# Analysis of Sheet Metal Tapping Screw Fabrication Using a Finite Element Method
## 2 Fundamental Theory
### Geometric Construction of Screw and Thread Plate  
The characteristics of the cone bolt (CB) screw thread plate (Figure 4) are as follows.  
1. Feed-in area: the portion where a light nail is fed in and compressed in the turning process. The feed-in area and shaping area are often within carved slip teeth.
2. Shaping area: the area where the light nail, after passing the feed-in area, is extruded and formed.
3. Forming area: the area where the light nail, after passing the feed-in area and the shaping area, is extruded into screws, in a manner that is determined by the thread grooves of the thread plate, the thread peak, and the cone bolt angle.
4. Discharge area: the portion where formed screws are discharged.
5. Back angle and elevation angle: the portion where the cone bolt thread plate is cut to form the tip. The back angle controls the feed-in of the material in that a large back angle allows more material to be fed in and a small back angle reduces material feed. The elevation angle is the essential parameter in the forming of the thread tip and thread peak on the thread tip. Small elevation angles result in trailing as the tail is not cut cleanly. Larger elevation angles cause broken tips because the light nail is cut too early.
6. Cut: crossing of the back angle and screw thread plate cone bolts. A sharp cut is required as a blunt cut results in trailing and inconsistent tip forming.
7. Thread depth: depth of the screw thread peak, which controls the outer diameter of the screw.
8. Crest angle: the angle of the screw thread peak. Specific screw crest angles are required to meet particular specifications; for example, a C/B chipboard nail has a crest of 40 degrees; a general BA round-tail screw has a crest of 47.5 degrees; a general triangular self-tapping thread has a crest of 45 degrees, and a general machine thread has a crest of 58-60 degrees. Crest angle is also critical in controlling the outer diameter of the screw.
9. Cone bolt angle: screw tip angle. General self-tapping screw tips have angles of 40-50 degrees. Specific screws must meet particular specifications; for example, general wallboard nail and chipboard nail tips have angles of 20-30 degrees. Customers sometimes specify tip angles.
10. Angle of chamfer: the feed-in area and shaping area that are milled sharply in the thread plate production process for easy feed-in. When the angle of chamfer is too small, the thread plate is not abrasion-resistant. When the angle of chamfer is too large, the tail feed-in area is too large, making screw forming difficult, and the outer diameter too small. When chamfer in the feed-in area is greatly worn, the screws are not sufficiently formed and the screw diameter will be too large to support slipping.
11. Slip teeth: threads that are specially carved on the thread plate feed-in area and the forming area to prevent a light nail from slipping in the feed-in process. Shallow slip teeth cause slipping, while deep slip teeth result in roughened screw bottoms, dim thread peaks, and even blocking of the extrusion of light nails, causing the outer diameters of the screws to be too small.  
1. Cone bolt height: the distance between the thread plate surface and the cut, which is generally half of the bottom diameter of the screw. Cone bolts that are too high cause the screw cut to bump together, causing insufficient screw forming. A cone bolt that is too low causes trailing because the tail is not cut off.