Dowels are very common and very useful to mechanical designers. First, let's introduce the positioning pin. Dowel pin is actually an informal name. The strict name is pins, connecting components used to precisely limit the position and coordinates of parts. Usually only the two components that need to be matched need to be added.
Positioning pins can be divided into many types according to their classification. As they are called dowels, they are usually round and have the diameter of the body. There are many types, for example: one end is round and the other end has threaded teeth. This is a guide pin typically used for two parts that are often separated and combined. Of course, some relative position tolerances are too large and the circle used is difficult to guarantee. Pins can be inserted.
This kind of tapered pin can be used, and the head is also round, but it is relatively small and can easily cause hole misalignment. Some products have a larger diameter than the bottom, which is smaller than the top. A small flange is also added in the middle. The difference in shape can be adapted to different occasions, but has only one purpose, which is to always hold the two parts tightly.
There are also tapping types for dowel types. The JIS standard refers to this as a fixing pin for the stamping die (B5062 straight type only). In addition, there are parallel pins (B1354) and tapered pins (B1352) as mechanical parts in JIS, and their accuracy is lower than that for mold applications.
The outer diameter accuracy, surface roughness and hardness of the material are necessary conditions for positioning pins. Dowel pins should be used by gently pressing them into the holes. When the locating pin is pressed into the hole, surface pressure and friction obtained by elastic deformation create a retaining force. Therefore, when using positioning pins, the relationship between the outer diameter accuracy of the positioning pin and the hole accuracy is very important.
Since the pin diameter must be pressed into the bore diameter, the bore diameter should be slightly smaller. If it is raw material and quenched material, if the hole diameter is not changed, the retaining force and pressing force of the positioning pin will be different. Therefore, the pore size of the raw material should be reduced by 10 μM, and the pore size of the quenched material should be reduced by about 5 μM.
For the basic usage of positioning pins, see: Press the two positioning pins into the board, taking care to prevent misalignment. The further away the location, the better, which is helpful for improving accuracy. The relationship between hole diameter and depth is that the length of the positioning pin clamping part is twice the diameter of the positioning pin. The minimum length should be the same as the diameter, and the maximum should not exceed 3 times the diameter. If the depth of the hole is less than the diameter of the locating pin, the positioning accuracy will deteriorate; if the depth of the hole exceeds three times the diameter of the locating pin, it will be difficult to perform hole processing with guaranteed accuracy.
Generally speaking, straight pins are most commonly used, but tapered pins are sometimes used. Tapered positioning pins may become loose due to vibration and shock, so please pay special attention to them. Furthermore, using a pin to support pressure exerted from the sides is not the purpose of the pin itself, and it is best not to rely on it to perform its full purpose. Please refer to the dimensions of the attachment screws used to determine the dowel diameter. Generally, the diameter is the same as the screw size, or one size larger than the screw size.
