Threading Index Varies With Speed

Could you plot the encoder position and velocity to see if the error is from the encoder side or LinuxCNC?

The jagginess may be from encoder wheel inaccuracies and inherent velocity quantization
errors from the 1 ms time stamp of the software encoder (this would be improved by using the hardware encoder)

 

I would think (in general) encoder resolution, axis acceleration and trajectory servo update rate all have potential to improve the tracking.

but changing the speed significantly is not going to track in the same place with the current code. I wouldn't think it would be hard to code absolute position tracking, but I bet it has some corner cases.

cmorley wrote:

I would think (in general) encoder resolution, axis acceleration and trajectory servo update rate all have potential to improve the tracking.

but changing the speed significantly is not going to track in the same place with the current code. I wouldn't think it would be hard to code absolute position tracking, but I bet it has some corner cases.
 

If that's the case, it's crazy and broken as any speed variation would  cause threading inaccuracies.
The time to get into sync with spindle rotation can of course vary, but the target Z position must be
synchronized to the spindle angle for the actual cutting path (or it's not spindle synchronized motion at all)
 

Take a look at this plot:

 

There is something pathological here as the Z velocity is never constant,
having a slow exponential rise, so the thread does not have a constant pitch.
This is not due to the acceleration constraints but either a hardware/encoder
bug or bug in the threading code.

 This is why I asked for a plot of the encoder position/velocity, to determine if this
is a LinuxCNC threading bug or hal/encoder bug.


 

Here's the plot you requested:
 

This cut is slightly different from the previous ones, as it has a tapered exit for the thread.

The comment about the non-constant exponential threading velocity got me thinking about the threads I cut. I made six. On about half of them, I had to rerun the program because there were tight spots in the threads and I could not easily get the nut on. Running the program a second time, at the same speed, still resulted in some cutting near the bottom of the threads.

I laid the treads up against a 8TPI tap. About half of them look OK, and the tap will sit in the threads without wobbling. The other half, the tap wobbles, or see-saws. Here's a photo of one of those. You can see that the tap is touching on the left side of the thread on one end, and the right side on the other. The reason some of them are OK might be because they were run twice. I don't recall which are which.

 

Well, naturally, that looks normal  (no exponential/varying thread pitch)
Was that at 200 RPM?