfectly matched thread and tight joint.
Tangent, thread rolling and thread forming screws have similar composition. They are made of carbon, stainless steel or aluminum, have a tensile strength of at least 100,000 psi, and have relatively high torsional strength. The screw can adopt any drive head design and can be used in a variety of materials.
Each screw rod has an asymmetrical thread, which generates opposite forces during installation to form a mating thread. Since these threads match the size of the installed screws, there is no gap between the material and the screws, thus realizing a dynamic and safe connection. Both thick and thin threads can be used, but for weaker materials, the former is recommended.
What makes these screws different is how they handle the material. The shank of the thread-cutting screw has a slotted point that cuts the material and forms a thread when pushed down, thereby generating chips that fall out of the hole. The shank groove can minimize the generation of chips. Some screws have recessed points similar to drill bits, designed to penetrate hard substrates.
These screws are usually used in wood, metal and plastic products that require regular maintenance, such as outdoor work bench seats and power tools. For plastic applications, screws perform best in thermoset materials with a flexural modulus of 500,000 to 2 million psi.
Simpson said that the rolling screws were introduced in the 1960s as a non-chip replacement for rolling screws in sheet metal applications. They have tapered leads and require the fastener to be harder than the mating material. During the installation process, the fixed roller will match the material into a thread shape.
Manufacturers in many industries choose these screws because they prevent cross threads, resist vibration, and are reusable. Another attraction is versatility. These screws can be used for aluminum, magnesium, plastic and sheet metal, and some suppliers provide threaded bolts.
Thread forming screws produce threads in a similar manner, although they tend to form materials faster and more efficiently. These screws have blunt ends and may or may not require pre-made holes. They are usually used in metal plates, plastics and composite parts that require high clamping loads to prevent loosening. Certain screws can be removed and reinstalled multiple times, or replaced with standard metric machine screws.
The largest users of thread forming screws are manufacturers in industries that use large amounts of high-performance thermoplastics, such as electronics, automobiles, agriculture, and office equipment. In these applications, the flexural modulus of the material is 150,000 psi to 400,000 psi. White goods and furniture manufacturers also often use self-tapping screws.
Semblex’s Rolok thread rolling screws and Polyfast threaded screws are available with or without blades. The Rolok thread has three asymmetric lobes that form a mating thread in the nut member. This design provides high performance by providing low driving torque and high peel torque in various materials and configurations (such as C-shaped extrusion).
The Polyfast screw has a 45-degree asymmetric thread, which provides a narrow shank and a 26-degree angle. The wide-angle thread can cut the material cleanly, minimizing material deformation and cracks, and the tool holder requires low installation torque, high peeling torque, and easy installation of the pointed tip.
Aware of the increasing use of carbon fiber reinforced plastics, EJOT developed the FLOWpoint Delta PT threaded forming screw, which does not require pre-made holes. The screw thread has a 30-degree side surface, which can produce high tensile strength, torsion strength and clamping load, but the radial force is small, which can ensure the integrity of the boss. The screws are available in steel, stainless steel, aluminum and titanium, perform well under high-stress vibration conditions of plastic components, and are easy to disassemble and reassemble.
In recent years, manufacturers have increased the use of self-tapping screws and rolled screws because they can effectively join different materials (such as steel and aluminum) without spot welding. Another reason for this change is that these screws have excellent joining properties in lightweight materials.
“With the increasing use of new materials, there is a challenge to decide whether to use thread cutting or thread forming fasteners for each application,” said Jim Graham, president of Weber Screw Drive Systems. “Design engineers need to solve four things to ensure the correct selection of fasteners: material properties, joint type, required fixture load and repairability of connected parts.”
A common but often overlooked problem associated with installing thread forming screws in thermoplastics is plasticization or softening of the material. This is especially true when installing fasteners in the system at 1,000 rpm or higher. The appearance of the plasticization is that the torque control system correctly installs the screws with a weight of 20 in-lbs, but a phenomenon called joint slack will soon appear. In a few seconds, minutes or hours, it will cause the initial torque in the joint to drop sharply. Sometimes up to 50% increase (in this example, 10 inch pounds of net weight).
Graham said that the plasticization problem is also challenging for value-added molding companies, which install screws in warm parts that have just been removed from the mold. Aware of this, some manufacturers use fasteners with double helix (high-low) threads, where the high threads are sharper than the regular threads. This design clamps more material between the threads, thereby increasing the peeling torque and allowing a lower main torque during driving.
Sometimes it is not easy to identify installation problems related to self-tapping screws or self-tapping screws. Cecil Morgan, an applications engineer at DEPRAG Inc., cited a recent example of a major medical device manufacturer experiencing a high failure rate for stainless steel threaded fasteners used in aluminum parts. The problem is that the screw is not fully secured in place, causing the joint to be reworked.
