If you’re going to create an exhibit highlighting the manufacture of a high-profile project, you might as well highlight the piece that has the sponsor’s name on it.

Cast stone is essentially concrete mixed with various aggregates, tints, and additives. Virtually any natural stone can be approximated, and in fact many unique “designer” looks can be achieved, in nearly any colour, with nearly any texture. As the name implies, cast stone is poured into a form, rather than quarried and carved, and an added benefit is the possibility to include UV protectants, moisture repellants, and physical and chemical binders that make the resulting product considerably more durable than the real thing.

Like concrete, cast stone takes the form of whatever it is poured into. If you pour it into a highly finished form, you get a highly finished surface. It is therefore important to orient the form in such a way as to keep surfaces that must be finished against a form face.

In the instance of the Wave Bench, as it is with much cast stone product, it makes the most sense to create each piece upside down:
The bottom of the bench does not require a fine finish. Pouring the piece in this orientation also creates a single level plane at the top of the form, which further simplifies the pouring process by reducing the complexity of the hand-troweling that must be performed on any exposed concrete as it cures.

The forms for the Wave Bench were created from expanded polystyrene (EPS) foam. Where possible, form segments were cut on a CNC hotwire. The hotwire is relatively fast, but the taut wire can only cut straight lines. Since the Wave Bench has many radiused faces, many segments of the forms had to be machined on a CNC mill, a much more time-consuming process.

Restrictions in the size of the CNC mill and the geometry of this piece dictate that the form must be made in two or more parts. The most logical location for a parting line between form segments is along the lowest point in the part.

The form (in blue), split in two parts at the lowest point of the casting.

The larger of the two form segments (in red) is still too large and geometrically complex to practically machine as a single piece.

The logical point to subdivide this segment is at the narrowest point of the segment. This generally ensures that no undercuts, inaccessible to the 3-axis mill, will exist in either of the segments.

Obviously, the goal when subdividing a form into manufacturable segments is to minimize the number of segments created. It is generally easier and faster to mill one large segment than two smaller segments.

Is it possible to machine this segment as a single piece? A number of factors go into that determination.

The Wave Bench will be installed outdoors, in the elements. To ensure that it sheds rainwater, the seating area is slightly sloped.

The corresponding face on the form must also have a slight slope.

The nearly imperceptible slope on the top of this form segment, as well as the small raised geometry at the back of it would be nearly impossible to machine from any direction other than the top.

For the same reasons, the front face of this form segment is best machined 90° to the top.

Since two faces of this form segment need to be machined from two separate directions, it is advisable to further subdivide this segment.

Can this segment be machined as a single piece? We must consider the direction in which it is being milled.

When oriented in the position that it must be milled from, the segment becomes too tall to fit within the CNC router’s machine envelope. It must be further subdivided.

The larger bottom portion of this segment contains no radial geometry. It can be cut quickly on the hotwire. The top portion must be machined on the mill.

Once the form is subdivided, manufacturing commences.

This single, relatively small segment of the Wave Bench formwork required 44 minutes of machining time.

In addition, roughly the same amount of time would have been spent modeling the blank, wirecutting the blank, writing machine code for the mill, and fastening the blank to the mill prior to machining.

Blockouts for the inset lettering on the bench are cut from dense extruded polystyrene (XPS), which holds a fine edge better than EPS.

The letters are mirrored, because the form is a negative. The name is curved because the blockouts will be applied onto a curved surface, where they will appear to be straight. The layout of the lettering can all be accomplished within Rhino.

Text is applied to the form segment using spray adhesive.

The text is further secured using a two-part casting epoxy.

The machined segments are assembled using a two-part epoxy.

Wirecut EPS foam endcaps fully enclose the form.

At 7,811 lbs. (3,551 kg), the piece cannot be poured directly into the foam form.

2x4 studs and a plywood face shore up the form and keep it from deflecting under the weight of the stone.

The wood box can be re-used many times for the remainder of the bench pours.