Generally refers to pointed, coarse teeth, texture hard wood screws, but also for aluminum alloys, plastic. A special small screw for threading a metal hole is called a tap.
Self-tapping screws
Stainless steel screw surface treatment: Most of the self-tapping screws are covered with galvanized or zinc phosphate coating. Cadmium plating is rarely used due to its high cost and toxicity. If appearance is important, nickel plating can be selected. Chromium. Appearance of the appearance of two questions: First, the appearance of the treatment will not only increase the scale (such as the reduction of holes, etc.) and the appearance of dissimilar treatment will affect the torque, tightening degree, the link between strength, so in Doing the experiment must be done in accordance with galvanization, cadmium plating or phosphate treatment to do the same rule. When testing the attack drill hole, the stainless steel screws used must be the same as the external disposal screws. The second is, the high hardness Small-scale self-tapping screws often cause failure due to hydrogen embrittlement. Self-tapping screws have to be carburized in order to attack the iron plate. High hardness and high carbon content will occur after carburizing and disposal. In this case, hydrogen embrittlement will occur during electroplating or pickling. The methods proposed in the specification to eliminate hydrogen embrittlement are: a). Baking within 1 hour after plating or pickling. b). Temperature 375 ~ 425 oF (190 ~ 220 oC). At least 4 hours.
Hydrogen embrittlement test: The hydrogen embrittlement test is mainly to test whether the self-tapping screws are residual hydrogen in the screw arrangement and whether it will make the self-tapping screw brittle, (usually brittle fracture is not a moment, and must be given The stress will be revealed.) The electroplated or coated screws shall be loaded into the test iron plate and the steel-made Pinghua Division as stipulated in Table 3, and the heads of the outstanding head-type screw head shall be regulated using the standard of Pinghua. Type (Oval) screws use a chamfered distance steel piece. The thickness of the flat washer or distance piece mounted on the underside of the head should match the maximum ineffective tooth length of the screw. The shank can be supported with a longer cold rolled steel distance piece or flat washer to ensure that the complete tooth part can be combined and maintained in the test plate thickness. The stainless steel screw should be locked to 80% of the maximum breaking torque The breaking torque is determined by locking the 5 screws to the breaking point and continuing to maintain the locking for 24 hours. After that, the screws are loosened and then rotated to the original locking torque. The screws should have no significant loss.