Experiments with laser-engraving continue.
The basic mechanism can be briefly described. Graphite is a poor conductor of heat, and a laser (of a wavelength the material absorbs well) focused on the surface can heat a small area to the point that it rapidly oxidizes in contact with the air, thus removing material. By varying the power of the laser, the amount of material removed can also be varied. By raster-scanning an area, a three-dimensional surface can thus be created. One problem which may arise is that this variation can be reflected in the width as well as the depth of the 'cut', although if the scanning pitch is fine enough, the resulting surface should be reasonably smooth.
While there are aspects of the tooling required which cannot be exactly determined without possession, or at least engineering drawings, of the press, the general outlines and even many of the details can be planned in advance. One of my concerns has been to create the most versatile possible arrangements of tooling, to allow doing more work at less cost. The arrangement of the dies is a key element.
I continue to mull over ways of producing dies, suitable for producing a reasonable number of pieces, at a lower cost than conventional methods. 3D modeling & some kind of CNC machining are obviously vital factors, but the question is how to apply them. A suggestion received over the weekend may prove valuable in this respect.
The Janvier machine is a specialized machine tool for making art. In the application most relevant to us, it is used to take a large-scale physical model of a coin design & reduce it to the final size. In the course of this, the detail picked up from a plaster or clay model (generally either plated or reproduced in metal or hard plastic) is cut into steel. This process economizes on the skill needed in the preparation of dies, since it does not require the sculptor to be able to cut fine detail directly into steel with no errors. And it means that any problems can be ironed out at the large scale, before they show up in the die or the finished piece.
That's a mouthful of a word, isn't it?
The Castaing machine, so called after its reputed inventor, a moneyer at the Paris Mint, is one of the three fundamental inventions of mechanized coining. Unlike the rolling mill, or the screw press (and the eccentric and toggle presses which have mostly supplanted it), the Castaing machine has few or no other uses. Its purpose can more easily be understood by mentioning that it is also known as an edge mill, or upsetting machine. It takes the flat discs of metal punched from sheet or strip by the blanking press, and prepares them for striking in the coining press.