Finally moving into a new place where I can have more than a shoe box-sized workshop. I decided to make this clock so I don't have to drop and or get my phone dirty to check how late I've been puttering around. Plus, being a solid chunk of concrete, I won't have to worry about it accidentally getting knocked off the shelf. Making this takes some patience, but it is doable. I may update or refine this Instructable down the line, but for now I like the more rustic look and feel of it. Enjoy!
Safety gear!! Definitely use hearing and eye protection, as well as gloves. A dust mask or better is a good idea as well. Working with the melamine and power tools, adhesive caulking, concrete and dyes can be all sorts of messy and dangerous.
1 1/2" drywall screws
drill/ screw driver
100% silicone caulk (not white)
razors/ exacto knife
scrap of rigid foam insulation
saw (table saw; (or circular saw and 4' straight edge and clamps)
framing and/or speed square
pencil/ fine marker
wood dowel 3/8"
clear packing tape
5 gallon bucket
trowel or similar (to mix with) OR concrete or mortar mixer (I used a hand held paddle type mixer)
inlays for hour markers (I used Euro coins)
wet stone polisher and diamond polishing pads
decorative aggregate (I used smashed black granite)
dremel rotary tool
compass (the geometry kind, not the "where am I?" kind)
Step 1: Design
It's always better to start with a plan; whether it's a drawing, or even just some ideas jotted down. Originally I wanted to go big and make a ~16"x16" wall mount clock, but then I thought about a few concrete table tops I have experimented with and decided it would be better to scale down to a 12"x12" and make it a self-standing desk clock. Besides, that made it more fun to come up with a design and then build the form. I figured that if I tipped the face of the clock back a few inches so it about lined up with the back end of the base, and made it thicker at the bottom, it would stand up fine. I tried playing around with some scrap plywood and 1/2"x3" strapping just to sort of double check.
I drew it up until I liked the look, sketched it on graph paper so it was proportional, then scaled it up to real size on a drawing pad. That looked good to me, so I measured everything and made a cut list for the form. For the hour markers, start with a line at the exact midpoint to make a cross between 12/6 and 3/9. Then take a protractor and mark every 30* and mark those lines, making sure they go through the center point so that the hours line up properly (e.g. 1 and 7, 2 and 8 etc). To mark the coins at an even distance from center, I could have used a compass if I could find it. Instead I drilled a screw vertically into the center point. Then I took some mason line (string etc will do) and looped one end around the screw, while wrapping the other end around a pencil. With a few minutes practice I nailed a nice circle. All these markings can go on the clock face form, but I wanted to sketch it up in detail first. This might be overkill, but my OCD tendencies were in full swing.