I’m always looking for another idea to experiment with. After working out the modular bolted die scheme I started looking for a simpler way too accomplish some of the same things.
A hinge articulation seemed appealing. The top leaf could swing open for redressing or revision. However, it wouldn’t have any intrinsic spring action. Gravity would close the die but something else would have to open it.
What is the function of spring in a traditional spring die anyway? One function is to open the die enough the workpiece can be inserted and to open the die between strikes so the workpiece can change position. The other important role is to keep the die faces aligned.
The idea of the hinge is that it serves primarily for alignment. It doesn’t constrain alignment as precisely as the bolted design but in most cases it is close enough.
I tested two versions of the hinge barrel which is welded to the end of the top leaf.
1. The tight fit - I cut some small pipe sections about 2” in length to serve as the barrel of the hinges. A 3/8” bolt could not be inserted but was close to fitting. I slit the pipe longitudinally and drove in a of 3/8” round coil spring mandrel and hammered the pipe to open it slightly and the 3/8” bolt then fit perfectly.
2. The looser fit - A quicker alternative is to use 5/16” bolts which have a slightly sloppy fit.
The short pieces of flat bar which carry the bolt axel serve to help control the alignment so the difference in the play of the bolts in the barrels didn’t seem to make much difference in performance.
As in the bolted box die design, a box serves as the spacer. I welded a 9” piece of 1/8” x 1/2” flat bar to the top of the box to provide the spring action. This can easily be bent to the desired angle to achieve the best opening.
In the testing phase I was working with three old traditional spring dies which needed revision to work properly. The top and bottom dies were salvaged and the remainder of the frames were scrapped. It is a little tricky to get the die cavities aligned when welding everything together. Two dies worked nicely but the third is a bit off and will need more work. After a die tests well I'll tack weld the nut to the flat bar carrier.
At this point, I’m thinking this is going to be my preferred method of initial closed die construction. If tighter constraint is needed it is easy to convert this design to the bolted design.