Lathe Treading Questions

[promisemachining]

Sirs,
I am having a threading issues that I need to resolve.
I am running a T39 Control with Centroid SEM 40 servos.
1.) The first issue is I am currently running a 1 1/8-12 OD thread in Steel with a single point carbide insert tool. RPM's 300. Programed with Intercon and using the provided thread information.
Report, Picture and program are attached.
The thread visibly looks good. When I check the pitch with a thread pitch gage it looks closer to the 11 1/2 pitch than is does the 12 for the full length of the thread. The thread runs 1.5" in length and I am starting it at +.200 before the end of the stock. Needless to say the nut will start for about 2 turns and then gets tight. The first thread I ran was a custom pitch metric in aluminum and I had to really work with the pitch to get the pitch to be right as well. Recently I ran a NPT in PVC and it is off too. I have double checked the Z axis for accuracy and it is within about .002" in 6 inches at the current threading location on the lathe travel and it repeats. I do not believe that I have an spindle encoder issue, because I can run the same program several times on the same part and it does not cut additional material on the extra runs. Is there a threading correction factor in the set up?
2.) The single point carbide threading tool needs to run about -.025" to get to a reasonable looking thread depth, otherwise the thread looks flat on the top. I usually run my part O.D. about .005" under size for threading. I used to run a Prototrak lathe and it ran the same way. They told me that it was the style of cutter I was using. If so is there an explanation for this.
I have a larger treading job coming up and need to get this fixed.
Thank you,
Tim

[cncsnw]

What are the chances your spindle encoder actually has 1024 lines (4096 counts), instead of the 1000 lines (4000 counts) you have entered in Parameter 34?

That would give you a uniform scaling error, where all spindle-slaved moves (such as threading) go 1.024x as far and as fast as they should, per spindle revolution.

[promisemachining]

Sir,
You are right! I adjusted the encoder count to 4096 and the 12 tpi is perfect.
Is there an explanation for the need to set the tool offset to negative value? I set it for -.017" on this thread and got a really nice fit.
Thank you for your help and the quick response!
Tim

[tblough]

You needed a negative offset because you were cutting with a sharp-V cutter. The minor diameter in the thread database assumes the correct thread form cutter for that thread and that includes the tip flat/radius.

[promisemachining]

Tom,
Thank you for the response and the information. I knew there had to be a reason.
Thank you,
Tim