I have been machining a part that has a draft angle of 20 degrees on two walls and I think I found a good method.
Here is a screenshot of one version of the part, where I tried using 'zigzag' machining with Heekscnc:
When I actually did machine the part, it took about an hour and 20 minutes. I knew that I would have to set my boundaries closer to the tapered walls, if I wanted to save any time on the next part. Also notice the extra tool paths that are cutting air. I wanted to eliminate them.
Heekscnc uses 3D machining methods from Opencamlib http://code.google.com/p/opencamlib/ . There are several strategies for 3D milling there and 'waterline' is one of them. Waterline machining makes the tool path run at a constant Z level around the part. This seemed like a perfect strategy for the part I was working on. I tried out the HeeksCNC implementation of ocl waterline using a flat bottomed end mill and got a tool path that looked like this:
I liked the approach, but I only wanted to machine the tapered walls of the part. There were some entries in the Heekscnc waterline operation for boundaries, but they weren't functioning. I looked at the source code and added some logic to 'ocl_funcs.py' to connect them and was then able to get the tool path to look like this:
Ahh! That's much better. When I actually milled the part, my cycle time was down to 20 minutes.