G-Wizard Calculator:
Fast, Easy, Reliable Feeds and Speeds




GCode is Complicated
G-Wizard Makes it Easy


CNC Gear Hobbing with a Fourth Axis: It’s Simpler Than You Think

Mar 24, 2012   //   by Bob Warfield   //   Blog, Software, Techniques  //  No Comments

I ran across this new post from Dave Decaussin (one of the founders of Fadal, now retired and doing other things) showing how he cuts gears on his 4th axis:

YouTube Preview Image

He makes it look easy and fun to try.

He’s cutting a 40 tooth spur gear with a diametral pitch of 24.  Diametral pitch is a measure of how many teeth there are per inch of circumference on the gear.  For more gear terminology, see this Carnegie Mellon University page on Gears.  David basically describes four things you’ll need to make a gear:

–  A Hobber, which can be purchased.  He is using one designed for use on a mill that cuts a single tooth at a time.

–  Calculation of Pitch Diameter, in this case 40 teeth / 24 Diametral Pitch = 1.667″.

–  Calculation of Major Pitch Diameter by adding 2 teeth and divide by diametral pitch = 42/24 = 1.75″

–  Calculation of Minor Pitch Diameter by subtracting 2 teeth and dividing by diametral pitch = 38/24 = 1.583″

The Major Diameter is the overall diameter of the gear and the minor is the diameter the gear hobbing cutter needs to cut to.  So, in this case, he is making a 1.75-1.583 = 0.167″ depth of cut.  You could use our G-Wizard Calculator’s Saw function to calculate feeds and speeds for such a cut.  Lastly, for each cut, advance the appropriate amount of the circle.  If we have 40 teeth, it’s 360 degrees in a circle divided by 40 or advance the 4th axis by 9 degrees each pass.

I’m fascinated by gears and will eventually add some simple gear calculations to my G-Wizard Calculator.  I’ll need to make quite a few of them on the road to realizing my design for an Astronomical Clock.  If you’re interested in gears, be sure to also check out Art Fenerty’s “Gearotic” software.  Think of it as specialized CAD/CAM software for designing and simulating gears of all kinds.  I’ll order a copy of it at some point and do a review with some screen shots.

Meanwhile, check out Dave Decaussin’s YouTube channel–good stuff!

And, if you like gear work, be sure to also check out Georotic Gear Design Software.


G-Wizard Calculator

Take Control of Your Feeds and Speeds.


Try It Now!


Like what you read on CNCCookbook?

Join 40,000+ Machinists, Designers, Engineers, and Hobbyists!  Get our latest blog posts delivered straight to your email inbox once a week for free. Just enter your email address below:


100% Privacy: We will never Spam you!

Leave a comment


G-Wizard Calculator

Take Control of Your Feeds and Speeds.


Try It Now!



  GW Calculator

  GW Editor



  Deals and Steals

CNC Blog








     Feeds and Speeds

     G-Code Tutorial

     CNC Machining & Manufacturing

     DIY CNC Cookbook

     CNC Dictionary

CNC Projects


     Machinist's Search


     Online Groups


     Reference Data


     CNC Dictionary


     Tool Brands


     Hall of Fame

     Organization: Soon!





     Our History

     Privacy Policy

All material © 2010-2014, CNCCookbook, Inc.