Teaching physics to teenagers is for sure one of the best jobs I've ever done. If you are touched by a project there are many chances that students will be excited too. I think this Instructable gives you a good example of that.
Assembling a kit composed by pulleys and gears, and based on a so essential and common in our life physics law, that is harmonic motion, could be an affective experience for both teenagers and adults. This will also give you a perfect tool to let students play and experiment with gears ratio, relations between forces and movement, and understand how a mechanical clock can be set changing pendulum length.
You can find the Italian version of this tutorial on WeMake.cc fablab website here (http://wiki.wemake.cc/Orologio_di_Galileo_(vers.1)).
Step 1: Galileo Galilei Design
The project takes inspiration from a video by Ken Kuo which explain how an Escapement design by Galileo works:
You will see that basic parts of my design maintain animation's original shape (thanks, Ken Kuo!), and are quite similar to Galileo drawings.
The escapement is a mechanism which prevents mechanical clocks to discharge in a big hurry, you can find very different types of escapements, but they all are based on the same principle. In a few words they take advantage of inertia of a pendulum and potential energy of a weight, so that at each oscillation a small portion of that energy can be transferred to the pendulum to help him keeping same amplitude forever, and another portion goes to gears, to move hands.
Step 2: Simpler Design
I tried to make the simpler design I could, so that also children can assemble it. On the picture right side you see my first version, and on the left is the second version, which makes a single charge last more than ten minutes. If you search for Galileo escapement on the web you can find many different designs, among them also a nice Lego version.
You probably can make gears by hand with a vertical drill and a jigsaw, if you have good patience and you already practiced quite a lot. I decided that lasercut is a good choice to obtain nice parts in short time, although it's not so praiseworthy in comparison with handwork.
Step 3: Materials
Materials needed are very common, cheap, and easy to buy. You need some 8mm and 4mm thick plywood, a piece of 8mm and a little piece of 10mm smooth wooden bar, a pair of little brass or aluminium pipes, one bearing (with code 6000), a 6mm threaded rod and some metalware. I also used a sheet of thin plexiglas, and I cut two circles of plexiglas about 3-8mm thick, but they are not essential. See next steps for more details about materials, dimensions, and sizes.
Step 4: Metalware BOM
Other than wooden rods and plywood and plexiglas plates, keep ready these metalware parts:
one 6000 ZZ or 60002RS (10x26x8mm) bearing
two small pieces of aluminium or better brass pipe 10mm (OD) x 8mm (ID)
a 25mm DIN-933 M6 hex cap bolt
a short screw for wood
Step 5: Tools
Actually you don't need many tools to assemble this mechanism. You should have an hammer, some fine sandpaper, vinyl glue, a little bit of cyanoacrylic glue, and a wooden block with two holes, 8mm and 10mm diameters, but this last one is not essential. You need also a fine saw to cut wood rods.
Step 6: Lasercut Materials and Procedure
You can cut most parts from a 58 x 18 cm plywood board, 8 mm thick. Then you need a smaller 4 mm thick plywood board and a small plate of plexiglas (about 50 x 100 mm), which can be from 3 to 8 mm thick. Find at least a small piece of good quality 4 mm plywood for the teethed washer. Use a 0.5 or 0.6 mm plexiglas plate to obtain thin washers.
Open the attached file which contains an .dxf drawing, upload it in the laser cutting machine, and choose the parts to cut referring to the notes on the red layer.
For detailed lasercut procedure please follow some other my tutorial, like the complete stamps instructable.
Step 7: Diagrams and Blueprints
Follow blueprints in attached PDF to assemble parts together, you will see top and front views there, with numbers and letters defining pieces. Look top view to determine gears and spacers order. Besides I drew a nice 3D model of the Escapement mechanism, to better explain the steps.
Step 8: How to Use the Straightener
I call it straightener, but it's nothing more than an MDF or wooden block, with perfectly perpendicular sides. If you insert the 8mm or 10mm pivot in the matching hole, while hammering it into the gears or the plywood base, you will be sure to keep it straight.
This is quite essential in this project, but you also can verify the perpendicularity with a small set square.
Step 9: Rear Panel and Pivots
First of all use the sandpaper to smooth every piece of wood, both gears (pay attention to every teeth), plates, and also extremities of wood rods you already cut at the precise dimension. Sanding is most useful especially where parts touch one each other.
With the straightener and the hammer insert all five long 8mm pivots into their holes, with a small amount of vinyl glue. Then glue in place the four 8mm spacers as in the image. Check the pivots are vertical, with a set square and also placing in position the top cover, but don't hammer it.
Step 10: The Ratchet
The ratchet is needed to charge the mechanism easily, that is to raise the weight with no need to disengage the gears. The saw-toothed part #20 should be a made by a good quality plywood, since is exposed to some friction with the plastic fins we'll see after. The circular transparent washer is made by thin plexiglas, and should be glued with cyanoacrylate. Other pieces may be glued with vinyl glue perfectly aligned one each other. To do that I suggest to use one of the aluminium pipes, place parts on it for first minute while the glue are drying, then take them off, and clean the pipe.
Step 11: The Pendulum Hook
The pendulum is made by a 6mm threaded rod because it's easy to take on and off, and also lets you hang on something as weight at a certain distance with a nut, and easily change it.
