I'm beginning to understand now. Yes I have to do the maths but applying TIG's methods make it relatively straightforward. I'm off to read more about tranformations and maybe have a look at fillets. Thanks.

Wow! Didn't expect such a timely response from a guru. I'll have a look. Thanks.

Hi Guys
Why is the standard Arc tool so different from the add_arc method?

I'm trying to write a script to import IDF data which defines arcs by startpoint, endpoint and include angle, to me this seems much closer to how the Arc tool works than the add_arc method (knowing that I can get the bulge distance from the include angle).

Can I mimic the Arc tool behaviour in my script or do I have to do all the mathematics to make up the parameters for a add_arc method?