Introduction: Mechanical Pocket Watch for 3D Print
In this instructable I'm going to show you how to print your own pocket watch! The thought of trying to make one is probably quite daunting, and in fact, it is quite the challenge, but lucky for you, I've already suffered through it so you won't have to!
Basic Pocket Watch Movement from Matthew "Rick" Shaw on Vimeo.
We'll need to look at how a mechanical pocket watch works so we know what we're getting ourselves in for. There are a few good videos online, this one is short and explains it very well, though it is just a demonstration and doesn't quite explain everything, while this one is considerably longer, but is still completely accurate and extremely useful for figuring out what we need to make. In case you don't want to watch them, or don't have the time, I'll give a quick explanation here:
The Balance Wheel is the key to the watch. It is attached to a spring, and is pushed by the Fork Pin. When pushed, it spins a little before the spring pulls it back, knocking the fork pin on it's way past, which gives it an extra little push to continue the momentum. By doing this, the fork pin is able to quickly move from side to side, allowing two paddles to catch and release a wheel with long pointed teeth called the escape wheel. This is the ticking sound you hear from watches, and is what controls the speed of the watch. A series of gears are attached which pass the motion on in a way that gives the correct rate for each hand. The power for the watch comes from a second spring inside the barrel of another gear. When you wind the top, the spring is tighetened, and releases slowly, pushing the chain of gears.
So, now that we have an idea of how the watch works, we can begin to look at how we're going to make it and some important facts to keep in mind.
Note: It is of course, impossible to entirely print a watch as there are two springs required which cannot be 3d printed as then they would not have tension. The spring for the balance wheel must be careful made to ensure the correct motion of the balance wheel. Similarly, if you want a glass front, you'll obviously need to get the separately, although with the rise in 3d printing, many companies are offering a clear material.
Step 1: 3D Printing Notes
Types of Printers
There are two main different types of 3d printers; FDM (Fused Deposition Modelling) and STL (Stereolithography). These names sound overly complicated, but they're basically; Melted plastic printed in lines, and liquid resin solidified by a laser. As you can imagine, lasers are far more accurate than a nozzle head extruding plastic, so Stereolithography printers are far better, and as a result, far more expensive. These are generally professional standard, so for home printing then you'd want a FDM, like the MakerBot Replicator 2. However, if you really really want an STL, MIT researchers have developed a small relatively cheap one called the Form 1. Now when I say relatively cheap, it is still about twice the price of the MakerBot, and with a smaller print bed, so keep that in mind if you're picking your own printer.
3D Printing Companies and Materials
Never fear however, there are plenty of online options for ordering 3d prints of your own designs (And even sites where you can download other peoples creations for 3d printing, like Thingiverse). Companies like Shapeways and i.materialise offer a range of materials that you can print your designs in too, which gives you much more control over the final look than printing something from home. It is important to note the specifications of the material you want to use though. Due to the nature of 3d printing, every material will have a minimum thickness and minimum detail it can print. Always check for the limitations before designing something in that material or you may lose important detail. With something as small as our pocket watch, we need as much detail to be kept as possible, so the High Detail Stainless Steel from i.materialise would be ideal, with a minimum detail of .1mm!
Sending Objects to Print
As for sending a file to print, there are a few important things to keep in mind. For those unfamiliar with 3d programs, objects are made using vertices, edges and polygons (Though they may be given different names depending on the program). Vertices are the points, edges connect them, and polygons are the surfaces that fill in between the edges. However, polygons can be deceptive. While in 3d they may look smooth and correct, in reality they may be trying to connect impossible shapes. Here are some key things to look out for (With further explanations below):
- Objects will be divided into triangles for printing, so make sure surfaces have enough detail, and are made of all quadrangles or triangles.
- Smoothing will not be printed, make sure objects are set to hard edges to see their true form.
- Holes in meshes will confuse the printer, make sure there are no gaps, and that everything has some thickness.
- Intersecting or Multiple objects will not be joined, but will confuse the printer as to which part is to be printed, and what is empty space.
- Surfaces have an outwards direction called normals, which must all point out from the object.
- Overlapping geometry will cause problems, make sure the mesh is clean.
