A free shank (see Fig. 1) is made up of two -- a shank holder and a shank head. The shank holder is installed in the slide of the press machine, and the shank head is installed in the die. The installation is done so that the top part of the shank head is caught in the groove in the shank holder.
This shank has the function of transferring the pressing force to the punch holder and of pulling up the punch holder. There is a considerable play between the two parts.
Because of this, the relationship between the top die and the bottom die should always be maintained by the guide posts and the guide bushes.
Fig. 1 (a) shows the relationship between the post and the bush when the die is at the top dead center.
Fig. 1 (b) shows the relationship between the post and the bush when the die is at the bottom dead center.
At this time, it should be made sure that the top end of the post does not hit against the bottom surface of the slide.
When the punch and the die are reground, the die height becomes smaller thereby making the gap smaller between the top end of the post and the bottom surface of the slide. The design should be made such that this gap is present and there is no hitting of the one against the other even after regrinding has been repeated several times and the life of the die is approached.
There are two purposes of using a free shank.
One is to maintain the relationship between the punch and the die using posts and bushes, so that the work can be done without this relationship being affected by the press machine. The other is to simplify the setting up of the die.
The relationship between the post and the shank is explained below. See Fig. 2.
Care should be taken about the post length in the case of a die constituted as a die set.
Normally, as shown in Figure 2 (c), at the bottom dead center, it is ensured that the top end of the post is lower than the top surface of the punch holder (even when the die height has become lower after regrinding).
If due to some reason, the top end of the post projects beyond the top surface of the punch holder as shown in Figure 2 (d), a flange shank should be used thereby maintaining the relationship "s" between the bottom surface of the slide and the post.
A very serious accident will occur if the top end of the post hits against the bottom surface of the slide. Sufficient care should be given in the design regarding this aspect.