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#055 Method of Using Standard Components (4) Knock Pin (Dowel Pin)

Category : Die Design
September17, 2010

A knock pin is also called by other names such as a dowel pin. This is used for positioning die components.

A knock pin can be of the straight type or can be of the tapered type as is shown in Fig. 1.

Fig. 1 Shapes of knock pins

Each of these come in tapped types also. According to JIS, this is called a dowel pin for press dies (B 5062, only straight type). According to JIS, apart from this, there is a parallel pin (B 1354) and a tapered pin (B 1352) for machines, but their accuracies are far inferior to those of the pins for dies and molds.

The accuracy of the diameter and the hardness of the material are the important requirements of a knock pin.
A knock pin is used by lightly pushing it in a hole. The retention force of a pin inserted in a hole by press fitting is generated by the surface pressure due to elastic deformation and friction. Therefore, when using a knock pin, the relationship between the accuracy of the diameter of the knock pin and the accuracy of the hole diameter is very important.

While the hole diameter will be slightly smaller than the knock pin diameter for the pin to the push fitted inside the hole, there will be changes in the retention force and driving in force between the raw material and the tempered material unless the hole diameter is changed. The hole diameter is made smaller by about 10m in the case of a raw material and by about 5m in the case of a tempered material.

The basic method of using a knock pin is, as shown in Fig. 2, driving in two knock pins in a plate thereby making sure that there is no position shift. For increasing the accuracy, it is better to make their positions as far as possible.

Fig. 2 Method of using knock pins (surface direction)

Fig. 3 shows the relationship between the pin and the hole.

Fig. 3 Relationship between the diameter and the length of the retaining part

A guideline for the relationship between the hole diameter and depth is that the length of the pin retaining part is about twice the diameter, the minimum length being about the same as the diameter, and the maximum being about three times the diameter. If the hole depth is shallower than the pin diameter, the positioning accuracy becomes poor, and if the depth becomes larger than three times the diameter, it becomes difficult to machine the hole while maintaining the accuracy.

The conventional method of using knock pins is to position two components as shown in Fig. 4(a). However, as shown in Fig. 4(b), although it is possible to use it by passing through three plates, the hole in the middle plate is left free as a dummy hole. Since in this form the middle part of the knock pin is not supported, the accuracy of positioning decreases. Therefore, passing a knock pin through four or more plates should be avoided because of accuracy considerations.

Fig. 4 Method of using knock pins (vertical direction)

Usually, although the straight type is used more often as the method of using knock pins, the tapered type knock pins is also used. Caution should be exercised because there is the danger of their becoming loose due to vibrations or shock.

Further, the method of using in which the pressure applied from the side surface is taken by the knock pins is not the proper method of using knock pins, and there is enough reason to think that it cannot serve the purpose sufficiently.

The method of determining the diameter of the knock pin is based on the size of the clamping screws used along with it. The size used normally is either the same as that of the screws or is a slightly larger diameter.

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