Designing a gear box

as seen in my previous post ,the gear reduction ratio (using spur gear) which we required was 128:1, and we achieved it through software and hardware. We made the 64:1 gear reduction box using three 4:1 gears, 4:1 gear reduction is achieved by using a 20 teeth and an 80 teeth gear. All the gears are cut using 1 module cutter. The basic formula involved in gear design is,

OD = (N+2) x M

OD is the outer diameter
N is the number of teeth
M is the module of the cutter used

So all the 20 teeth gear has outer diameter of 22mm ((20+2)x1) and all the 80 teeth gear has outside diameter of 82mm ((80+2)x1). Before starting with the manufacturing process, you must draw a neat sketch of your gear box in a paper, showing its front view and top view, it will be good if you know a mechanical enginner who knows AUTOCAD or SOLIDWORKS. Knowledge of Engineering graphics is essential because you are going to have your gear box in your mind and draw its front and top view.

Three machines where used for gear box manufacuting, which are

• Gear hobbing machine - used for cutting gears automatically, you have to just feed the information to the machine and it will cut the gears perfect. The gears made using the gear hobbing machine will have perfect finish, less time consuming. Best way to make a gear, but the problem is you have to convince the owner to make the gear for you because they will have big orders. Changing the setting for the gear requires more time and manual work. The below figure shows a gear hobbing machine.

• Milling machine - used for cutting the gears manually, requires more time and labour cost for making a gear. They will cost Re.1 for each teeth i.e, 100 teeth gear will cost Rs.100. Please go for this machine if you are not finding any Gear hobbing machine in your locality.The below figure shows a milling machine.

• Lathe - Before making a gear, you must make a circle having the specified OD, so you go for a lathe. Only after the turning and facing operation the circle must be taken to the gear hobbing machine or milling machine for making gear. Be careful in marking the center hole in the circle. The figure below shows a lathe machine.
  

The materials which we used for making gears are

• acrylic material - half the weight of normal plastic, will look transparent and nice, strength is less when compared to nylon material. much attention must be paid while cutting the gears in this material, failure will result in breakage of gears.
  
• nylon material - completely made synthetic fiber, has high strength and flexibility. less smooth when compared to acrylic material. no problem in cutting gears through nylon material.
  
• brass metal - kind of bronze, an alloy of copper, tin and some zinc. Its a soft metal which can mesh easily with an acrylic or nylon material. Gear which is attached to the shaft of stepper motor is made using this gun metal.