Comparing the architect’s drawings to my CAD (Rhino) file, you might notice some small dissimilarities. As the cast stone manufacturer, we are often obliged to make minor adjustments, generally to make the pieces more manufacturable, or to correct a design feature that might result in a durability concerns (overly thin walls, or concrete that comes to a sharp point, for example).

In this case, the base has been sectioned into 5 separate segments. The inclusion of the conic details at the bottom of the base presented the only real formwork challenge among the base pieces.

The negative forms for each of the base pieces will be cut from expanded polystyrene (EPS foam). Once assembled, the form for the bottom segment must look like this:

There is no way to manufacture this form as a single piece. It must be manufactured in segments, and then assembled.

Subdividing the form into straight extrusions wherever possible allows the majority of the form to be cut and mitred with a CNC hotwire, which is faster, and creates a finer surface, than cutting foam on a mill. Additionally, joints between form segments should be kept to corners and edges wherever possible: butt joints between form segments on planar surfaces quite frequently telegraph through to the cast part.

Of the 7 individual pieces that make up the form, only the conical segments (in magenta) require milling. As well, the far rail (in blue) requires secondary cuts on the hotwire to create the miters.

The anthemion at the top of the plaque posed considerably more of a challenge in creating the negative form.

The negative for this part must look like this:

This form can be made in just three pieces:

The upper rail of the form (in green) looks complicated, but is, in fact, a simple extrusion that can be cut in minutes on the CNC wirecutter.

The lower rail (in blue) requires a small amount of milling, but is similarly straightforward.

The base of the form (in magenta) poses a couple of conundrums:

• The outer perimeter of the geometry is quite complex, and must match perfectly with the inner perimeter of the mating rail segment. Of course, if the rail can be cut easily on a CNC wirecutter, then so can this base. However, CNC wirecutters can only cut EPS foam. EPS foam does not hold detail well enough to adequately cast the part.

• A denser substrate such as MDF can be milled to the level of detail required, but the perimeter of the segment cannot be cut due to the presence of sharp inside corners, between the “leaves” of the design. A milling tool long enough to cut the required thickness of material would simply have too large a diameter to adhere to the perimeter:

(All corners between leaves had been slightly radiused, as even a hot wire, at a diameter of 0.014” (0.36mm) cannot cut a perfectly sharp corner.)

Instead, we chose to create a master (positive), in MDF. The master poses the same obstacle as the negative, in that a milling tool cannot get into the tight corners between leaves. And so each individual element of the anthemion was cut out separately, from a single blank of MDF:

There is enough space between each element to accomodate a 1/2” tool. Elements were secured to a backing board to ensure that they could not come loose during machining. Drill points for screws in the backing board and the blank are in blue.