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Author Topic: THC moves on the Z axis...  (Read 6894 times)

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Offline BR549

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Re: THC moves on the Z axis...
« Reply #20 on: February 13, 2016, 10:54:52 AM »
What you are trying to acheive is lowering teh normal RPM range of teh z to push it back down into teh higher torque range.  Doing that you can acheive faster accel before loosing control of the stepper.

If neccessary you can lower teh G28.1 speed in Homing Config.

Always test and tune in G00 mode Then rerun test under Gcode/THC control to verify it all.  The first signs you run into with teh Tuning Z is Z will start to loose steps IF you have gone too high.  On teh other end IF you get too fast for teh THC you will see Z occilations .

(;-) TP

Offline Davek0974

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Re: THC moves on the Z axis...
« Reply #21 on: February 13, 2016, 11:01:07 AM »
So i should put the speed back down and take advantage of lower motor speed but increase the acceleration rate?

Currently 3000mm/min and 3000mm/s/s

The speed in homing for Z only adjusts the final stage - the slow bit before contact, it was the first rapid stage that had the issues - like it missed to use acceleration sometimes.

Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives.
Plasma table, Mach3 V062, Step motors, C&CNC THC.

Offline BR549

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Re: THC moves on the Z axis...
« Reply #22 on: February 13, 2016, 11:38:35 AM »
Rememeber that you are moving the Z under THC control in VERY small moves so having a high VEL is useless as you are NEVER going to get there with a normal cutting move.

On the other hand you are using FAST Accel with EVERY move (;-) so it really becomes the  more important element with plasma.

Re: THC moves on the Z axis...
« Reply #23 on: February 13, 2016, 11:40:24 AM »
Dave,

Remember the THC Up and Dn Acceleration is near infinate (it is shunted / slapped) to whatever your percentage of rapid velocity you have set for the THC

If you look at post #3 on page one...

You listed your maximum (rapid velocity as 1500mm/min) and the THC setting as ~20 to ~30 %...

Say its 20%.

this means your THC slap and go velocity is 300mm/min (5mm/sec) .....

Given acceleration = (Velocity initial - Velocity final ) / Time

In post 6 you gave the steps / mm as 500

hence the time to accelerate from 0 to 5mm/ sec = 1/500 of a second...


You acceleration is 2500mm/s/sec.... for THC


(providing your z axis does not miss a step or two in the process (note 1 step is 1/500 of a mm..... if you can measure that I'll be impressed .... but if it was to miss a step or few the error would be cumulative.... hence if you had a way to toggle the z axis up and down to match the THC UP and THC DN signals.... you shunted it up and down 1000 times (and it missed a step on every cycle) you would accumulate an error of 2mm... which can be a little more visible


But I've read that steppers don't tend to loose one step at a time but the step loss tends to be greater or the motor just stalls.
(can't find the post now,  but it was on here regarding encoders, closed loop and steppers missing steps about 5 years ago)


Also you will note that the 2500mm/s/sec acceleration correlates with your Mach3 target smooth curve acceleration nearly of 3000mm/s/sec..

Hence you could probably up the THC % or rapid speed to ~24%   (if your rapid speed is still the same as post #3 at 1500mm/min)...... however at 3000mm/min.... your THC speed setting will be at 12%


Try it see if your THC works at 15% / 20% etc if your rapid speed is 3000mm/min.... but according to the calc above it may not.



______________________________________________________



If you had an arduino ..... you could always programme it to toggle two outputs and interface it with two pins on your motion controller, swap your THC pins around and try it.... (I use an ESS so have a load of pins but dod thing about this as a potential THC UP and THC DN testing philosophy)






Rob

Albert Einstein ― “If you can't explain it to a six year old, you don't understand it yourself.”

Offline Davek0974

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Re: THC moves on the Z axis...
« Reply #24 on: February 13, 2016, 11:50:17 AM »
So reduce speed again, increase acceleration and check it under THC control?

Back to 1500 again or compromise at 2000 maybe?

I can use the keyboard as THC inputs again and just flick the U/D keys.

Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives.
Plasma table, Mach3 V062, Step motors, C&CNC THC.
Re: THC moves on the Z axis...
« Reply #25 on: February 13, 2016, 11:50:37 AM »
After reading Terry's post above....

if the maximum acceleration you can get before stall is 3000mm/sec/sec.... and you are running 500 steps / mm

Should you not be better setting your Z axis rapid velocity at say 500mm/min.... and your acceleration at 3000mm/s/sec

That way your THC Speed setting for THC Up & Dn motion would be ~72% of rapid z axis motion...

(3000 mm/sec x 1/500 ) * 60 / 500 mm/min

Hence you can always tune your setting a little more (72% is an easier number to adjust by 1% up or down than 12% is as a 1% change at 12% is 6 times greater than a 1% change at 72%)

...... Sorry I hope the maths logic makes sense it may take a few times to read and understand what I'm getting at .....
Rob

Albert Einstein ― “If you can't explain it to a six year old, you don't understand it yourself.”

Offline Davek0974

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Re: THC moves on the Z axis...
« Reply #26 on: February 13, 2016, 11:55:40 AM »
I see what you are getting at - reducing Z max speed to increase THC rate ?

I can see that might be worthy of testing, will slow probing down a tad but whats a second here or there???

Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives.
Plasma table, Mach3 V062, Step motors, C&CNC THC.

Offline BR549

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Re: THC moves on the Z axis...
« Reply #27 on: February 13, 2016, 12:04:52 PM »
ESS verses  teh LPT. NOW you are talking apples and oranges As teh ESS has to handle teh THC internally in its on way.  

When you retune for best performance you normally leave teh VEL where it was because it was stable at that point. Then you run teh ACCel up to teh fail point and back it down 25% and retest teh THC function.

IF you are chasing teh divot on pierce. You have several things to work with . Normally nothing can move unitl you get teh Arkok signal. Then normally you move DOWN in Z to teh cut height. You can mkae that a GO move instead of a G1 OR speed up teh G1 move to maximum.

OR seeing how you are cutting thin guage metal simple make teh pierce height eh same as cut height and eliminate teh Move to CH move all together. That way as soon as teh Arcok signal is seen XY are moving.

For ME here I override teh Arkok signal ( turn it ON all teh time) so as soon as teh torch gets teh fire signal teh torch is moving. That give teh ark no time to burn a divot.  NOW that does not work well with thicker metals as you DO need teh dwell to fully pierce. But with thinner sheets the ark start and pierce completion are almost instant.

Re: THC moves on the Z axis...
« Reply #28 on: February 13, 2016, 12:11:50 PM »
Dave, yup, that's what I was getting at.

Terry, as always, thanks for the tips and guidance.
Rob

Albert Einstein ― “If you can't explain it to a six year old, you don't understand it yourself.”

Offline Davek0974

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Re: THC moves on the Z axis...
« Reply #29 on: February 13, 2016, 12:14:07 PM »
I forgot about the divot issue - a good reason not to reduce the Z speed i think.

Will have a play with this tomorrow

;)
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives.
Plasma table, Mach3 V062, Step motors, C&CNC THC.