Rhomba Bench Design

Adding inlaysThere are a few things we're going to do to this form, though, before we start casting in it. One is we're going to add a little bit of artwork. In this case, I've got just a small ammonite that I want to place so that it is visible on the top of my batch after I mold it. It's important with this cube to keep in mind that what we're seeing -- the bottom of the box -- will be the top surface of our bench. This wall here is actually going to be what is underneath the bench. Imagine that the wood portion of the bench is going to extend or sit into the pockets created by these knockouts. I want to just talk a little about small artwork and inlays. In this case, we have an ammonite that we're going to be placing. It's relatively small, so we can use an off-the-shelf spray adhesive. In this case, we're using 3M Super 77. I'm just going to spray a light mist of adhesive onto the face of the ammonite. You need to let this dry for about 15 to 20 seconds before we can place it in the mold. It looks pretty good; we'll go ahead and try this. There, we've set the ammonite into the base. Before I get casting, I'm going to go ahead and turn this up on the bottom side. In the casting process, we're going to start by placing our concrete material down in the base.

Hand packing the casting blendThe material we're going to be working with is the CHENG D-FRC casting blend. We're going to be mixing the casting blend a little on the thick side, because we're going to hand pack this entire box. You might wonder why we're not going to spray into this box with the casting blend like we did in some of the other projects, and the main reason is this box is too small for us to be able to position a sprayer and actually get up into all of the corners. So for this, we're going to hand pack the casting blend into the form. We're going to mix it a little on the dry side so that it'll hang on the vertical surfaces without sagging or slumping. One note, when you're doing a hand pack technique like this, and you'll see when we demold this project, it's going to leave a nice variegated surface with lots of random air bubbles in it.

The instructional drawing for this bench can be found online (Rhomba Bench How-to Project Drawings).

Pressing the mix into the moldSo we're going to start on this project, down at the bottom of this cube, which is going to be the top of our bench. Now when I'm placing the material into the mold, I'm pressing it into position and this is about a quarter of an inch thick. I'm placing it down into the mold, kind of working it up the edges, but making sure that I work the casting blend into the tight corners so that I don't get the air bubbles trapped in those zones.

As I'm packing the D-FRC around the knockout, I want to make sure to mention that this edge, right here, needs to be fully covered with the casting blend. So I'm wrapping the edge, but not pressing so hard that I actually thin this edge out here. We want to ensure that when we're done, we have a full ½ inch coverage over all faces of this form. As I'm placing these handfuls of the D-FRC, I'm trying to keep it kind of random because where I'm setting one pod or pocket of material, and it's coming in contact with the D-FRC that I've already placed in the mold, there's a chance that where those two come together, there's going to be a line of air bubbles, so I want it to look very random, not in any set pattern. So it's almost as if I'm pressing cookie dough into a pan.

We've now filled the bottom of this mold with 1/2 inch of the casting blend. Instead of working the sides vertically, which will take a little longer, I'm actually going to roll this over on it's side, so that I'm always working on a horizontal plane. Makes the project go faster.You can see here everything that I've applied on the top of the mold is actually staying put in place. I don't want to do any heavy vibration or shaking or bumping to the mold while I'm applying the rest of the concrete, because that might cause the concrete that's hanging on the vertical surfaces to peel off. So we're going to be doing this rather gently. You see how the casting blend -- I've been working with it now for 15 minutes -- has started to get a little stiff? So before I place it in my mold, I'm just going to knead it a little bit in my hands, which loosens it right back up and makes it super-pliable again. I'm placing about 1/2 inch.

We've filled the form here and we're now ready to actually finish off the top edge of the form, which is going to be the bottom, or the feet, of the bench. In order to give myself a nice consistent, level surface on the bottom of this bench, I'm going to go ahead and add a couple strips of wood here to my form, so that as I bring the D-FRC material up, I have a flat, level surface that I'm going to be fitting it to.

Creating the feet of the benchNow real quick, I want to just show you we're going to have quite a bit of the casting blend built up along this edge. To create, almost, kind of a structural L-shaped leg that the box is going to fit on. I'm just going to gently press this up into position. I need to be careful that I don't add too much at once, or it will want to peel off of this. It’s a very delicate gesture of bringing this material up the wall, to create the feet, or the foot of this bench. If it does start to drop, don't keep pressing it back into position. Just kind of let it sit for about 5 minutes and then come back and the material should have stiffened up enough that you can actually, you know, press it into place.

The instructional drawing for this bench can be found online (Rhomba Bench How-to Project Drawings).

OK, we've got this popped out of the mold and I've got these knockouts here that I want to talk a little bit about. They were sized to hold the bench. We've got this bench that's going to be bridging between this cube and another cube a distance down. But the bench itself is actually just made out of 2x4s. In this we've case got a little cedar here, and the knockout is sized to fit a standard 2x4 on end. This area here is actually sized to hold the 2x4s for the bench, basically the bench slats, like so. So that's how these knock uts were sized, to accommodate the bench that is going to sit between the left and the right cube for this bench.

Filling the voidsLet's move onto slurrying. I've mixed up a little bit of the platinum acrylic slurry here. The consistency of this is almost like a soft putty or clay. I'm going to work it with my hands into any of these voids that I would like to see filled. I could, you know, leave these voids open, for a slightly more rustic look. That's pretty much it. We'll let this sit for a day or two and then come back and start polishing.

PolishingWell, we're ready to start polishing the top here, but before we do I want to just talk a little bit about the tools that we're using here. I'm going to be using a wet-dry polishing system by Alpha. For this we're going to start off with a 500 grit, and polish the surface just to knock off a little of the melamine texture. Then we're going to switch it out to a 1,000-grit polishing pad, and then go up to a 2000 and finally finish off with a 3000-grit pad. We are going to be dry polishing this piece, using the Alpha WDP 120, which is my new favorite toy. This is a great polisher. But one of the real benefits of it is we can use it both wet and dry.

Now when you're working with a dry polishing system like I've got here, you're going to want to make sure that you have this hooked up to a HEPA vacuum. In this case we've got Alpha's HEPA vac, which is a great tool to use in conjunction with the WDP 120.

The instructional drawing for this bench can be found online (Rhomba Bench How-to Project Drawings).