Trying to program a helix in G18 (which would be XZ plane, axis of arc parallel to Y axis).  An example of the code would be:

G00 X0.375 Y0. Z0.375
G18
G02 X0. Y0. Z0. I-0.375 F10.

IJK is set to incremental, and I do get an arc on the toolpath display.  However there is no difference shown between G02 and G03. Both would generate what would be a CCW arc.  Now it's entirely possible that I missed something stupid, and I did try multiple computers with multiple profiles and the issue repeats itself.  Although I mentioned cutting a helix, which is the end goal, I have cut back the code to just get an arc to cut.  I also tried with R values. 
I know arcs (let alone helix's) in planes other than G17 aren't that common so maybe this hasn't come up before.

Looks like a bug in the display. But if you run the code, it runs correctly. The G3 will travel in the gap of the displayed arc.

Btw, I have cut G18 arcs before and they were displayed correctly, but I found another bug at the time. The feedrate of the G18 arc was limited to the Z axis velocity in motor tuning. I reported it to Brian, but don't know if it ever got fixed.

I am unwilling to run code that won't display, just doesn't seem like a way to be able to check yourself.  Does not instill me with confidence.  I think it may be time to move on to other software options..

I reported it on the Yahoo group.

You may want to take a look at Mach3 Turn Arc motion write up which can be found in the Members Docs as it goes
into a lot of detail and explaination about arcs for the lathe which uses G18.

As far as cutting a helix as a continous motion, i have done that using an indexer. In a sense you can cut a helix using
a threading code. Some of the wrapping programs provide code but it breaks the helix into small straight line segments.

FWIW,
RICH  

There really is no other way to run the code I need to run other than to just get the machine to run as it's documentation suggests. Additionally to the above requirements I need to make a move in the A axis while the helix is going on.  For testing purposes I have removed the A axis moves to just try and generate a correct arc (to no avail).  While it *may* run the code but display it wrong, it doesn't help me to think that it might at some point display correctly and not cut correctly or any one of a number of bizarre scenarios. 
Pretty much the final nail in the coffin for me, time to look elsewhere for reliable software...

I wish you luck in your search for alternative software and programing endeavors.

RICH