The title pretty much sums up the question, but basically I need to create 3 sizes of gears. I have tried 2 shapes, the first being basic rectangles. However, according to my Engineering teacher, neither of those designs will work. So now I am going to try a spur gear, but I have no idea how to get the right angle and everything else, especially seen as the shape of each spur will change with the gear diameter. So how do I go about designing a spur gear in autocad? Any (free) auto cad plugins or calculators are appreciated as well.
Gear design from scratch is a fairly major process in itself and so the vast majority of gears are made to standardised tooth profiles. Metric gears are usually designed to the module system. This specifics gears according to pitch circle diameter and number of teeth (the module number is the ratio between the two). So a larger module means finer teeth.
This means that the fiddly calculations of tooth profile, clearances etc are covered by the standard so you don't have to reinvent the wheel every time you design a part with gears in it.
This leaves the designer to specify thing like the thickness of the gear (which determines its torque capacity) as well as the arrangements for fixing it to a shaft. Although in practice this often means picking an off the-shelf gear set rather than getting them manufactured as stock gears will cover almost all requirements.
Note that in the module system you need to work out your gear rations by number of teeth, you can't just have arbitrary diameters as for obvious reasons you can't have fractions of teeth on meshing spur gears.
For assembly drawings it is common to draw meshing gears as circles of the appropriate PCD and use a note to specify the module. AutoCad probably has a feature to draw the teeth for you based on this information but I don't know for sure. In any case, in modern context, it is not usually necessary to draw the detail of a gear in an engineering drawing as it will be specified elsewhere.
Also be aware that for meshing spur gears the dimensions and profiles of the teeth must be the same regardless of the diameter of the gear ie you cant just draw a template gear of one size and scale it to give different rations. Similarly for a given tooth profile there will be a lower limit to the diameter which will physically accommodate the number of teeth required.
For an academic project, properly specifying stock gears should be perfectly adequate as long as you set out the criteria used (unless of course the assignment is to design the gear profile form scratch, in which case you would hope that the basics would have been covered by tuition...)
Involute spur gears have involute shape. This means that the point of contact between meshing teeth will move in a straight line (called the line of action). Visualized it will look like a string is unwound from one wheel and wound on another wheel.
image captured from woodgears.ca free gear template page
It is the number of teeth and the angle of that line that will decide the shape of the spurs.
Is a cheap solution an option? If so there is a gear generator on woodgears.ca that can export to DXF.
P.S. No affiliation to woodgears.ca. It's just a site I know that had a decent gear generator.