Well it's been a
LONG time in the gestation, but at last, as it hasn't stopped raining for days, I've found a bit of time to sort out a suitable 'Post Processor' to allow me to engrave text on the outside of a cylinder mounted in the 4th Axis.

So what's the issue

Well, if you use a CAD/CAM set of programmes to generate the bits you want to make on a CNC mill or lathe, the program has to spit out code that meets the specification of the controller that drives the machine. So most CAD/CAM set ups use an intervening set of definitions and code that take in the symbolic actions, and translate them to actual code that the controller can understand. Once defined and installed, from then on the CAD/CAM should (hopefully) produce the right stuff for the controller.
Now there wasn't a 'standard' Featurecam Post Processor suitable for the TNC355 controller in my mill that understood the 4th axis. There were ones that drove 4th axes. And there were ones that were close to the TNC355 (as the TNC150/151/155 is very similar, so it was a case of making a Frankenstein version, with a few tweaks of my own.
It didn't help that some of the 'standard' Post Processors made mathematical errors when wrapping stuff round a cylinder

All sorts of Gotcha's appeared - like the one that made moves all the way round the bar backwards to get to somewhere close to where it started, cutting a nice groove as it did it

Anyway much

and rather a lot of

got me there in the end.
So what are we talking about: well the first picture below is a screen shot of a very small fraction of the program like structure of the Post Processor, and the second picture is code suitable for my Heidenhain TNC355 to obey