How to Develop a Curved Gear Profile

Traditionally, gears used to change the direction of rotation have been bevel gears with straight-cut teeth. While these are not the most mechanically efficient gears, they are less costly to scale up and down in just two dimensions, and they can be manufactured with traditional milling machines. Curved or helical gear profiles are far more complex to model and manufacture using traditional techniques. However, vector-based modeling software simplifies the process, and CNC (computer numeric controlled) milling machines take much of the complexity out of milling complex gear profiles. The two technologies makes it a far less onerous task to develop gears and gear ratios to maximize horsepower in racing differential, for example, in curved rather than straight profile.

Things You'll Need

  • Vector-based software
  • CNC mill


    • 1

      Determine the gear ratio you want to start with. The ratio is the number of times a primary gear will turn relative to a secondary gear, expressed 1:4, for example, for a primary or drive gear that turns one revolution for every four turns of the secondary gear.

    • 2

      Input the primary gear's dimensions into a vector-based or computer-aided drafting program designed specifically for gears and mechanical engineering. These first dimensions will be the same as a straight-toothed gear — outside diameter at each end of the gear.

    • 3

      Input the number of teeth you want in your gear profile.

    • 4

      Adjust the vectors using software points and "wires." The software will display points representing corners or intersections of planes of a gear's profile. Axis points can be adjusted to model complex curves, or helixes. The software will calculate functions for you, translating a graphical image into coordinates of the curve or multiple curves that can be read by CNC mills. Gear software will also impose any physical constraints onto your model. For example, in a beveled gear with 14 teeth, the software will constrain the ends of the larger and smaller diameters to a width of 1/14 of the gear's circumference, only allowing you to adjust what is possible to be machined. When you design one gear tooth, the software can replicate it and fill in the remaining gear with the profiles.

    • 5

      Create a derivative secondary gear. The software will allow you to create a secondary gear based on the same basic dimensions and you started the primary gear with. Given a ratio and or dimension, it will create an inverse vector-form that will mesh with the primary gear profile in the desired ratio.

    • 6

      Scale the profile up or down. Once you've designed the profile in vectors, you can scale the entire gear up or down. You can also distort it, increasing the diameter of one end of the gear but not the other.

    • 7

      Output the computer files to a CNC machine, usually by loading the files on a networked folder, then downloading them from the CNC's interface.

    • 8

      Place an appropriate-sized blank or piece of steel in the CNC machine. Like other lathes or mills, CNC mills vary from machine to machine. Blank or pre-milled metal must be installed to the specifications of the particular CNC mill.

    • 9

      Execute the software program. The mill will automatically cut the entire gear profile. Or, in some gear profiles, it may cut a portion automatically, require a cutting head change, then cut the remainder. The mill's ability to move along three axes, however, will cut all the complex curves as specified in your vector model. Then, cut the corresponding gear in the same fashion.

    • 10

      Develop profiles by keeping detailed logs of the gear's performance, including heat, power loss, friction and tooth damage. Based on the feedback, alter the vector and wire-frame model and cut new, matched gears, refining the designs for your specific application as you go.

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