- SPECIAL REPORTS
When spot welding or fasteners prove to be unsuitable for joining sheet metal parts, assemblers have another alternative.
Clinching is a high-speed, mechanical fastening technique for joining sheet metal from a total thickness of 0.1 to 10 millimeters. This inexpensive and easily automated process requires a punch and a die. The punch pushes the sheet metal into a die, forming a mechanical interlock. The result is a joint similar to a press stud.
There are two main methods of clinching—single-stroke (displacement-controlled) and double stroke (force-controlled). With single-stroke clinching, the punch deforms the overlapping sheet and profile material plastically inside the die cavity. This means the material undergoes a permanent change in shape or size. The die wall, which is typically split in two or four parts, remains closed.
When the lower material sheet contacts the anvil (the bottom of the die cavity), the material flows laterally. This creates a mechanical interlock. During this phase, the die parts are pushed outward, sliding on a base, until the punch-to-anvil distance reaches a preset value.
After the punch has been pulled back by the stripper and the die has disengaged, the die walls close again.
In double-stroke clinching, the punch will deform the overlapping sheet material plastically inside a rigid die cavity.
The anvil is locked mechanically, and the joint is created as the material is squeezed between the punch and anvil, outside of the rigid die, until a preset clinching force is reached.
According to Anna-Lena Bergkvist, vice president of marketing at Attexor Tools SA (Ecublens/Lausanne, Switzerland), the double-stroke technique is especially insensitive to variations in material thickness because the material squeezing occurs outside of the rigid die.
Why Clinch?Clinching is a viable substitute for mechanical fasteners and spot welding for several reasons.
Clinching requires no fasteners. Therefore, the cost of fasteners and equipment to feed them to the correct location is eliminated.
As a replacement for spot welding, clinching can join coated materials. Clinching is mainly used in the manufacture of white goods, heating and ventilating components, and automotive parts. These industries routinely use precoated, enameled and galvanized materials. Spot welding can destroy the protective coatings on these materials. However, clinching does not. Also, clinching requires none of the peripheral equipment needed for spot welding, such as high-current electrical and cooling systems, and ventilation installations.
Clinching can also join dissimilar materials. "You can join aluminum to steel, brass or copper; galvanized to a prepainted; or prepaint to prepaint. You can use prepainted material or galvanized material with a coating on it, and dissimilar metals," says Bob Kent, application salesperson for BTM Corp. (Marysville, MI).
Materials of different thickness can also be joined with a clinching system. However, to successfully clinch materials of different thickness, it is best to have the thicker material on the punch side. The thicker material must be at the top so that enough material can flow into the die cavity. Otherwise, the neck area will be very fragile. The thicker material should not be more than twice the thickness of the thinner material. The combined thickness of the two plies should not exceed the combined maximum thickness recommended for the die.
By the same token, if one material is considerably harder than the other, the harder material should be on the punch side. If the softer material is on the punch side, the punch will go right through the softer material, instead of deforming.
Maintenance is also limited with a clinching system. "It is a low-maintenance way of joining two pieces of metal together. Typically you get 250,000 cycles without any maintenance," says Kent.
Clinching does have its limits, though. "Clinching is more suitable for ductile material. If the material is more brittle, it doesn’t work as well," emphasizes Bruno Maczynski, engineering manager at Tox Pressotechnik (Warrenville, IL). Typically, when material ductility is greater than 20 percent, clinching can be used without any problems. That includes steel, copper and aluminum. By using special punch and die sets, it is possible to clinch material with lower ductility.
The strength of a clinched joint depends on four major factors: material type, material thickness, clinch point size and surface condition. Generally, a joint in steel is stronger than one in aluminum. Also, a dry surface will provide a stronger joint than if it is oiled or greased.
Ensuring a Successful ClinchTo get the most out of a joining process, the parts to be assembled should be designed with that process in mind. Clinching requires open flanges with good access to both sides for punch and die tooling. The flange width must be sufficient to accommodate the interlocking button produced during clinching. The clearance between the center of the joint and the flange outer edge should be 1.5 Arial the punch diameter. Also, the clearance between the joint and the flange inner edge must be large enough to allow tooling access to make the joint.
Clinch joints should be spaced to avoid previously formed joints, as well as the immediate area around them. Clinching in or near prior joints may result in unsatisfactory joint appearance, excessive thinning of the bottom sheet and accelerated tool wear. Also, joints should not be placed so close together that the material distorts. However, there must be enough joints to ensure that the assembled component meets its strength requirements. A minimum joint spacing of two to three Arial the button diameter is recommended.
Accurate fit-up and alignment help form good joints by ensuring that the sheets are drawn together between punch and die in the correct manner. Joints should be fully closed after the clamping stage. Poor fit-up and alignment are major contributors to inconsistent clinch quality. "Alignment of the tools is very critical for clinching. They have to be perfectly aligned," says Maczynski.
Types of Clinching SystemsClinching systems come in all sizes and types of operations and speeds. Options range from handheld units to multihead systems with double-acting punch and dies, and self-centering heads.
Many portable or handheld units are cordless. Standalone units, which are similar to spot welders, require an operator. The operator stands in front of the machine with a foot pedal, holds the part and does all the joints.
Die set tooling can be installed in existing presses. The die set will have all the necessary tooling to complete the required job.
And as clinching does not require separate fasteners, robot-based clinching installations are attractive options.
Tox Pressotechnik also offers a unique process that is a secondary operation. Because clinching produces a protrusion on one side of the material, where the die is, that protrusion might be an obstruction. The secondary operation flattens the joint. It is called the Tox flat joint. The die is replaced with a flat punch that flattens the joint.
Monitoring Clinch QualityClinched joints result from interplay between the clinching machine, the material, and the punch and die. With the resulting clinch joint, the material has been geometrically changed in comparison to the original flat sheet metal. Therefore, the joint quality can be monitored by measuring the bottom thickness of the joint, or through the button width or diameter.
As Maczynski states, "With spot welding, you don’t have a very good idea of the strength of the spot weld, unless you do a chisel test and destroy the joint." However, a thickness gauge or go/no-go gauge can be used to provide a visual assessment of a clinched joint.
Tox Pressotechnik measures the remaining bottom thickness inside the joint with a caliper. The company also provides process monitoring systems that can be incorporated into the customer’s clinching process.
Process monitoring consists of measuring force and displacement of the punch, as joints are being made, and checking that the values of these parameters are being correctly maintained by the clinching equipment. Values are obtained using sensors built into the punch and die assembly.
The width of this acceptance range can be altered to suit the requirements of specific applications. Results outside the acceptance range normally indicate faults or variations in process operation or materials, which could lead to unacceptable joint quality.
BTM Corp. gauges joint quality by measuring the diameter of the button, which will determine the strength of the joint.
Source ListAttexor Inc.
Norlock Technology Inc.
Brantford, ON, Canada
Tox Pressotechnik LLC
TWI Ltd.World Centre for Materials Joining Technology