Hesitation between two consecutive G33 codes

I wrote some code to thread NPT pipe. I used two consecutive G33 commands to cut the effective and the vanish thread. When I run the code on my lathe, I get a slight hesitation between the two tapers. I can see the effect on the cut. If I run about 100 RPM, it's not too bad. More speed makes it worse.

I thought it might be because the threads didn't quite match up, so I made sure that the length of the first cut is an Integer multiple of the thread pitch. This made it better, but didn't completely eliminate it.

Here's the section out of the loop that cuts the tapers:

If LinuxCNC generates the code on the fly, maybe there's some processing time between the commands that results in a small delay?

Anybody have a solution or any ideas?

G33 waits for index so will likely pause between passes
but a pause (of any duration) should not matter as both cutting
paths should be correct

What is the actual problem?

I believe that G33 waits for an index. If I rerun the program, it starts on the same point in the thread.

The names come from Machinery's Handbook. A vanish thread is the second part of the thread that blends the tapered thread to the OD of the pipe. Without it, the thread will stop at a point where it is still deeply cut into the pipe. It's a section of steeper taper, if you will.

The problem is that I get a hesitation in Z movement while the thread is being cut. If it is great enough, the Z pauses for some or most of a spindle revolution, and a non-threaded circumferential cut is made.

The explanation that there is a deceleration and acceleration between the two G33 codes may be the problem.

If it hesitates during the cut , thats either a G33 bug or perhaps an encoder issue

Edit: Maybe its related to his bug: (blending during G33)

github.com/LinuxCNC/linuxcnc/issues/68

Nots sure if this is fixed or if you could work around it by setting exact stop mode
or disabling the circular arc blend mode (ARC_BLEND_ENABLE = 0 in the ini)

Just to be clear, the hesitation is between the two G33 commands, not during a G33 command.

Edit: The link you provided appears to be similar to what I've experienced. Thanks! It'll take me a while to digest / try out the suggestions.

But since G33 waits for index (at least that's what the docs say) , it will always pause at the beginning
so It may not be possible to use it the way you are trying to use it
(merging 2 G33s instead of a separate pass), that is because of the wait for index sync
you cannot be cutting at the beginning of G33

Here's a picture of the threaded pipe:


There's a somewhat puny thread about three threads from the left end. That's the transition between G33 commands. It's not too bad on this sample, since I ran at about 100 RPM. If I run the lathe faster, it's much more pronounced.

I agree that the vanish taper is an artifact of the pipe die, and that this cut is an approximation. However, some transition is needed, or the thread would have to be cut too long. A steeper taper can help make up the pipe joint if the tolerance between the pipe and fitting is a little off.