Polygons and smoothing
Place a sheet of paper flat on your table. This is our polygon. Now lift up two opposite corners. See how the paper curves? Our polygon is trying to create a flat surface that connects those points. In reality, we would need to fold the paper diagonally to keep the surfaces flat while lifting those corners, which is really creating two triangles. For this reason, it is always best to keep your models made of a maximum of four sided polygons, and ideally in triangles to see it's true shape, as this is how it will be divided up when it goes to print. Also, do not be fooled by any smoothing that the program does. Many programs will try to adjust the surfaces to give an illusion of a curve when it's made of tiny squares. Turn off this to give it hard edges so you can see exactly what will print.
Holes in Objects
When printing something like a cube, The program understands that the interior is to be filled. However, if you remove one of the sides, the printer will not know where the printing should stop and will provide messy results. Ensure that there are no gaps in your model. If you need to have an opening, you will need to fill in any interior detail (eg, if you want a hole in a block, you will need to make the interior tube connecting the two sides, and have it attached).
Similarly, intersecting objects will confuse the printer as to what is 'inside' and what is 'outside'. Always make your object out of one mesh, or print it in separate parts that you can attach together later. Boolean functions can add/subtract multiple objects, but the resulting meshes can be very messy and require some cleaning up to ensure there are no gaps or incorrect edges/vertices.
Another problem that can cause these messy prints are the normals of your surfaces. Normals dictate what direction a surface is facing. Taking our box example, if the normals point outwards, it will print the box with a filled interior. If they point inwards, it will consider what we see as inside the box to be outside, and will consider the entire rest of the printing area as the 'inside'! (However, without another surface to tell it where to stop, it will be confused and likely just make a mess!).
Sometimes when creating edges/vertices/polygons you can easily accidentally create ones on top of one another. The printer will try to print all of these, so you'll get some very bad results. Selecting all vertices and welding at a very low value is useful for removing any double vertices (and often their adjoined edges). Programs like 3ds max have options like XView (in the Tools menu) for checking for any irregularities on the object.
Step 2: The Mechanics
Alright, there are a lot of gears to think about and it is probably quite hard to decide where to start. Firstly, we'll need to know what ratios we want our gears to be to pass along the power so that we get exactly the movement we want. As we all know, 60 seconds to 60 minutes, 60 minutes to an hour, 12 hours to one revolution of our hour hand. So, our gears for our seconds will want to rotate 360° every 60 seconds, our minute gear every 3600 seconds, and our hour gear every 43200 seconds (or 12 times our minute gear).
If you watched the previous videos, you'll know that there should be 5 ticks per second. As the fork pin moves, it creates two ticks for each one pin of the escapement wheel it allows to move. So, 5 ticks = 2.5 pins. To save everyone a headache here, I'll type up the number of pins and ratios below, but feel free to try work it out for yourself!
Escapement Wheel: 15 pins (15 pins = 30 ticks = 6 seconds)
-Attached Gear: 6 pins
Third Wheel: 60 pins (6 pin wheel turning a 60 pin wheel means it will take 10 times as long, so 10 x 6 = 60 seconds)
-Attached Gear: 8 pins
Secondary Wheel: 60 pins (8 pins turning 60 = 7.5 times slower = 450 seconds (wait for it...))
-Attached Gear: 8 pins
First Wheel: 64 pins (8 pins turning 64 = 8 times slower = 3600 seconds, which is our minute hand)
-Attached Gear: 11 pins
Intermediate Gear: 36 pins (11 pins turning 36 = 3.2727 recurring times slower...)
-Attached Gear: 9 pins
Hour Gear: 33 pins (...and 9 pins turning 33 = 3.666 recurring times slower. Multiply 3.2727 x 3.666 and you get 12 times slower, giving us our 43200 seconds)
Phew! While I'm certain there are other ways of getting the same results, these seem to be the most common ratios used. Also, it took me a very long time to figure them out so they're what we're sticking to! Now then, all we have left is to add in our mainspring and winding gears. These don't necessarily need to be any specific size, as they will simply decide how long it takes to wind up the watch, and how long it holds the power for. Keep in mind the basic rules of gears (a smaller gear turning a bigger one turns it slower, while vice versa makes it faster) and you can come up with suitable ratios.
Next up, Designing the gears.
