# Drawing and cutting gears



## rickharris (Apr 3, 2008)

I am not sure how applicable this is to everyone but there are so many free CAD programmes out there and various ways to use the output on CNC systems I though i would post this method to draw and make gears.

This is a simple gear structure suitable for light weight operations and best suited to cutting out in plastic (acrylic) on a laser cutter.

The picture below tells most of the story but to amplify slightly:

The idea uses semicircular gear teeth, these work well in low power/small scale applications and run smoothly.
Decide on the gear ratios you need and the diameter of your gears. Select diameters that reflect your required ratio e.g for a 3:1 ratio you could use 100mm and 30 mm diameter. OR 50 and 15 mm.

You can operate from a given centre to centre if you like to fit a given design. Your diameters may be awkard numbers though.

Calculate the circumference (dia * 3.142)
Divide circumference by the number of teeth needed. (tooth diameter)
Draw a major circle with your CAD system at the diameter of your gear.
Draw the tooth circle at the tooth diameter and set exactly on the major circle.
Use the CAD system to replicate and rotate the tooth circle round the major diameter.

Delete the top of every other tooth circle and the bottom of the other tooth circle (i.e. alternate tops and bottoms - see drawing)

Add a shaft hole in the centre - 

Delete the major circle leaving your gear. Cut out using your handy CNC or Laser cutter and you have it done.


We have used this on a number of projects at school since I devised the method to calculate both given gear ratios and to fit existing centres. the next thing is to get the CNC mill to cut them in Aluminium.







Edit to add pictures of cut gears


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