The nut which keeps the rod is crowned by three plywood layers, labeled from 17 to 19. Look at the image and pay attention to glue everything in the right order. On the rear side, in the piece #19 you will insert the head of the bolt which will keep the hook clasped to another part (the "finger" #4).
Step 12: The Main Gear
You should also have 12 rivets, to be cut at the right length, that is about 8mm of stem. With assist of an hammer beat them in the tiny holes. Pay attention they don't lean or stick out from behind.
Then glue 8mm spacer and small gear on the rear side of the main gear, insert the aluminium pipe in the holes, and let it protruding 1mm on the rear. Always verify that wood rods rotate freely inside the pipes. Also verify that gears, especially the big ones, don't swing at all in the rotation around the pivots.
Step 13: The Fingers
In the detail from the bottom side you see there are two reference cuts on parts 4 and 5, and they have to stay aligned so to glue parts in that position. Between those two parts remember to glue spacer 13. After the vinyl glue is dry, push the bearing in the bigger hole, placing the pieces on a planar hard surface, making sure the bearing will be perfectly flat. Then you can push the assembled piece on the 10mm pivot marked as "f".
Step 14: The Multiplier Gear
This gear has the function to multiply the time needed to unroll completely the weight cord, even if you need a bigger weight to move the main gear. The cord has to be rolled around the racket, and it has to be a thin twine since it must not roll on itself.
Push the aluminium pipe in the gear hole, leaving only 1mm of tube outside the gear's bottom edge. You can also glue the tube to the gear but it's necessary, the only important fact is that this gears like the others, must not swing while it turns.
Place the little plexiglas fins in the grooves, as in the picture, then glue the extremity which is further from the center, with a drop of cyanoacrylic glue in the groove. leave the glue drying completely before sliding the racket on the tube. You see that over the bigger plexiglas washer ther is a free smaller spacer, needed to reduce frictions.
Step 15: The Arm and Counterweight
This long arm must rotate quite loosely around the shorter 8mm pivot, and as you can see in the pictures, you should place two thin washers between fixed 8mm spacers of the base and the arm. The last spacer (#15) should have a tight hole to keep the arm in place.
The metal washer at the arm extremity acts as counterweight, anyway it is not always needed. You will understand that you need it if the gears sometimes speed up and make two or three teeth at a time. Anyway if you keep a light counterweight or if you don't use it at all, the clock weight can be a little smaller.
Step 16: Slide Pads
Now, to reduce frictions, you must glue a three little Plexiglas pieces where parts slides one over each other. This is between arm and one finger, on the other finger extremity, and on the extremity of the arm where it touch the rivets gear, like in the pictures. Glue pieces with cyanoacrylic glue, or other strong glue.
you can do that also after closing the cover, but it's simpler that you take note of position, and glue them now.
Step 17: The Cover
Before hammering the cover in place, you must push the circular windows in the holes. Take off any protection from the plexiglas before inserting the discs, and look if you can determine the face a little smaller than the other. This is usually because the laser ray is not exactly cylindrical, but a little conical, and so the pieces. Place smaller face down, and insert discs in the holes. Then, cover them with a rag, use the wooden block and hammer them in position, until the upper face is at same level of wood surface..
Step 18: Tollerances
Before assembling the pieces together and closing the cover, you should well understand where to place every thin spacer, as you'll never be able to open the cover unless you'll cut the rods.
Gears have to move very softly, but they must not have play more than half or one millimeter. Washers are not essential but they help to reduce frictions. Insert one #18 washer at every extremity of the gears pivots, as in the images. In the animation you only see the top ones, but consider you need also two more spacers below the gears.
Step 19: Hang It
Now that assembling is mainly completed, it's time to think how to hang the pendulum mechanism, and where. It's easy to use holes to screw a board on the rear side, as in the photo, so that you can place it under some books in your library. Another option is to use the two holes to fix it on the wall.
Once the mechanism is vertical, you can screw in the threaded bar which acts as pendulum. At the lower extremity screw also two bolts (one should be a butterfly bolt), and a washer between them. You'll need them to attach a mass to the pendulum.
Step 20: Give It Power
Wrap the ratchet with a thin rope, in clockwise direction, and knot a metal washer on the right extremity.
Then you'll need a nice weight on the other extremity of the rope. Since Mechanism is quite flat the weight cannot be too big. I suggest a metal pipe, made from aluminium or brass, maybe one inch diameter and 20cm long. You can close bottom face and put some gravel inside. Actually you should not have too much weight, it depends on the construction quality of the pieces and assembling. Try with an empty pipe, then add more weight if you're not able to make it working.
Step 21: Setup and Details
Setup is not simple, but follow instructions and you'll be able to make it working like a Swiss clock! Furthermore, if you're a lucky person, you could already have it working without setup at all.
Now you can charge the ratchet, and push the pendulum. The fingers should touch alternatively the arm and one rivet, between one oscillation and the other. If this is not happening, you must loosen the bolt which keeps the pendulum bar, and vary the angle.
The two elements which act on the rivets wheel, should touch gently it in one position or the other (on teeth or rivet), when the pendulum is steady and vertical.
Try changing angle with very small adjustments, then when mechanism works for a while and you're satisfied tighten the bolt quite hardly.
Step 22: Show It!
Time to show the Galileo escapement mechanism to your friends! I hope you'll share and make this project, which is so much enlightening for physics phenomenons and mechanics operations!
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