This example is based on the following resources:

http://blog.makezine.com/archive/2010/06/make-your-own-gears.html
http://repraprip.blogspot.com/2011/05/inkscape-to-openscad-dxf-tutorial.html
http://www.iheartrobotics.com/search/label/OpenSCAD



Gear basics
If you know any two things about gears—outer diameter and number of teeth—you can use some simple equations to find everything else you need to know, including the correct center distance between them.

Number of Teeth (N)

How many teeth are there in the gear

Pitch Diameter (D):

The circle on which two gears effectively mesh, about halfway through the tooth. The pitch diameters of two gears will be tangent when the centers are spaced correctly.

Diametral Pitch (P):

The number of teeth per inch of the circumference of the pitch diameter. Think of it as the density of teeth—the higher the number, the smaller and more closely spaced the teeth on a gear. Common diametral pitches for hobby-size projects are 24, 32, and 48. The diametral pitch of all meshing gears must be the same.

Circular Pitch (p) = pi / P:

The length of the arc between the center of one tooth and the center of a tooth next to it. This is just pi (ï€ = 3.14) divided by the diametral pitch (P). Although rarely used to identify off the shelf gears, you may need this parameter when modeling gears in 2D and 3D software like we're doing here. As with diametral pitch, the circular pitch of all meshing gears must be the same.

Outside Diameter (Do):

The biggest circle that touches the edges of the gear teeth. You can measure this using a caliper like Sparkfun.com's # TOL-00067.

Note: Gears with an even number of teeth are easiest to measure, since each tooth has another tooth directly across the gear. On a gear with an odd number of teeth, if you draw a line from the center of one tooth straight through the center across the gear, the line will fall between two teeth. So, just be careful using outside diameter in your calculations if you estimated it from a gear with an odd number of teeth.

Center Distance (C):

Half the pitch diameter of the first gear plus half the pitch diameter of the second gear will equal the correct center distance. This spacing is critical for creating smooth running gears.

Pressure Angle:

The angle between the line of action (how the contact point between gear teeth travels as they rotate) and the line tangent to the pitch circle. Standard pressure angles are, for some reason, 14.5&Acir;° and 20°. A pressure angle of 20° is better for small gears, but it doesn't make much difference. It's not important to understand this parameter, just to know that the pressure angle of all meshing gears must be the same.

Addendum:

how much the tooth protudes beyond the pitch radius

Dedendum:

how deep goes the tooth before any fillet take place

Pressure angle:

The angle between the radial direction and the tangent to the tooth at the pitch circle. 20 is a good value.

Some Rules

The value of the base circle is based on pressure angle.

• 1.The smaller the pressure angle, the smaller the base radius
• 2. The addendum must be less than the dedendum
• 3. The dedendum must be more than the difference between the pitch and the base radius.

All of these gear parameters relate to each other with simple equations.

InkScape to OpenSCAD

1. 1. Open Inkscape (www.inkscape.org — free, open-source vector based drawing program ) and create a new document (CTRL+N).
   
   

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2. 2. Under the file menu, go to Document Properties and change the default units in the upper right hand corner to inches (SHIFT+CTRL+D). This will change the rulers from pixels to inches.
   
   

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3. 3. Create a gear: Extensions>Render>Gear.
   
   

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4. 4. The dialog box allows you to specify
   • Number of Teeth
   • Circular pitch, px
   • Pressure angle
   Leave Pressure angle alone — the 20° default is standard for off the shelf gears so is a good place to start.
   
   Chose 28 teeth with a circular pitch of 24. Click Apply, then Close the dialog box, your gear should be displayed.
   
   

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5. 5.From Dustyn Roberts's Gear Tutorial
   

Note on circular pitch: In Inkscape, the circular pitch is given in pixels, not inches, as we're used to using in the equations in the above table. You can get different gear ratios by just choosing a circular pitch that looks good and varying the teeth number, but if you want to make gears that interface with off the shelf gears, you need to pay a little bit more attention. By default in Inkscape there are 90 pixels in 1 inch. So if you set circular pitch to 24px in the gear tool as done above, that rounds to 0.267 inches (24/90 = 0.2666...).


Since diametral pitch (P) = π/circular pitch (p), the diametral pitch (P) in inches is = π/ 0.267 = 11.781. You will not find any off the shelf gears with a diametral pitch of 11.781. Common diametral pitches are 24, 32, and 48. So if you plan to make gears to play nice with off the shelf gears, start with the diametral pitch of your off the shelf gear and use the equations in the table to work backwards to what your circular pitch should be in pixels in Inkscape.

This looks much better. The lines are straight, but due to the sheer numbers, you see a curve!

linear_extrude(file = "something.dxf", height = 10, center = true);

• Both of your gears have a circular pitch of 24 px, a pressure angle of 20°.
• The big gear has 28 teeth and the small one has 14.
• The circular pitch in pixels to a diametral pitch in inches of 11.781.