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Cracks in semi-tubular rivet clinches: can they be eliminated?

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Cracks in semi-tubular rivet clinches: can they be eliminated?

Often in the riveting industry, rivet and riveting equipment manufacturers are faced with a specification requiring crack-free clinches of semi-tubular rivets. While a desirable goal, the manufacturer of the rivet or the rivet clinching equipment is unable to guarantee 100% elimination of this condition.

Most cracks can be avoided by both the rivet and equipment manufacturer through proper quality assurance measures in the following areas:

Rivet manufacturer

1. Visual inspection of all wire to assure that all material accepted is within standard industrial specifications for surface imperfections and/or cracks.



2. Twist test of all incoming material to test for seams.



3. Maintenance of cutting tools in the header (quill and chisel) in a good, sharp condition to reduce the step at the interface between the shear and brittle fracture of every cut-off blank.



4. Quality assurance inspection of concentricity of rivet hole to shank as a characteristic critical to product quality.



5. Where it does not interfere with performance of the fastener in the customer's product, provide fasteners with a wax coating (transparent and, usually, water soluble) to act as a lubricant between the rivet and setting tool (anvil) surface.

Riveting equipment (or tooling) manufacturer

1. Where spring loaded pin type anvils are used, manufacture the anvils with proper discontinuity (pin height offset from anvil body height in forming position) to assure best possible clinch quality and anvil life.



2. Produce anvil forms with sufficiently 'tight' roll radii to limit cracks, yet not so tight as to cause "stalling" of the rivet clinch.

Will this eliminate all cracks in the rivet clinches?

No, but it will reduce the number of cracks to a minimum. Some cracks are caused by seams or cracks in the wire from the steel companies. Seams come from the initial pouring of the ingot. As the ingot cools down a crack may appear in the side, and this crack is drawn down from ingot to billet to rod to wire to fine wire, etc. There is no way to remove the crack other than turning the initial billet down below the surface irregularities. (This is sometimes done in brass, but for most types of rivet cold heading materials, it is not done.) Rivet manufacturers are not major accounts in the steel market, so threatening to re-source their purchases is not of significant consequence to them.

Rivet manufacturers can reject material when the seams are large, however, there is a specification allowing a small percentage of the wire diameter. Material with seams within tolerance is not rejectable and there is a strong possibility that there will be seam type cracks in the clinches when the rivet is rolled.

Cracks can also be caused by lack of or poor maintenance of the rivet setting equipment and setting tools. In impact riveting, since the rivet strikes and rolls against the anvil form surface, the anvil (also called roll set) can be expected to wear and require regular replacement. The anvil is a vital part of the proper setting of a rivet and both the stocking of replacement anvils and an inspection program are strongly recommended.

Is there an acceptable standard for cracks in rivet clinches?

An automotive component manufacturer had a problem with leakage in their fuel sender assemblies and tried, for years, to achieve crack free clinches unsuccessfully. Consequently, they developed an internal specification that states, "No more than three (3) cracks in 360 degrees, or more than two (2) in 180 degrees, and these cracks cannot travel up and over the clinch and down into the unclinched hole.

Another manufacturer established the following criteria for acceptance of a semi-tubular rivet clinch for use by their machine set-up and assembly line inspection personnel:

ACCEPTABLE	NOT ACCEPTABLE
1. Up to three (3) cracks.	1. More than three (3) cracks.
2. No more than one crack per ninety degree quadrant.	2. More than one crack per ninety degree quadrant.
3. All cracks hairline.	3. "Slice of pie" type cracks.
4. All cracks must be radial.	4. Cracks are not radial.
5. Cracks do not extend into solid or 'hole' portion of rivet shank.	5. Cracks extend into solid or 'hole' portion of rivet shank.

As manufacturer of the assembled product, you are in the best position to evaluate the effects of clinch cracks in relation to performance of your product. Considering that a reliable "across the board" standard cannot be formulated, we urge you to establish an acceptable, but feasible, standard that will allow proper function of your product.

If, after failing to achieve acceptable results per the above with impact riveting, perhaps orbital riveting should be considered. Typically, orbital riveting can produce better quality rivet clinches though rivet feed must be approached different than with impact equipment.

It is our hope that this material will provide you with a better understanding as to what industrial manufacturers of quality rivets and rivet setting equipment can provide in addition to providing assistance toward the development of acceptable quality standards for your assembly process and product.

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