Preliminary tests showed that the friction torque levels were inconsistent. Some joints have a level higher than the specified tightening torque, while other joints only reach 40% of the tightening torque. After further analysis, the manufacturer found that the hole diameter in the part was slightly inconsistent, and the amount of thread locking agent on the fastener was also inconsistent.
To overcome these problems, the company began to use DEPRAG’s Minimat EC fixed electric screwdriver with the AST11 controller. The software routine “friction torque value program” in the controller calculates the friction torque during fastener installation and applies a consistent clamping load to the joint.
The company also provides an AST40 controller with clamping force control (CFC) for its Minimat EC-Servo screwdriver. CFC is an adaptive technology that eliminates all friction effects until the head is fixed in place, thus ensuring that the fastener provides a consistent clamping force.
“Because these screws usually require a torque equal to or greater than the final tightening torque in rolling, forming or cutting operations, it is very difficult to ensure a strong connection,” said Kevin Buckner, Director of Design Tools Engineering at Design Tools. )recognition. Automatically feed and drive all types of fasteners. To solve this problem, we often use a DC screwdriver equipped with a sensor to install these types of screws. The DC drive allows multiple tightening steps during the screwing process to ensure that the thread cutting has been completed and the final required torque value is within tolerance. ”
Simpson said that feeding these screws is often a challenge, especially those with profiled heads and shorter lengths. His suggestion is to use designs with defined side walls, such as pan-head screws, whenever possible. Another challenge is to convince customers that they need to focus more on total assembly costs instead of parts costs, and work closely with fastener suppliers throughout the fastener selection process (including joint design).
Simpson explained: “Traditional fastener methods are often used where future cost savings need to be considered, which is different from the cost avoided at present.” “Because fasteners used to work on many parts, the company immediately hopes Use it in new parts as well.” But what if the parts are installed in the new plastic that was just used? Maybe there is a better option. ”
Graham said that screw rejection rate and screw reusability are important factors to consider. The installation success rate of self-tapping screws exceeds 99%, and the installation success rate of self-tapping screws is usually about 85%. The lower number is due to the possibility of cutting screws and damaging the parts in the final stages of manufacturing, which is quite expensive.
Regarding reusability, thread forming or rolling screws can be reused many times because they produce high-quality threads during the initial installation process, thereby ensuring zero clearance in the mating threads. However, since the cutting screw forms another thread in the material each time it is installed, it is usually only used once or twice. This may permanently damage the component, prevent it from working properly, or make the screws difficult to remove.
“Unlike thread rolling, thread cutting also produces contamination during the cutting stage, which is not always satisfactory,” Graham points out. “This is why cutting screws are not recommended for parts or components that need to be opened occasionally for repair or maintenance, such as the lid or housing of a vacuum cleaner or other consumer products.”
By pairing Weber’s C30S advanced process controller with a motor driver and sensor kit, engineers can install and inspect thread rolling and tangent in core holes of injection molding, casting, extrusion and multi-material stacking (requires 0.01 to 60 Newton meters) fastener. Torque.
The controller can program up to 31 different tightening recipes or strategies to best fit any given joint configuration. In-depth review of torque, angle and local simulation is possible, and a large number of communication or bus interfaces ensure compatibility with most factory communication and MES systems.
The HET series handheld electric screwdriver is often used in conjunction with the C30S controller, which can provide a torque of 0.1 to 30 Newton meters. These screwdrivers can be used for installation and joint audits, and have the function of recording and drawing joint data when installing and removing fasteners to achieve a higher level of quality control.
In order to ensure the correct installation of the screws, Lori Logan, marketing manager of DEPRAG, stated that it is important for assemblers to use professional fastening tools instead of off-the-shelf pneumatic or electric screwdrivers. It is also important to perform routine tests, such as joint analysis after the fastener is damaged. The analysis will create a detailed torque map, allowing the assembler to determine the exact torque level at which the screw head is in place and the thread is stripped.
Jim is a senior editor of ASSEMBLY and has more than 30 years of editing experience. Before joining ASSEMBLY, Camillo was a PM engineer for the Facility Engineering Magazine and Milling Magazine Association. Jim has a degree in English from DePaul University.
The COVID-19 pandemic has a major impact on all industries, and the manufacturing sector has been hit particularly hard. As the industry resumed operations after it closed, leaders worked hard to create a “new normal.” Experts believe that automation and technology are the two key factors that help companies move forward successfully. The leader in automation suites is collaborative robots or “collaborative robots”, which are robots that are safe, easy to program, and cost-effective.

Post time: Aug-04-2020