4 Axes CNC how wire foam cutter

[CarCan]

I have some trouble in UCCNC setup.
The cnc machine is composed by two indipendent "tower" moved by two motors each (XY axes and AB axes)
I wanna use constant velocity setting and I work with inverse time mode feedrate.
With standard settings, sometimes, the trajectory algorithm change the speed of the motors in strange way.
I have tried to configured the AB axes as rotary, to exclude they, from the trajectory algorithm but nothing is changed.
In other words, I need that the trajectory algorithm is performed for XY and AB independently.
There are some settings that I can use?

[CarCan]

I made some test with a simple gcode,

out.tap

The GCode

(874 Bytes) Downloaded 376 times

the left "tower" is XY and the right is AB (AB as rotary to exclude from trajectory algorithm)

TGA.pro

The profile

(35.58 KiB) Downloaded 375 times

As shown in these two picture

the feed is correctly 240 and it's ok.
But next something goes wrong as shown in this picture

the feed is now 160 that is not ok.
All the movement should be done at the same speed.
Someone has ideas ?

[CarCan]

I think that is a bug, I do a test with feed mode in unit per minutes, the feed remain the same also when, in the previous test, the feed become slow (that is wrong).

[Klembaris]

Try all axis as linear. Don't think so its a bug, its just not suitable for foam cutter especially for tapered cuts.
CncPlanet has an option for foam cutters like speed calculations. Maybe its possible with plugins.

[CarCan]

What Is not clear in my post?

[cncdrive]

And what velocity does the AB axes moving when the feedrate drops from the programmed 240 to 160?
When the axes are in rotary mode then the feedrate is calculated on the linear axes (except if there are only rotary axis movements), the rotary axes then only following the XY in a way that when the XY finishes the movement with the programmed feedrate on the XY axes plane the AB finishes the same time.
This is how the motion is planned when rotary axis are in the motion.
The only exception is if the A or B or both axes cannot finish as fast as the XY, then the XY plane feedrate will drop, because then the A or B or both cannot finish that fast and so it is nessessary to slow down the XY. This can happen in cases when the AB acceleration and/or velocity is too low compared to the XY and/or the AB path is too long and so they cannot run that fast even with the set acceleration and max. velocity.
So, this is a physical constrain due to the settings and/or the motion code.

[CarCan]

I understand what you say, but you need to know that the axes are identical, and also the movement XY AB are the same, and when the trajectory is bottom to top the speed is right but when the movement is top to bottom the speed decrease. Test it with the attached profile and gcode. Consider also that If you do the same test with g94 and not inverse time the speed is always right, so is not an axis limit

[cncdrive]

In G94 the trajectory is planned on the XYZABC plane depending on which axes are used in the motion, so then the feedrate is correct on a multi-dimensional plane and not on a max. 3 dimensions plane with the other 3 rotary axes possibly just following.
But it is possible that something is not right, we will test it out soon to see what's going on.

[CarCan]

Thank you. I think that is a good improvement to consider an option to select the type of trajectory algorithm xyzabc (with the rotary options like now) and XY AB for foam cutting machine, like cnc planet do.