EVERY die-struck item – coin or medal – has two important hard and fast rules. I call these “undeniable truths.” They cannot be overlooked at any step of modeling, pattern-making or die-making in the medallic field.
Dies need to strike and withdraw. They must be made to insure that ability to withdraw from the struck piece. Otherwise the struck piece clings to the die. Pressmen call this a “hang-up.”
In a coining press a hang-up with a struck piece attached strikes the next blank that comes into position with two blanks between the two dies. The struck pieces have no design on one side and a mangled surface on the other, what mint error collectors call a “brockage.”
If it continues to hang on to the die and the coining press continues to feed blanks that first struck piece will wrap around the die. Mint error collectors call this “capping” or “cupping.” It is one of the worst situations for a coining press operator to experience.
Even if the die isn’t damaged by all this, it should be rejected anyway. It wasn’t made properly in the first place. It provides a devil of a time for the pressman. Reject that die. Its problem was an improper bevel.
The problem with the die started with the modeling of the design. Two rules govern here – two undeniable truths – no undercuts and proper bevel of all lettering and devices. The two rules are so closely related we discuss them here both at the same time.
An undercut is modeling of relief between the design and its background; the carving of overhang of design relief; a negative slope of relief. Metalworkers call it back draft. Relief sculptors call it under bevel. Everyone calls it undercutting and everyone connected with medal making attempts to avoid it right from the beginning for any die-struck or electroformed reproduction..
[Undercutting is a sculpture technique of full-round sculpture even though it can be attached to its background; it intensifies a contour line or relief by casting a shadow behind the relief. In the medallic field undercut designs can only be reproduced on bas-relief cast plaques, and then only made by rubber or flexible molds.]
For new artists who want to model coins and medals, I recommend hanging a sign above their workbench: “No Undercuts. Bevel All Relief.” Hopefully they would see it every day and burn it into their memory.
All relief requires a proper bevel. The sides of all relief and lettering must have a slight bevel. Each medal making process has its own requirement. It is ideal to model a bevel (also called draft or taper) to accommodate any process used.
Four boundaries must be considered here:
- Vertical relief from 0° to 2½° is called holding taper. Not only is that taper impossible to cut into a die, or strike, it would be impossible for the die to withdraw from the struck piece after striking.
- Hand engraved dies can accommodate a 5° to 10° bevel where the dies can strike and the struck piece release from the die.
- Reduction on the die-engraving pantograph, as the Janvier, requires a minimum 15° bevel. This is required for the shape of the cutting point that mills the design into the face of the die.
- Reduction by computer generated models, requires a minimum 20° to 25° bevel, draft or taper. This also is determined by the shape of the cutting point that mills the design into the face of the die.
Early in the modeling career of every medallic artist it would be wise to create the sides of all relief and lettering with a minimum 20° bevel and maintain this throughout their career. A 20° bevel on relief or lettering is about the slope of a sharpened wood pencil.
Here’s a tip for all medallic modelers: check the bevel of relief by holding a pencil upright next to your modeled relief. Light will show at the base of the relief if the relief is too steep.
The slope in which the relief rises from the background has the proper bevel of at least 20° it will carry forward in all the die-making steps. Anything less than 5° draft will cause a formed piece to “hang up” or freeze in the die or mold.
While steep vertical relief without any bevel is impossible to strike, relief with minimum bevel creates stress in the dies. The displacement of surface metal of the blank is greater at that point and the wear to the dies is at its maximum (which leads to diecracks and diebreaks).
Humans like the sharp, crisp detail in their medallic designs. Unfortunately they also like sharp rises and falls of the modulated relief to give emphasis to the design. So the designer and modeler must balance the need for a superior design with the requirements of the medallic technology.
As the artist shapes the sides of the relief in his design during modeling he must be aware of this angle or bevel at all time.