In prototyping mechanisms I find that I need to print parts that include RC servo spline geometry. A google search didn't point me to useful geometry, at least not when I wrote this.
Before you get too excited you'll need access to a high resolution 3D printer, I used the an Objet 500. The good news is service bureaus are popping-up everywhere and small parts are quite affordable to print.
I tend to use Hitec servos with ball bearings and metal gears for most of my mechanisms. They cost a bit more and are worth it. Stripping plastic gears on a newly assembled project feels almost the same as finding out someone ate your last piece of chocolate.
Step 1: Printing and Testing for Proper Fit
Sadly, not all 3D printers are alike. I've even found that different materials, on the same printer, produce different results. Given the size of the RC servo spline you'll want to run a few test parts and check the fit.
I've included the test parts that I printed in a zip file. Servo Spline 000.stl worked perfectly for me. 001, 002, and 003 are .001, .002, and .003 of an inch oversized.
Remember to clean out all the support material before testing. I've found that remaining support material can give the impression of a tight fit. Assemble and dissemble each test part a few times as well.
Step 2: Creating Your Part
Here's an example part incorporating the spline geometry.
I've also included the solid part files to work with.