Given a 12V coffee grinder motor, a manual on mechanical movements from 1898, a wide range of junk and a laser cutter, students must create a working clock. The only other stipulation is that it can’t resemble a normal clock. The project brings in some history (how timekeeping changed societies), design skills, math, mechanical engineering, CAD, and laser cutting skills. It’s a long project – this one took the better part of two months.
Full writeup and photos of the project on our maker site: http://lindylabs.org/recipe/clockmaking/
Riverdale’s Learning Research Team made a short film on the project here: https://vimeo.com/154866341
Materials and Tools
Basic wood shop
Wood scraps, 1/4 to 1/2″ dowels, 1/8″ plywood for laser cutting
Acrylic for laser cutting is handy
DC gear motors – ours were intended for coffee grinders, ran at 2rpm, 12V approx 1A
Cheap power supplies for the motors (amazon or ebay)
http://507movements.com/ – the manual from 1898
http://geargenerator.com – website that lets you design gear combinations
http://gravit.io - simple and free Adobe Illustrator replacement
Bins or tupperwares for each student are crucial – projects have lots of parts
- Understand gear ratios (to get the rpm’s right so the clock keeps time)
- Understand torque, how gears and other mechanical movements work
- Understand basic (or complex, depending on the clock) electrical circuits
- Use woodshop tools, learn to solder electrical connections
- Learn about the history of timekeeping – why was it important? how has it changed the world?
- Learn about the history of mechanical innovation and its effects
- Learn to use 2D CAD (we used gravit.io mainly) to design, edit, and size mechanical parts
- Get comfortable using a laser cutter
- Learn, most likely the hard way, to plan carefully and think through your designs.
While there is some theory and skill development at the beginning of this project, the projects themselves end up taking over. Most students got pretty obsessed with their clocks and the teachers acted more like coaches.
We start with a discussion about the history of timekeeping. See if students know or can guess how the earliest clocks worked (water, sun). Why was timekeeping so important? (religion, sea navigation) How did it change the world (allowed the industrial revolution to happen) etc. Look at some early design problems (e.g. at a time when clocks all used pendulums, what needed to be invented to make clocks work on the moving deck of a ship? Springs). Look at different ways to tell time from Chinese water clocks and sundials to the present. Include wacky examples. Brainstorm ideas and start sketching.
First shot at design process – identify your user and think about user’s needs. Think, sketch, adjust.
Introduce geargenerator.com and how gear ratios affect speed of rotation and torque.
A gear ratio is: number of teeth of gear 1 : number of teeth of gear 2.
10:5 goes 2x fast, 1/2 times as much torque torque. Easy to demonstrate with lego gears or in our case, some laser cut demo gear sets.
Learn the different ways gears are measured:
- Circular pitch: distance between teeth (length of arc). Must be same for all gears that mesh.
- Number of teeth: number of teeth on gear. this and circular pitch determine diameter.
- Pressure angle: determines shape of teeth. 20 degrees is a good value. Must be same on all meshing gears.
Introduce 507movements.com and talk about the time period it’s from – turn of the previous century – near the end of a period when many thought most important technology had been invented already. Explain how the index works – you can look up 145 year-old mechanical solutions – many of which haven’t changed – that fit specific needs. Show students how to grab images of mechanical parts and save them for tracing in Adobe Illustrator to use them as templates for laser-cut parts.
This year we took a break here and had groups of students build wooden boxes to store the clock parts they would soon be generating. Each group of three students was given a biggish scrap of plywood - most were at least 3x3 feet - and challenged to make the biggest (volumetrically) box they could. We required exploded drawings (after showing them what these are and what they're for) and when they were ready, taught them to use the bandsaw, tablesaw, drills, and screwguns. The takeaway was that having a perfect drawing doesn't necessarily lead directly to a perfect product. For most it was the first time using these tools and for many, introduction to recalculating, fixing, and fudging on the fly.
Once students have made gears using geargenerator, show them how to export and open them in a 2D CAD program (http://gravit.io, free and online, works for most operations. Go over editing shapes and the output format for laser cutter (usually no fill, lines stroked at .001″). Next, show them how to trace an image grabbed from 507movements so as to make a laser-ready file. At this writing this requires Adobe Illustrator or perhaps Inkscape (free).
Work on design sketches that include measurements and gear ratios. Once the sketches have turned into plans, the building / testing / redesigning process begins.
At the end of the project we display the clocks in public as an art exhibit.
This project was inspired by High-tech high's combined mechanical / history project, where students mechanically represented the rise and fall of civilizations.
- Quiz on gear ratios and torque
- Quiz exercise on using the software - e.g. in 10 minutes hand in a laser-ready pdf of a gear with X teeth
- Participation and final product
- Reflection at end of project
- Placing meshing gears just the right distance apart is key – too close and they lock up, too far away and they can skip. The trick to this is to laser cut the supporting panels (with holes for the gear axles) on the laser cutter as well, using the radius guides (rings around each gear that indicate how close the gears should be to each other) that geargenerator provides for gear placement.
- 1/8″ plywood may warp, rendering it useless for gears. Store it flat with a heavy weight on top of the stack.
- Cheap gear motors are rated at a certain speed (rpm) but in practice the speed varies, which isn’t good for accurate timekeeping. This doesn’t need to matter, but a variable speed circuit that adjusts voltage within a small range would be useful. This could be accomplished with a belt drive mechanism with properly sized wheels (good math exercise).
- We often had a bottleneck at the laser cutter when students used slow-cutting materials (acrylic or thick wood) or engraved hi-res designs. Long engraving jobs should be done at lunch or after school, not during class.
- Students often tend to rush to construction without enough planning. Try to minimize this.
- Structured as above, this is a long project. It took us the better part of two months, meeting 4x a week for 45-minute periods. It would go faster with fewer, longer periods, as it takes a bit to get set up, get back into where you were when you left off, and then clean up.
- gravit.io notes: stroke width is the box to the right of the word "Border" in the right panel. When you set border to .001" for laser output, the outlines disappear on screen. Change view to "outline" and you'll see them again.