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

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:

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.