I can't seem to add this video- the error says it is password protected, but it isn't:
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.
Materials and resources:
- cheap surplus DC gear motors - http://www.surpluscenter.com/Electric-Motors/DC-Gearmotors/DC-Gearmotors/?page_no=1&sort_by_options=price+asc
- cheap power supplies for the motors (amazon or ebay)
- http://507movements.com/ - the manual from 1898
- http://geargenerator.com - website that let's you design gear combinations and save as .svg files, which can be outputted on a laser cutter
- basic woodshop tools and scraps
- whatever junk materials you have handy
Learning goals / skills acquisition:
- Understand gear ratios (to get the rpm's right so the clock keeps time), 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 the world changed as a result?) and mechanical innovation
- Learn to use 2D CAD (we used Adobe Illustrator) to design, edit, and size mechanical parts
- Get comfortable using a laser cutter
- See https://vimeo.com/154866341
- Talk about the history of timekeeping. See if students 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.
- 1st shot at design process - think about user’s needs, think, sketch, adjust
- Introduce geargenerator.com and how gear ratios affect speed of rotation and torque. E.g. gear ratio: number of teeth of gear 1 : number of teeth of gear 2. 10:5 goes 2x fast, 1/2 torque. Learn the different ways gears are measured:Circular pitch: distance between teeth (length of arc). Must be same for all gears that mesh. In inches, 1 is a good value to use. 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 show students how to grab images and save them.
- Once students have made gears using geargenerator, show them how to export and open them in a 2D CAD program. Go over editing shapes and output format for laser cutter. Show them how to trace an image grabbed from 507movements so as to make a laser-ready file.
- Work on design sketches that include measurements and gear ratios.
- Build. Test. Redesign. Iterate.
- Display the clocks in public - make an art exhibit out of it.
- Quiz on gear ratios and torque
- Participation and final product
- Reflection at end of project
- 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.
- Bottleneck at laser cutter when students use slow-cutting materials (acrylic or thick wood) or engrave hi-res designs
- Students often tend to rush to construction without enough planning. Depending on how much time you have, try to minimize this
- This is a long project. It took us more than a month, 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, then there's cleanup.
I co-taught this project with Melissa Belardi, physics teacher.