Step 3: Designing the Gears
Designing gears can be tricky, but once you've got the hang of it, they're quite simple to do. Due to the great size differences in our gears, we'll need to be extra careful in how we make them.
For each gear, we'll need to find out it's Pitch (P), Number of teeth (N), and Diameter (D). For any gears to mesh smoothly, they must have the same sized teeth and spacing. To save ourselves some pain, this site is fantastic for laying out what we need to know for making our gears. It might look a bit worrying, but don't worry, all we really need to worry about is the Pitch, Number of Teeth, Diameter, Tooth Thickness, Addendum and Dedendum (We then add and deduct these respectively to the radius of our gear). Now, to begin making our gears, we're going to make a very quick gear shape. Depending on your choice of 3d program, there may even be built in 'gear' options. However, if you're using 3ds max like me, then your best bet is to use the Star spline option.
Go to Create > Splines > Star and create it with the same number of points as teeth you need, and use your inner and outer radius (as calculated from adding / subtracting the addendum / dedendum from your radius). Use the fillet options to round off the edges (But make sure to adjust your inner/outer radius by an equal amount so they do not shrink!) and then applying an Edit Spline modifier. Using the spline selection mode, select the star, right click and set set points to corner. Ta-da, we now have a good start to our gears. While normally this could be enough, we're going for a precision object so we may need to tweak them a bit.
The best method I found was to place the gears in correct contact (You can work out the distance between their centers by adding their Diameters and dividing them by 2) and then adjust the teeth that are connecting. Put a test animation on to see how they blend together. When you're certain they're meshing as good as you can get, apply an extrude modifier to give the spline a solid form. Next, use an edit poly modifier to join up the surface into quads (remember our 3d printing notes?). Delete the rest of the gear except for the one tooth. Now you can use a tool called 'Array', found under the Tools dropdown menu. Because you made the tooth from a gear of the correct size, it's pivot point will be in the center of where you want your gear to be, so by creating a number of copies equal to the number of teeth you need, and rotating each copy an equal division of 360°/(number of teeth), you will get your gear shape. Use an edit poly to weld the teeth together and tidy up the inside and congratulations, you now have your gear.
Step 4: Positioning and Casing
Trying to pack all the gears into as small a space as a pocket watch means two things. Firstly, we're going to need some really small gears! But more importantly, by carefully placing them we can maximise our space. We also need to consider where we need our gears to line up. Obviously, we'll want to attach hands to our gears so our clock-face will actually tell us something, so we'll need to keep in mind how we're going to reach something through the jumble of gears to attach all our dials.
The minute and hour hands are nearly always kept together at the center of the the dial, though the second hand is often placed separately in it's own circle just above the '6' or to one side. This is a far easier pattern for us to arrange. In hindsight, I should have designed the front of my clock-face first so I knew where I wanted to place my second hand, something which I certainly advise you do as I accidentally left it a bit low and had to adjust the numbers right to the edge to fit everything. Because our gears will need to be exactly the right distance from each other to work, you can calculate their positions by making a triangle of the distance between the center of the watch to the center of our seconds hand gear, and the two distances between their intermediate gear. Where these two distances meet, place your middle gear.
The casing for the pocket watch is divided into two main plates, a top and bottom. This is so that we can place all our gears and keep supports on either side of them. The hour gear and it's intermediate gear will be in the top plate, while the rest are held in the bottom plate. Keeping in mind our restrictions for printing that require our meshes to be relatively tidy, creating a casing to hold all our gears can be quite tedious. The best method I found was to create a circle for each gear, but slightly larger to allow room for it spin freely, and then adjoin them. Make sure to keep a hole at the center position of each gear so we can tighten them in place. In a real watch, jewels weren't just used for decoration, but because they allow for much smoother movement, and therefore more accurate timekeeping. If you happen to have jewels lying around, then great! Otherwise, you'll need to come up with your own system to keep the gears spinning in place.
To create the time keeping hands, the hour gear will have a hollow pipe attached that allows the shaft of the minute gear to rise through to the clock-face, which the hand will be attached to, while the hour hand will be attached to the pipe.
Keep in mind when making your casing that it not only needs to hold the pieces, but you'll need to be able to take it all apart and put it back together. There's no point in have a hole for a gear if there isn't a way to get the gear into it!
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