[Maker’s Note]Concrete Magnetic Clock

by Yun-Han Lin

Earlier this year, we promised to hand-make clocks for people who subscribe our page. Now, we’ve made it- “Magnetic Concrete Clock.” The magnetic display allows users to change the number anytime at will! In fact, there are some details omitted from the video. Today, we’ll show you everything!

Bolt v.s. Magnet

We have tried using bolts and magnets. Bolts leave small holes on the clock face, while magnets retain the integrity and beauty of the clock face, so we opt for the latter.

Magnet Specification

The neodymium magnet has better magnetism. After the test, we embed a magnet, 10 mm in diameter and 4 mm of thickness, into the movement. For decoration and numbers display, we use smaller magnets in 10*2 mm for they feel great when you move them with hands. Some magnets are in 5*2mm, because the patterns stand out more easily with magnets with different thickness.

Choosing Movements

Movements can be categorized based on their functions and quality as follows: Long Shaft, Short Shaft, Sweep Second, Jump Second, Mute, Short Hand, and Long Hand.

  1. The shaft length is determined by the thickness of the clock face. Most of the hand-made clocks, such as those made of solid wood or concrete, have thicker clock face than regular ones bought from stores. Because of this, movements with matching length is crucial. The shaft length should be the same as that of the metal thread. There should also be room for nuts and gaskets for future use if needed. In addition, if the shaft length is longer than the clock surface, it may not look as appealing.
  2. The difference between Sweep Second and Jump Second is that the second hand moves in different ways. Sweep Second causes the second hand to move in a smooth sweeping motion which human eye cannot see the intermittent, while Jump Second causes the second hand to move in individual ticks, usually along with noticeable movement sounds.
  3. Clock pointers mainly come in different lengths, styles, patterns, and materials. Pointers sold in stores are usually made of metal, and it is harder to find plastic ones. Therefore, we decide to 3D-print our own pointers.

Plate for Positioning

There are two reasons to use a foam board as the plate for clock positioning. One is to help position magnets, and the other is to reduce the weight of the clock face. The key to embedding the plate is to dig some holes, which help share the weight on the two layers of concrete surfaces so that the clock face will not crack. We’ve tried foam boards in 3mm and 5mm. Although the one in 5mm can effectively share the weight, it is so thick that it can possibly crack the second layer of concrete, so we opt for the one in 3mm.

Concrete powder

We use Lotos Concrete Pour, which is different from other brands as it features fine aggregate that can form a smoother surface and faster drying. It is suitable for making small figures and easy to get started for beginners.

However, we also accidentally discover that this kind of concrete contains metal or other materials that are magnetic, which can cause the concrete above the magnets to swell when we vibrate to eliminate air bubbles after the concrete is poured. Polishing is also needed after drying.

Design the Numbers

As shown in the video, smaller magnets that make up numbers must be placed in the middle of the clock surface. Only in this way will the numbers be correctly positioned after these magnets are attracted to the one on the clock face. This is how we come up with a clock with outstanding functions, appearance, and precision.

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