Cement Casting a Wall Clock

Hi Instructees! After years of lurking I'm finally contributing the the wealth of knowledge on this site.

I've been seeing a lot of interesting objects made out of nature materials like concrete and cement, so I wanted to design and make something beautiful that isn't too complex out of the material. With all the tools at my disposal, I instantly thought I would start with cement casting.

Using Fusion360, I modelled several versions of this clock concept until I came across a form I was satisfied with.

Designing your clock

• Fusion 360

Master Copy

• I used a resin 3d printer, but you could also use a fdm printer, cnc, laser cutter, hand carving, or many other methods depending on the geometry of your clock design.

Making the mold

• Master Copy - The master copy that you will used to create a mold from
• Vytaflex 30 (Urethane rubber) - It's very durable and will give you a great surface finish. The number is how flexible the rubber is, so the lower the number, the more flexible it is
• Mold release - I used dry lubricant spray which honestly wasn't the best but it worked out. I'd recommend real mold release spray, or mineral oil.
• Planter saucer - They come in a variety of sizes
• Flexible plastic - To make a boarder around your part to contain the Vytaflex. You could also use cardboard with a smooth side.
• Scissors - This will be necessary to trim the border to the right size
• Hot glue gun - To secure your master copy and the flexible border to the saucer
• Power drill & mixing attachments - I found this set of three that included an epoxy and cement mixer here
• Mixing Containers - The size and quantity will depend on your model
• Nitrate gloves

Casting the cement

• Mold from previous step
• Cement All
• Dust Mask
• Nitrate gloves
• Water
• Power drill & mixing attachments
• Optional. Pigment powder - I used Supra Black from GlobMarble
• Optional. 3D printed void jig - I wanted the mechanism to sit inside the body of the clock, but this isn't necessary
• Mixing Containers

Post Processing

• Sanding tools - Depending on how neat the edges of your pour is you could be fine with a simple sanding block. For rough areas a rasp would work great.
• Optional. Spray Lacquer - To protect your new clock from the elements or to just give it a beautiful shine, use a clear spray sealant.
• Drill Press or Power Drill - To create the holes for the handing gear. You can avoid if you make the holes while the cement is drying.

Clock Parts

• Clock Mechanism - Find a clock mechanism you like with the appropriate shaft length as well as the design and length of the hands.
• 5 Minute Epoxy
• D Hooks - The clock will be heavy (mine was around 6lbs), so make sure to get one that can support the weight (These support 40lbs so it should be more than enough)

Additional Tools

• I had access to a small shop, so I used various tools like a hammer and wrenches to remove the cement from the jig
• Writing tools - A simple sharpie is always helpful

I want to give you the information to design your own beautiful and unique clock, so I'll go over various factors to consider in the design.

1. Size. How big of a clock you want to make will impact everything from the size of the clock mechanism to how much supplies you need to buy. I recommend first looking at what clock mechanism you want to use, and then design the rest around that.
2. Geometry. Be wary of undercuts. An undercut would be an area that the rubber and cement would not be able to flow into from above because the air would not be able to escape (example). I wanted a void in the back for my mechanism, but that can't be achieved with a one-sided mold, so I would have to design a void jig for when I pour the cement. Not doing this would make this project easier but I wanted to go the extra mile.
3. Analysis Tools. Fusion 360 made it easy to plan out the whole process. You can plan out your project by modeling the urethane mold and cement pour, and then viewing the properties to see how much volume of material you will need. (ex. 1.469E+06mm^3 is equal to 1469 ml of liquid). Using that, I was able to estimate how much Vytaflex and cement all I would need and plan my mixing vessels accordingly.
4. Planning. By modeling the urethane mold and cement cast, I was able to preview what my each step would look like before I progress. I added a layer of complexity by designing a jig to create the void for the clock mechanism. The void screws into a base piece, so the depth can be easily adjusted.

Initially I wanted to cnc the master copy, but I couldn't find somewhere in the area on a short time frame. I'd recommend CNC milling and Fusion360 has some great tools to help with that.

