I am actually wondering if the spindle pulleys ratio will be observed in the ESS plugin and that may be why there is a prescale in the Plugin itself.
Ask Greg if that is what the Index Prescale is for in the Plugin, ie set to 2 for a single slot rotating at half spindle speed.

Hood

Its actually backwards from that. The spindle rotates twice for every 1 disc rotation. I thought about tuning it back in the ESS dialogue but then I'd have half the threading resolution and I already consider 1 tick per revolution to be barely adequate. The slots in the disc are hand ground. I'm going to throw it in the mill and cut two precise slots to get rid of that differential you saw in the scope on the video. Do you feel like that is the core problem here?

It certainly wont help.
Hood

OK. I milled two precise slots with the CNC mill and eliminated the shift in the signal between slot 1 and 2. I turned the index debounce to 0. As a result the threading cycle seems to be working significantly better. It seems to want to cut an 11 pitch thread instead of an 8 pitch thread but I think I need to go back and revisit the Z axis steps per inch calibration since it seems to have also cut another surface on the component at 5.95" when it should have cut it to 6.00". I also need to go back and read the threading manual again. I'll report back once I have these last few bugs straightened out.

Thanks for the help.

SOLVED:

I was able to turn some very nice threads last night after figuring out the last of my issues. As I mentioned above the error between the two slots in my timing disc was the source of the inconsistent pitch issue. I suspect this particular problem was rather unique to my system since my index wheel rotates at exactly 1/2 the speed of the spindle. I have two slots on the disc so that my timing index still reflects a 1:1 ratio. Most people would have one slot because they are running the disc at the same speed as the spindle.

The problem with the pitch being wrong was also related to ratios. At first I thought maybe my Z axis calibration was wrong but it was spot on. In my pulley setup screen I had modified my pulley ratio to make the VFD and the RPM readout match. This was WRONG. The pulley ratio will reflect the ratio between the index signal and true spindle speed. I could have used this to keep just 1 slot on the disc with a 0.5:1 ratio, but instead I had the ratio set to 1.5:1. I only realized this error when it occured to me that the pitch I wanted was exactly 2/3rds of what I was actually cutting. Well, guess what 1/1.5 is? ) 0.667. When I changed my pulley ratio to 1:1 it cut a perfect 8tpi thread.

Since I changed the ratio on the pulley the VFD and the RPM readout were now significantly off. I fiddled with the rather miserable automatic spindle calibration function in Mach3 for an hour before I got the numbers somewhat close. They still are not the same (500rpm setting reads 460, 600rpm setting reads 600, 1300rpm setting reads 1100) but at this point I don't care much.

How are you controlling the VFD?
Hood

 The VFD is being controlled through the C32 breakoutboard via analog 0-10V PWM. Using the auto spindle calibration feature in Mach did work, it just isn't as accurate as I'd like. I can manually tweak the voltage signal on the BoB to tune in a particular setting, but if I get 500rpm right on, 1000rpm is compromised. It seems like the auto calibrate routine should work better than it does...

Ok, have b=never used an CNC4PC stuff so not sure whether its PWM or not but have you seen the info Greg posted for the SmoothStepper and spindle control, I would imagine its the same for the ESS.
http://www.machsupport.com/forum/index.php/topic,9523.msg58400.html#msg58400

Hood

 Been there done that. The spindle motor tuning tab doesn't seem to have any effect on the relationship between the 0-10V output and the RPM actually commanded. I wish there was a way to manually set the modification values which the auto spindle tune is coming up with.

Is it a VFD thing or a C32 thing, ie if you command your top speed is the voltage 10v and if you command half speed is it 5v?
I have only used 2 methods of spindle control, one is step/dir capable servo drives and the other is step/dir via Peter Homanns controller to an analogue servo drive. With either of them it has been a simple setup, no messing around with spindle cal, just set the numbers and go.
If however the voltages are correctly being output and it is the VFD then I can see why you may need some form of calibration.
Hood