Stitchfolding and Clinching – assembly techniques for the 21 st century
Clinching and stitchfolding are methods for joining sheet material and profiles without rivets, screws or other added fasteners. These assembly methods are technically and economically attractive. Additionally they are ergonomically and environmentally sound. This means increased productivity without health hazards. The quality of a clinched joint can be controlled at any point in time without destroying or disturbing the assembled structure.
Clinching works perfectly with pre-coated or galvanized material as well as with steel+aluminum combinations and gives a finished assembled product without pre- or postwork . This is typically the situation in the automotive, appliances and light gauge steel construction sectors. Clinching can also be combined with sealant or intermediate layers acting as sound dampers. Clinching does not build any thermal stresses into the workpiece which gives a clinched joint an exceptionally good performance in situations of thermal fatigue or fire.
The fact that the joint is created from the base material itself, without any addition of alien parts or alien material, means that a clinched structure can be recycled in the simplest and most straightforward way without any complicating separation processes.
Stitchfolding and clinching are simple and cost-effective methods for sheet material assembly. They do not require any rivets or screws or other separate fasteners. They do not require any complex systems for high power and cooling which are typical for spot welding installations. The reason is that clinching and stitchfolding equipment generally are driven by simple compressed air or from the mains.
Stitchfolding , as the name suggests, literally stitches material together, very much like we do when stapling paper. But in contrast to stapling, the stitchfolding method generates its own staples from the base material.
Clinching generates a rivet-like joint from the sheets to be assembled through plastic cold flow in a punching and squeezing sequence.
First punches cut out tabs of the overlapping sheet material, Figure 1. Then these tabs are folded back to give a staple-like joint from the base material itself .
There are two main principles, single-stroke and double-stroke.
Single-stroke clinching – displacement controlled. In the first part of the process, (see our video) , the punch will deform the overlapping sheet material plastically inside the die cavity. The wall of the die, typically split in two or four parts, remains closed.
When the lower material sheet gets in contact with the anvil, i.e. the bottom of the die cavity, the material has to flow laterally which creates a mushroom shape which in the end will develop into a rivet. In this phase the die parts will be pushed outwards, sliding on a base, until the punch-anvil distance reaches its final pre-set value.
After the punch has been pulled back by the stripper and the die been disengaged, the die walls will be pulled together by a spring, thereby closing the die.
The result is a high-quality rivet in terms of shape, resistance and reproducibility.
The advantages of stitchfolding and clinching include:
- Simple and reliable non-destructive testing of the joint quality at any point in time
- No need for rivets or screws or other added consumables. No need for tracking the individual batches of fasteners as often prescribed by ISO 9000 or building code requirements. These factors alone often represent savings enabling the amortization of a stitchfolding or clinching installation over a few months
- Sorting, orienting and feeding devices for the fasteners can be avoided. No need for costly strip-mounted or magazined fasteners. Such devices are notorious sources of problems, which means that stitchfolding or clinching techniques give simpler and more reliable installations. This has proven to be of vital importance particularly in the automotive, the appliances and other high production rate sectors such as industrial paneling shops
- Short assembly cycles, typically 0.6-1.2 seconds. Compared to riveting or screwing, which may take 5-10 seconds per cycle, this represents even greater savings in the cost of manpower than the cost of the rivet or the screw itself in hand-held or semi-automated situations
- Possibility of assembling pre-painted, enameled , galvanized or heat sensitive materials without complicated preparation and without post-work touch-up tasks
- Superior fatigue strength and corrosion resistance over spot welded joints
- Possibility of joining material couples that are difficult to weld, for example aluminum and mild steel or stainless steel and copper. This opens a lot of new opportunities of combining structural and decorative elements or elements of different materials for optimum compound performance
- Possibility of handling a total material thickness from 0.004″ (0.1 mm) to 3/8″ (10 mm)
- Possibility of joining three or more layers or material members that do not have the same thickness such as sheet panels and profiles without any risk for burn-through
- Possibility of including intermediate layers acting for example as sound dampers in structures like floor joists
- Flat top surface without protruding head avoids gypsum board cracking risks
- Less risk for hand injuries and secondary damages from smooth back surface and no sharp screw tips
- Integrated clamping means no gap, no deformation and no burrs
- No need for long and costly training of operators. In fact using a hand-held clinching device is as straightforward as using any standard power tool
- Simple compressed air from an ordinary factory net, standard mains or battery as prime mover, instead of complex and costly transformers, high current electrical systems, cooling systems and ventilation installations
- No strong electromagnetic fields that may be dangerous for operators of equipment involving high currents. Even though it may be too early to draw definite conclusions concerning the potential dangers, a way of avoiding gambling with operators health is to choose the clinching and stitchfolding techniques which rely on environmentally sound and safe compressed air as prime mover
What a clinch joint looks like
Depending on the exact configuration of punch and die, the clinch joint will take different shapes. The two most common types are round, Figure 3, and rectangular, Figure 4.
Key parameters of a clinched joint
Clinching points have a high repeatability. Their quality can be easily controlled thorugh elementary measurement operations.
For the round point the control of the diameter of the lengthen is realized with a callipers. The value obtained has to be compared with the preconceived value reported on the Instructions Manual of the machine depending on the diameter of the punch and on the thickness of sheet metal.
For the trapezoidal point it is important to control the diameter of the point that is realized in the inferior part of sheet metal. This value has to be compared with the predetermined values included in the Instructions for Use of the machine depending on the width of the punch and on the thickness of sheet metal.
From the above can also be seen the importance of the orientation of uneven material thickness, often expressed as the rule Thick-in-Thin , meaning that whenever possible the thickest layer should be on the punch side.
Jurado Clinching tools gives advice and supplies easy-to-read tables guiding the tool kit selection process. As an example, under normal operating conditions, one and the same tool kit will cover assembly tasks ranging from 2×22 gauge (2×0.75 mm) to 2×18 gauge (2×1.25
mm) without changes or adjustments.
Non-destructive testing of a clinched joint
Once the tool parameters have been selected and optimized and the corresponding residual bottom thickness value ST corresponding to a certain clinching force has been established, this ST-measure can be verified systematically, offering a non-destructive quality control.
As a simple rule of thumb, the ST-value of a good quality round clinch joint is typically 1/3 of the total material thickness, and about 1/2 of the total thickness for a rectangular joint.
Strength of a clinched joint
The strength of a clinched joint depends essentially on four major factors:
- The material type. A joint in steel will be stronger than one in aluminum
- The material thickness. A joint of 2×14 gauge (2×2 mm) will be stronger than one of 2×20 gauge (2×1 mm)
- The clinch point size. A Ø 5/16″ (8 mm) joint will be stronger than a Ø 3/16″ (5 mm) joint
- The material surface condition. Generally speaking a perfectly dry surface will give a stronger joint than if it is oiled or greased. However, in steel these effects are relatively minor while they can have a considerable influence in aluminum
Static strength of a stitchfolded joint, indicative values
A stitchfolded joint in 2×20 gauge (2×1 mm), mild steel, has a shear strength, perpendicularly to the main axis of the joint, of some 270 lb (1,200 N).