If you're able to 3d print your clock in one piece, that would be ideal, but there aren't a lot of printers with a big enough build volume. If you have to split it, here are my tips to 3d printing your master

• Avoid splitting across complicated geometry. For my clock, splitting down the middle would disrupt the hour marks, so I angled the split plane to avoid any weird mismatching.
• Less material. Resin printing can be expensive, so to reduce the amount of resin used, I hollowed out each part to a 2mm shell of material. This way I saved print time, money, and didn't waste too much resin.
• Interlocking pieces. I created pieces that would lock each quadrant into place with each other. I still glued them together, but it made it easier to keep them in place while the glue set.
• Imperfections. My 3d print wasn't perfect, so I used modeling clay to fill in the small gaps between the parts.

Now the fun can begin!

1. Using the hot glue gun, secure your master to the planter saucer. Make sure its flat against the bottom so the rubber doesn't get under the master.
2. Measure and mark where the border will be with a sharpie. You should leave about a minimum of 0.5" inches of space between the border and the model. You should also consider either coating and edges of the model now because it might be harder to get around the edges after you glue down the border.
3. Use the hot glue to secure the flexible border around your master. Make sure to fill in all the gaps!
4. Get your mixing tools ready and mix the Vytaflex according to the instructions. Using the drill attachment, mixing should be a breeze.
5. Coat your master model and the inside of the border in the mold release of your choosing.
6. Pour, starting from one side and let the rubber spread and flow around the model.
7. Wait. Vytaflex takes around 16 hours to cure. If you have extra vytaflex, you can use that as a gauge to know when your main mold has set.

1. The dried hot glue should be weak enough that you can peel it off the base. (If not use a hair dryer or heatgun)
2. Peel and/or cut the border away from the cured rubber.
3. Release the master removing all the linkages and working your way around the edges.
4. Inspect the mold and make sure it's free of any debris and then it's ready to be used!

SAFETY: Cement dust in your lungs is EXTREMELY bad, so use proper ventilation protection. Wear a mask and do this in a shop or outside.

• The instructions for Cement All call for a 4-1 ratio, but I found a higher water ratio was easier to work with, and left a smoother finish on the back side. Measure out a 3-1 ratio and add more Cement All to get a smooth consistency. It shouldn't be watery, but it also shouldn't be too thick. (I suggest practicing a small amount)
• A tip I learned was to have small extra molds on hand, so pour extra cement into so you don't waste anything. You can find coaster molds at hobby store or Amazon.

1. Optional. If you decided to do extra credit, assemble your void jig into the mold.
2. If you calculated how much you would need with Fusion360, make sure to mix more than enough.
3. Add your pigment powder to the Cement All, mix, then add it to the water
4. Use the drill with the mixing attachment to mix the cement. Check the drill attachment for clumps and remix if necessary.
5. Pour, starting from one side and let the cement mixture flow underneath your optional void jig.
6. Vibrate the mold (careful not to spill) to remove bubbles to have a smooth finish on the top.
7. Optional. If want to avoid drilling, wait a few minutes while it semi hardens, and use a stick to create the screw holes for the clock hook.
8. Cover with a camp paper towel and leave to dry.

1. Carefully remove the clock from the mold. If you used a jig for your void, double check the dimension to make sure everything is where you want it to be, otherwise you might need to buy a new mechanism.
2. Break off what you can with your hands, and then use some sanding tools to smooth out the rough edges of the back.
3. Optional. Sand the outer surface of your clock. I used 600 grit wet sand paper because my master was already very smooth. If your master was made from multiple pieces, there might be seams you may also want to sand out.

If you want to avoid having to drill into the cement, you could make the holes while the cement is drying. Otherwise, follow these steps.

1. Mark the centerline for your holes
2. Cover the spot with tape to keep dust down
3. Measure the length of your screw, and use tape to mark that distance on the drill bit
4. Mark the holes, and get to drilling. You may need some lubricant oil make it easier on the drill bit.
5. After the holes are drilled and clean, mix the epoxy and put a bit into the holes.
6. Place the D-hook and screws into the clock
7. Cover it with tape to ensure that they stay in place while curing.

The cast cement looks a bit dry and dull, but after a coat of spray lacquer, the color darkens and shines.

• Use an old box to protect the ground from the spray

Measure the gap for clock mechanism. The Cement All pushed my void out a bit, and ended up making the gap go from 1 cm to almost 2 cm, so double check the gap and buy a new mechanism if needed (or account for the gap).

• When you put the clock hands on, align them all at 12 so they are in sync, and then set them to your current time.

Congratulations! You made a clock! Now what will you do with all this time on your hands?