self-tapping screw
| Screw thread |
2 |
2.3 |
2.6 |
3 |
3.5 |
4 |
4.5 |
5 |
6 |
8 |
| Outer diameter (d)/max |
2 |
2.3 |
2.6 |
3 |
3.5 |
4 |
4.5 |
5 |
6 |
8 |
| Outer diameter (d)/minimum |
1.9 |
2.2 |
2.5 |
2.9 |
3.4 |
3.85 |
4.35 |
4.85 |
5.85 |
7.8 |
| Valley diameter (d1)/max |
1.5 |
1.7 |
2 |
2.3 |
2.7 |
3 |
3.4 |
3.8 |
4.6 |
6.1 |
| Valley diameter (d1)/minimum |
1.4 |
1.6 |
1.9 |
2.2 |
2.6 |
2.9 |
3.3 |
3.6 |
4.4 |
5.9 |
| Tip diameter (d2)/max. |
1.4 |
1.6 |
1.9 |
2.2 |
2.6 |
2.9 |
3.2 |
3.6 |
4.4 |
5.9 |
| Tip diameter (d2)/min. |
1.2 |
1.4 |
1.7 |
2 |
2.4 |
2.7 |
3 |
3.3 |
4 |
5.4 |
| Thread diameter (25.4mm)/min. |
40 |
32 |
28 |
twenty four |
20 |
18 |
16 |
16 |
14 |
12 |
| m (maximum) |
0.1 |
0.15 |
Self tap fastening method
1. For screw heads with a countersunk screw structure, a wedge effect (radial component) tension acts on the circumference of the resin part that comes into contact with the screw head. Accordingly, since plastic is less rigid than metal, we advise against using such screws.
2. If there is a gap in the self-tapping joint of a resin-molded product, a tensile force will be applied to the back of the molded product and boss pull-out force will be applied to the boss when the screw is tightened. Accordingly, the contact surfaces of the resin-molded parts to be fastened must be positioned in close contact so that compressive stress is applied.
3. Using the wrong hole diameter, washer and or screw diameter may cause stress to build up due to the screw or washer digging into the hole or uneven tightening. Select a screw that matches the hole diameter and use a hard and thick washer capable of withstanding the tightening torque and without deforming.
[Reference] Troubleshooting : Molded Part Cracking
Basic dimensions of self-tapping boss
January
Pilot hole diameter for self-tapping
(d) The optimal approach involves taking the average value of the outer and root diameters (d1 and d2) of the thread being used (generally approximately equal to the effective thread diameter). While numerous screw threads increase the breaking torque and pull-out strength the screw-in torque is also higher, hindering workability worse and rendering the units prone to cracking.
Increasing the screw-in depth is the best way to boost workability breaking torque and pull-out strength.
The boss thickness is usually 1/2 to 1 or more times the thread diameter, but if it is thicker than the base material, sink marks are likely to occur on the back of the boss. Accordingly, the ideal boss wall should is be slightly thinner than the base metal wall as a general rule. (However, although this helps prevent sink marks, an excessively thin part may result in , problems like silver streaks and flow marks.)
The outer diameter of the boss varies depending on the self-tapping function provided, but a range of 2.1 to 2.5 times the pilot hole diameter is considered appropriate as a general rule.
The boss height changes depending on the depth of the self-tapping screw, but must generally be at least three times the diameter of the pilot hole. The depth of the hole in the molded product should slightly exceed the screw length, so that the shavings removed by the screw can collect at the bottom.
The thread diameter must be at least twice the nominal thread diameter.
- The radius of the base of the boss must be 0.3 mm or more to reduce stress concentration due to distortion and external force (bending moment) while forming.
- The entrance to the pilot hole should be dish-shaped or curved to facilitate screwing in and also prevent chipping or cracking of the hole.
Stress and deformation acting on the boss due to self-tapping
Tapping process
Screwing a screw into a pilot hole that is smaller than the outer diameter of the screw has two consequences. The wedge effect generated by the thread angle acts to expand the boss, while and tensile stress tends to be generated on the boss in a circumferential direction.
Tapping process
Fastening process
As well as the internal pressure generated in the tapping process, tightening the screw thrust generates tensile stress in the circumferential direction of the boss and compressive stress at the upper end of the boss. Shear stress due to the tightening thrust is also generated in the threaded portion of resin-molded products.
Fastening process
Important Note:
We do not assure the quality or safety of products derived from our materials or suggestions. It is essential to determine the appropriateness for your product independently. Ensure adherence to laws, regulations and industrial property rights.
