THC always hits THC Velocity Lock

Plasmac cutting beautiful parts - but THC just keeps dropping out due to Velocity Lock as cutting begins.
Running Mesa 7i76e w/THCAD-10, Linuxcnc version 2.8, Axis - see screen shot of Config and .ini Attached. Tried lots of variations of Voltage threshold and VAD Threshold - no luck. ARC Voltage reading about 80 volts with 'Use Auto Volts' during cutting. Read lots of forum posts thinking maybe Max Velocity or Max Acceleration not correct in .ini ????? All feedback much appreciated.
Dennis

What cut velocity are you trying to cut at?
Your ini file seems to have very low max velocities (1500mm/min)
This could be below your cut velocity so your machine never gets to cut speed.
eg. I cut 2mm steel at 2219 mm/min
VAD might never get enabled.

thanks for quick reply - these THC tests were run on 1/8" steel (3mm+) at either 1500 or 1600 mm/min about 45A

kramerda wrote:

thanks for quick reply - these THC tests were run on 1/8" steel (3mm+) at either 1500 or 1600 mm/min about 45A

Exactly. Just to be clear. THC Velocity lock (or corner lock) is enabled if the current velocity is a certain threshold percentage below the cut velocity (where cut velocity = book speed). Normally we would set the threshold to be 80%-90%. I think I use 90%. You had it set to 40%

So I saw your max-velocity in your ini is 25 mm/second (1500 mm/min) so how are you going to cut at 1600mm/min? You can't! The Gcode might be accepted but it won't be achieved. Also, the currnet velocity needs to be at or near the feed rate (cut velocity) befor auto sampling and THC enabling is executed.

So your current velocity will always be less than the cut velocity feed rate so the THC can never be turned on.

You need to be able to get up to about 10500 m/min (175 mm/sec) to cover everything in your cut charts.

Sounds like I've got a couple of issues. Let me paraphrase so I understand.....so its just the Max Velocity for XYZ in ini that's wacked.
1. For cutting movements the .Ini variables for X and Y max velocity need to be twice what I'm actually cutting at. i.e. cutting at 1500 mm/min set max velocity to 50 (mm/sec).
2. For Z axis THC same applies but that movement is likely slower (???) but ratio must exceed % threshold set by VAD. i.e. Z axis movement
750 mm/min set max velocity > 25 mm/sec. (2:1).
3. Like you said - set V Threshold % at something like 80 or 90 %.
4. Not sure about my cut chart settings - will have to read up on that topic - for now just testing on the 3mm steel.

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Since my system with steppers is 'open loop' speed control I think i'll be OK if I can get the numbers here in line.
Thanks again - we'll give it a go today.

THANK YOU VERY MUCH. That fixed it - doubled all occurrences of max velocity from 25 to 50 mm/sec (1500 to 3000 mm/min). Left accelerations alone.
Attached is new .ini file

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is it necessary to also double stepgen velocity if no backlash comp. used??

For future reference --- is the parameter MAX_VELOCITY there just to put a reliable boundary on the stepper motor limits or is there some other reason?
Dennis

Great to get you going.

You need to determine the maximum velocity your motor is capable of without loosing steps

You then need to determine the maximum accelleration your motor is capable of without loosing steps. I have found this is much more critical. It is when accelerating that the load is on the motor. When you get to speed, the load falls to about 30% of that used when accellerating (motor/driver dependent).

You need to test this in the real world as I found simple oscillating back and forth does not test it out.
I also found that when a motor gets hot, that is the time that it will start to loose steps.

You also need to heed the comments here and adjust

Thankyou - watching my Z axis stepper go back and forth during cutting is a thing of BEAUTY.
Now that everything is 'basically working' we'll tune and tweak it. Since my duty cycle is only one part at a time - pushing temp. on motors or drivers is not an issue. That said the dynamics is very important for 'accurate' parts, corners, etc. Nice catch on the STEP GEN values. Thanks.

Hi, I read many many post to setup my baby-cnc-plasma (thank you all! Great help!.
(Sorry to write here but it's the same problem as the first author, so I thought not to open a new thread.)

After setting up many things on a completely diy cnc machine, after cutting some test part, I'm now trying to cut some actual parts for a project and I have the same problems of THC blocked. (I swapped to shielded nozzles and the shield is critically near the metal to respect the cutting height... so I want the THC to work well.. I don't want to brake the new 400 USD torch for crashing into the metal... )

I read all this thread: I have a "max speed" in LinuxCNC config panel of 1999 and I was trying to cut at 2200 (2mm thick part), so I increased the MAX_VELOCITY X and Y in the .ini in my config dir and I reopened LinuxCNC, but the "max speed" parameter in the config panel has not changed.

I saw there are 2 MAX_VELOCITY parameters for every axis...

So my question is: which of the two MAX_VELOCITY (the first one or the second one) do I have to increase? and in which file? I changed these in my plasmac_06.ini (plasmac_06 machine) but it seems it's not changing the "max speed".

The .ini file was generated by the pncconf (than I used the python for installing plasmaC) I use a 7i96 and a THCAD-10.

Thank you for the reply,
Alberto.