Để có thể lắp ráp cần phải có ít nhất một bước tiến của ren trên miếng tôn.
Với bề dầy của tôn trong khoảng 0.5 đến 1.0 mm thì đúng là vấn đề.
Ví dụ với bu long M4X0.7,bước tiến của 1 ren là 0.7 mm.Không thể có đủ 1 ren với tôn dầy 0.5 mm.
Vấn đề này được giải quyết bằng cách gia tăng bề dầy của miếng tôn.
Một bài viết tiếng Anh khá rỏ ràng ở link này :
http://findarticles.com/p/articles/mi_qa5357/is_200601/ai_n21408593/pg_1?tag=artBody;col1
Trích dẫn từ trang này một đoạn cần thiết :
Solution five
Finally, make an extrusion in the sheet metal. This can be done in the stamping operation with the use of a punch. Often the punch is a secondary operation off-line.
Punches vary from single blow extruding tools to a nail-like device called a spud. The stamping plants are inclined to use a spud, as it requires little maintenance. It acts like a nail that, when driven through the sheet metal, produces a c
extrusion
..................
While the extrusion solution appears to be unusable, it has the best potential for fixing the problem if modified. If the extrusion could be made by other than stamping with a punch, it would have sufficient wall thickness and height to solve the problem. There is such a method. It is called thermal friction welding. The process, illustrated here by the Flowdrill, Inc. company's diagrams, consists of spinning a c
, high
arbor against the metal under pressure.
The tool can be run in an arbor press (drill press) as part of stamping operation, or even a h
tool. As the part "rubs" against the metal, heat causes the metal to become molten and the tool penetrates the metal. The arbor tool is sized to produce the exact diameter of boss ID required. Heat effect is limited to a few millimeters around the boss extrusion. Flowdrill's website,
www.flowdrill.com has an excellent video showing the operation. The formed boss is three times the metal thickness and, when coupled with a thread rolling screw, is as strong as or stronger than the parent metal.