7 Replies Latest reply on Mar 2, 2012 12:29 PM by Robert Crawford

# involute tooth profile

Hello,

It looks like a silly question though. which of the following is bigger in size when im drawing involute splines ( when Im drawing a external spline).

base circle or addendum circle? and from which circle the tooth profile starts??

• ###### Re: involute tooth profile

A quick google search yielded a ton.  Just grabbed this link as a "saw a good picture first"

http://www.helicron.net/workshop/gearcutting/involute/

• ###### Re: involute tooth profile

Hey Mike,

Thanks for your link. So, according to the link you provided, base circle is bigger than deddendum circle. right?

• ###### Re: involute tooth profile

It can be either; the base circle is a simple formula:

base_circle = pitch_circle * cos ( pressure_angle )

- - -

The deddendum (for a standard involute) can typically be 1.25 or 1.335 times the gear module ( 1 / DP )

deddendum_circle = pitch_circle - 2 * 1.25 / DP (for a 2.25 cutter)

- - -

So for example, a 60 tooth, 20° and 6 DP gear with a 2.25 cutter would have a pitch_circle = 10.00000", a base_circle = 9.39693" & and deddendum_circle = 9.58333"

A 18 tooth mating gear would have pitch_circle = 3.00000", a base_circle = 2.81908" & a deddendum_circle = 2.58333"

• ###### Re: involute tooth profile

Base circle is mathematically determined, usually a result of by pitch diameter and pressure angle (as Joel said).

Dedendum is an engineering decision based on the amount of clearance deemed necessary.  The numbers Joel uses for dedendum are abitrary, but should work.  Also important to consider the outside diameter of meshing gears and spacing tolerances.  Sometimes it is also necessary to have space for lubricants.

• ###### Re: involute tooth profile

Here is a model which uses a graphical method to produce a true involute from a base circle.

http://www.esoxrepublic.com/models/

Gear-A-out.zip

• ###### Re: involute tooth profile

Here is a gear model using an equation driven curve for the involute profile.

You will need to unfix and update the value for t2 in the equation driven curve in the construction sketch if you change the OD.

I haven't put the model through many changes, so it may fail if changed significantly.

Feedback on any errors would be appreciated.