What is the procedure for the fine thread self tapping screws inspection and determination?

The inspection of the fine thread self tapping screws generally has two types: destructive inspection and non-destructive inspection.
Non-destructive inspection: Randomly take samples from the acceptance batch according to the requirements in the table, perform visual inspection or other non-destructive inspection, such as using magnetic technology or eddy current. If the defective sample is found not to exceed the permissible limit, the batch is accepted. If the number of defective samples is found to exceed the allowable limit, the destructive inspection is performed in batches with these defective products.
Sample size for visual and non-destructive inspection
Batch (N)Sample size (n)
N≤120020
1201≤N≤1000032
10001≤N≤3500050
35001≤N≤15000080
Note: 1. Batch refers to the same type, specification and performance level, and the number of fine thread self tapping screws submitted for acceptance at the same time.
  1. The sample size is based on the inspection level S-4 specified in Table 10 of GB/T 15239.
Destructive inspection: After non-destructive inspection, if a non-conforming product is inspected, the product with the most serious defects is composed of the second sample according to the provisions in the table, and a defect perpendicular to the defect is taken at the maximum depth of the defect. The section is checked.
Second sample size for destructive inspection
The number of defective products in the sample (N)Sample size (n)
N≤82
9≤N≤153
16≤N≤255
26≤N≤508
51≤N≤8013
Note: The second sample size is specified in GB/T 2828.
Judgment: In the visual inspection, if any quench cracks on any part or wrinkles on the inner corners are found or wrinkles on the non-circular shoulder fasteners that are lower than the “three-leaf” shape on the support surface, Reject the batch.
In the destructive inspection, if there are forging cracks, bursts, cracks and streaks, dents, cuts, and damage that exceed the specified allowable limit, the fine thread self tapping screws are rejected.

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