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  1. #21
    Community Moderator Jim Dawson's Avatar
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    Default Re: CNC plasma setup. Lots of questions.

    I think the radial acceleration (at the cutter) is a vector calculation based on sqr(X^2 + Y^2) at any given instant as the cut progresses around the arc. This should all be handled in the motion controller, so the vector acceleration value would be what you would plug into the radial formula.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: CNC plasma setup. Lots of questions.

    Jim, I don't think the motion controller's calculations are particularly useful unless it publishes it so you can use it in some way.
    There is a recommendation from Hypertherm to reduce cut velocity by 60% on holes < 32mm dia. This is often done by rules in the post processor.
    But surely this is a bit crude. What if the table can cut a 15mm dia hole at the cutting speed because it has higher acceleration than the average 0.1G to 0.3G?
    Say your designers habitually add a 5mm radius fillet on corners but the calculation says they should be using a 7mm radius to maintain cutting speed? A small tweak at their end will improve cut quality.
    Usually when you set the motion controller up for a machine, you define the maximum acceleration for each axis. Thats the number you need to work back from. Then if your motion controller is kind enough to publish the arc radius in real time, you can modify your cut parameters if it can't maintain the desired cut speed.

    Rod Webster
    www.vmn.com.au


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    Member peteeng's Avatar
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    Default Re: CNC plasma setup. Lots of questions.

    Hi - I'm not sure why the centripetal (or centrifugal) acceleration is important to the motion planner or the path. The linear (or path) acceleration is. The linear accel becomes the tangential accel if the machine is travelling on an arc. The centripetal accel creates radial inertial loads that should be resisted by the machine. The analogy is driving your car around a corner. The radial acceleration is trying to spin your car out and the path acceleration (or your foot or cruise control) is trying to get the car through the corner as fast as possible (or at least at constant speed). The machine can't spin out as its mechanically constrained. The motion controller has path tolerances that creates a wide pathway unless its set to "exact stop' in which case it will go exactly through every calculated point (this is slow though). If the controller is set to constant velocity mode the smoothest path is chosen within the tolerance to maintain the speed. Just like when driving a car we cut a corner to smooth it out. A cars limiting factor is road friction which is the centripetal part and a min radius at set velocity. A machine does not spin out but does wobble due to inertial loads and the relative stiffness of the machine. The total inertial load is the vector sum of the path acceleration and the centripetal accel but I don't see this as a concern for the motion controller, it is for the machine structural design and the machine capability. Unless someone out there can tell me otherwise, like does the plasma stream respond to centrifugal forces ? If the controller has a centripetal accel set limit then it will use the limiting value (path accel and vel vs radius and Cp). So does the Hypertherm system have a set centripetal limit? cheers Peter

    Many many moons ago I worked for ESAB programming welding robots. We had a formula for volts, speed and amps and the controller adjusted amps vs speed for the specified fillet size and velocity. I'm learning about plasma at the moment. So does a plasma controller do the same? Adjust the voltage ( for torch height changes) amps for speed changes and is this coupled in the controller with the motion planner? cheers Peter

    Last edited by peteeng; 08-19-2020 at 05:49 PM.


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    Default Re: CNC plasma setup. Lots of questions.

    Is the screen set for Linux available ? Or we have to create it ...



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    Default Re: CNC plasma setup. Lots of questions.

    Quote Originally Posted by peteeng View Post
    Many many moons ago I worked for ESAB programming welding robots. We had a formula for volts, speed and amps and the controller adjusted amps vs speed for the specified fillet size and velocity. I'm learning about plasma at the moment. So does a plasma controller do the same? Adjust the voltage ( for torch height changes) amps for speed changes and is this coupled in the controller with the motion planner? cheers Peter
    If you bought your plasma controller from ESAB, Hypertherm or Thermal Dynamics, you would get those features (and a much depleted bank balance) but they are not available in the normal run of the mill controller. Linuxcnc has some of the features and eventually I will have those features in a variant of Linuxcnc one day. I just don't have much time to play with this stuff.

    Quote Originally Posted by asuratman View Post
    Is the screen set for Linux available ? Or we have to create it ...
    Yes Plasmac has two GUI's available. The traditional Linuxcnc Axis screen and a touch screen compatible screen set (Gmoccappy).
    Plasmac is included in Linuxcnc 2.8 and above. The dev team are working hard on final testing of V2.8 pending it becoming a release version
    In the meantime, there are a few ways to upgrade to 2.8

    Rod Webster
    www.vmn.com.au


  6. #26
    Community Moderator Jim Dawson's Avatar
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    Default Re: CNC plasma setup. Lots of questions.

    Making some progress with this project.

    Just a teaser, a partly assembled model. Still designing parts. Gantry target mass <50Kg, so far I'm up to about 25Kg and will be removing as much unneeded mass as possible. Work area is 48 x 48 inches, main frame is 4 x 4 x 1/4 wall steel tubing. The gantry tube is 4 x 6 x 1/4 wall aluminum tube. No cutting forces so it does not need to resist that, only the acceleration forces. Single motor rack & pinion drive.

    CNC plasma setup.  Lots of questions.-plasma-table-v27-jpg


    CNC plasma setup.  Lots of questions.-plasma-table-v272-jpg

    Attached Thumbnails Attached Thumbnails CNC plasma setup.  Lots of questions.-plasma-table-v27-jpg   CNC plasma setup.  Lots of questions.-plasma-table-v272-jpg  
    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: CNC plasma setup. Lots of questions.

    Jim sounds like you are off to a great start. Nice to see somebody understands light and fast is the way to go. 25-30kg for the gantry complete is quite achievable. Thats where mine is. I used 40mm x 80mm x 4mm rectangular section for the gantry and it worked out quite well so I think you will be fine.

    From my recent experience, I would add some bracing/triangulation on your gantry ends if you are going to design for high acceleration. The forces from inertia are significant.

    Recently, I compared notes with a CNC router technician who was next door. He had made his own router and thought I should have used two rails each side but I'm not convinced there is movement around the linear rail.

    Rod Webster
    www.vmn.com.au


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    Default Re: CNC plasma setup. Lots of questions.

    Here is what I mean. 0.5G acceleration with a notch on the foldline and the cut is along the Y axis. The leadin was straight into the notch. So the sudden and immediate reversal caused the gantry to oscillate.
    CNC plasma setup.  Lots of questions.-gantry-flex-jpg

    My gantry ends are laser cut 6mm ally with folded edges. Initially I wanted to use 10mm but the laser cutters thought it would crack when they folded it. I always knew this was a potential problem but it has not been an issue until I did a massive upgrade on the motors recently. Bracing this up is high on my priority list.

    Attached Thumbnails Attached Thumbnails CNC plasma setup.  Lots of questions.-gantry-flex-jpg  
    Rod Webster
    www.vmn.com.au


  9. #29
    Community Moderator Jim Dawson's Avatar
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    Default Re: CNC plasma setup. Lots of questions.

    I see the problem. My gantry ends are currently 19mm thick aluminum. But they will be pocketed into I beam sections with a max 6mm web section. Doing this and machining the cross tube into a truss bridge section will substantially reduce the mass of the components without sacrificing strength. If need be, I'll add some cable tension members to the cross tube.

    Jim Dawson
    Sandy, Oregon, USA


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