why need optical sorting for screws?

When it comes to automatic assembly, many customers will put forward the need for optical sorting. In this article, we will briefly explain why the optical sorting is needed, what items can be fully inspected, and the defect rate after the optical sorting.

Since screws are products produced on a mass scale, even if the production tolerance is strictly controlled, there will still be a certain defect rate. In the automotive industry or electronic assembly industry, automatic production lines are widely used, and the high defect rate is easy to cause the blockage of the production line, further causing the stagnation or delay of the production line. The need for inspection of critical dimensions or indexes arises.

The inspections are composed by three industrial CCD cameras as following diagram shows. The CCD1 is a head-up camera used to inspect the multi-segment height and outer diameter of the products. The CCD2 is a vertical bottom view camera used to inspect the cross slot or flat slot on the head of the product. Some cases will using additional 1 to 4 CCD cameras for inspecting thread damage or other defects of the product with a complete 360 degree view.

However, even after the optical sorting, the defect rate still exists. We need to correctly recognize that 0 defect does not mean that the PPM is 0. A PPM rate of 0 does not exist. The defective rate of products formed through mass production naturally is about 200ppm, when products go through manual screening is about 100ppm, and after sorting by machines can reach about 30-50ppm. Even for the strictest control with only one feature, the defect rate can only reach 10ppm max. Moreover, the accuracy of sorting will greatly affect the production cost, which is the result of greatly prolonging the sorting time.