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This Instructable will show you how to cast a fiber-reinforced concrete clock in a melamine box, using a foam knockout to recess the clock hands and mechanism.

About Fiber Reinforced Concrete: CHENG D-FRC is a fiber reinforced concrete mix that is ideal for large or small projects where strength and thinness are desired. Fiber reinforced concrete doesn't contain rock aggregate, instead it contains a range of fibers that act as the aggregate and help bind it together. The 1/4" thickness at the face of this clock is less of a concern with D-FRC because it's designed to be cast thin. The aggregate in regular sacked concrete can be up to 5/8" in size, and getting small forms to fill without large voids can be tricky (a 5/8" rock won't fit in a 1/4" gap). Concrete without aggregate (cement by itself) is brittle, but with that said, try this project even if you're not using fiber reinforced concrete, just take out the larger aggregate, don't mix it too dry, and vibrate the form to remove as much air as possible.

It takes some time to shape the foam knockouts and then they can only be used once, but the foam is relatively easy to shape with hand tools, and the total cost of forming materials is pretty cheap.

Because I didn't have so many pictures of this project, I modeled it in sketchup to help explain the process, hope it helps!

Materials + Supplies:
Tools:
  • Hand Saw / Table Saw
  • Screw Gun + Pre-drill bit
  • Drill + 7/16" Drill Bit
  • Caulking Gun + 100% Silicone
  • Sanding Block + Sandpaper (60,100,220)
  • File / Rasp
  • Acrylic Shaping Rods for tooling silicone beads

Concrete Mixing:
Concrete Finishing:

Step 1: Build Form

Step 1: Build Box
Cut the Base and Form Walls from 3/4" Melamine-coated particle board or equivalent. The melamine coating is water proof and provides a good surface to cast against. Alternatively you could use plywood that has been sealed or covered with clear packing tape, but if any water is absorbed by the form walls, the concrete will have a chalky finish in those areas.
1x - Base: 3-3/4" x 3-3/4"
4x - Walls: 2-3/4" x 4-1/2"

The size of the base and height of the form walls determine how large the clock will be. Any changes in size must have corresponding changes to the size of the Knockout in Step 3.

Place walls around base. Pre-Drill and assemble with 1-1/4" drywall screws. Use a ruler / square to keep the form walls at a 90 degree angle to the base.

Step 2: Seal Box
After the box is assembled, seal all inside edges with 100% silicone. This will make the form water-tight as well as give the edges a nice round-over. Sloppy silicone work will be obvious in the finished concrete piece. Focus on making the vertical edges look good as the seams at the base will be covered by the knockout and won't be visible in the finished piece.

Tool the silicone (use something like the acrylic shaping tools for best results, not your finger...) and remove excess with a razor blade or wait for the silicone to cure and peel up the remaining lines.

Work on the foam knockouts in the mean time.
What clock set was that?
<p>I had a whim and wanted to do some stuff for my desk at work and thought of a clock and came on here - got me thinking about a few other things I could make as well.</p><p>Thanks for the steps, now that I know how the process works a little better with the form work Ill be making a version 2.0</p>
<p>Thanks for the share! A classmate and I are making a scale model of the Kimbell Art Museum and wanted to get as close to the real thing as possible. This is really given me some ideas as to how to proceed!</p>
<p>Great! The Kahn building or the new Piano Pavillion?</p>
<p>HEY! Sorry for the delay, I have just now caught up with how to check replies on Instructables!<br>We were doing specifically the Kahn building, and due to a change in time frame constraints, we had to go a plaster over CNC foam route. <br>I am still planning some other projects, specifically using CHENG D-FRC!</p>
<p>Love this project! Simple and stunning.</p>
<p>Dear CHENG,</p><p>I've been seriously interested in trying out the D-FRC Casting blend (the one used in this project). Although, according to the instructions on the CHENG website, the all D-FRC blends are to be used with the &quot;Activator&quot; product. Yet, here on 'instructables', the activator is never mentioned. </p><p>What is the difference in performance when not using the activator with a casting?</p><p>Thank you! </p>
<p>The liquid activator was used on this project. It is a necessary component of the mix. Not using it, or using less than required, will have negative effects on the strength and quality of the piece. Thank you for pointing this out!</p>
Looking all over these concrete things I started to wonder - is there any substance to cover the mold, so after curing the clock would just drop out nicely without disassembling the box? <br> <br>As an example - when working with glass (i.e. slumping bottles) the furnace floor is preferably lined with some kind of special paper to prevent sticking.
There are commercial products used for helping concrete release from a cast. They do seem to be oil based - here's a start http://www.fuchs.com.au/categories.asp?cid=26&amp;c=10441
Thanks. Seems like oil will do the trick :)
A shape with no draft, like a box, isn't ever going to just drop out of a rigid form, no matter how much form release you use. If the shape is more like a pyramid, you might have better luck. A very light coating of vegetable oil can be used as a form release, but it's not usually necessary.
Oh, that sparks an idea or two, thanks! (:
Im really liking the visuals you have there, really makes things clear!
Very cool! I've been wanting to do some small scale concrete casting for a while now. Great tips in this instructable!
I like ur design... thanks for sharing
That looks pretty good. And great concrete casting info for folks who may need something similar. Thanks for sharing!
Awesome design! Love the simple, modern look :)

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