Head cracks are taking the industry by storm! Why is it that head cracks are so hard to control, inspect for, or even eliminate? First we should talk about what causes them. Sometimes head cracks are a result of asking the metal to do more than it is capable of (plastic deformation). This can result from poor material selection. Sometimes cracks are a result of a flaw in the raw material (often called a seam and sometimes called a lap). These flaws can be from the billet that the wire originally came from or they can be from the wire processor that draws and anneals the wire or they may be from the cold heading operation in the form of tool marks that slightly break the grain of the steel and create a starting point for the crack. If it is caused from tool marks at the cold heading operation, the defect will most likely be on all parts produced. Similarly if it is plastic deformation ( exceeding the maliability limits of the steel) the defect will most likely be on all of the parts. When all of the parts have the defect it is more easily caught by in process visual inspection. Rarely, however, thermal processing will open up a crack that might have been difficult to see with the naked eye prior to thermal processing.
By far the most difficult head crack to detect is the one which is caused by a flaw in the raw material. This is often a short section or sections of a 4000# coil of wire that has a defect in it. Unless that defect drops into the inspector’s hands at the precise time that he is catching parts for his in process inspection, it will not be seen until it is caught during 100% post production inspection or until it is caught during installation at the assembly plant. The fastener producer is then caught in the middle of a raw material supplier that offers replacement value as a guarantee and a tier one or two manufacturer that demands sorting costs, assembly costs, administration costs etc. etc.
While varying technology exists to catch cracks through 100% inspection machines, each technology has it’s limitations.
Eddy current coil – limited to large obvious defects
Eddy current probe – very effective but requires 100 degree rotation of every part resulting in slow sort speed, limitations on part shape, and higher cost.
Back lit camera – effective when the crack is open and light penetrates through crack. Not as effective on defects that are more visible from one side.
Front light and conical mirrors – very effective for a specific size range of product but currently limited to smaller parts.
Customer demands for “certified” shipments then place the fastener supplier into this difficult spot to try and find a solution. Often when dealing with very high volumes, a dedicated solution can be found and afforded. When volumes do not support a dedicated solution the interim solution is usually 100% visual sort. This is labour intensive and while costly can be relatively effective but still carries the high risk of eventual human error. Moving forward, they are now faced with the challenge of making a profit on a job that has added unforeseen cost. Got to love the fastener industry!! Never a dull moment!!!