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Thread: CNC Plasma Table Questions - Suppliers

  1. #13
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    Default Re: CNC Plasma Table Questions - Suppliers

    Quote Originally Posted by Razzmeister View Post
    I am planning to go and visit RodW in Archerfield - sounds like he is pretty experienced. Let us know please.
    Happy for any of you guys to drop in at Acacia Ridge. Just contact me though the contact us page on my website in my signature.
    I won't say I'm experienced with plasma cutting but I've learnt a lot about the software side by building my own machine and building a THC in Linuxcnc.
    Perosnally, I'm not convinced there is any such thing as a quality external THC as there are things you can do in the motion controller that is impossible externally.
    So if you want quality, buy a table with a Hypertherm control that has fully integrated motion controller based THC control or try your luck with LinuxCNC and build something similar like I have.

    Rod Webster
    www.vehiclemods.net.au


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    Default Re: CNC Plasma Table Questions - Suppliers

    @RAZZ

    I've got no where with this. Mainly because I have several designsa in my head and I need to flewsh them out and narrow it down to one. I have just downloaded Solidedge as I get a free license through work (like Solid works) to nut out the mechanical design.

    The electronics is still a puzzle to me. I'd like to do it cheap, but also want to do it well. Conflicting goals.

    @RODW

    Are you saying that THC works best when your plasma unit has a THC output, or is it more than that.

    This was a timely update as I've just cleared a bunch of projects and was ready to get back to this.

    cheers
    Damo



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    Default Re: CNC Plasma Table Questions - Suppliers

    Hi,
    since my first post in this thread there has been two new developments.

    WarpTD is days away from releasing its new plugin with THC support for Mach4. Warp9 don't charge an extra license fee like Vital Systems and their Ethernet
    SmoothStepper board is only $190US, you can get one from Homann Designs in Melbourne. Note this THC support is a genuine realtime solution, ie fast response.

    Mach4 has not officially released, but is available as a development version, a programed THC support. Because Windows, and therefore Mach, is not realtime
    the response may be a bit slow. In a non-demanding situation I think it will work OK, but with a really wavy piece of material you will need a realtime solution,
    ie the Hicon by Vital Systems which is ready to go OR the ESS from Warp9 due to be released within days.

    Craig



  4. #16
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    Default Re: CNC Plasma Table Questions - Suppliers

    @Razz, I hope you got value from your visit today. For the others we pulled up a small DXF file designed in Solid works, ran it into Sheetcam, had a quick look at the nesting features and then cut one of these parts in 2mm steel at 40 amps and about 1600mm/min from memory. Then we checked the 10mm hole for fit with a nutsert as that gets fitted. We decided (and I had proven previously) that it would be a good fit. The 6mm clearance hole was not 100% but we decided it was fit for purpose but not as nice as a similar laser cut part.

    I also told Raz that I had a fairly brutal learning about the importance of clean dry air which necessitate me spending about $3k on a refrigerated drier and a precooler due to our humid climate here in Brisbane



    I suggested to Razz that as he wanted a plasma cutter to do production paying work that he was probably better of buying a table and shared a recent quote that had options starting at $20k

    So now we get to @Damo

    If you follow some of Jim Colt of Hypertherm's posts on various forums, he says the best plasma CNC controllers do the THC internally where it is tightly linked to the trajectory planner. This is how Hypertherm do it. This is how I do it with Linuxcnc and I've written a lot of software to achieve it.

    Before we go on, its important to understand that in a Mach3/4 environment the motion controller is generally a smooth stepper or some other board but in the case of LinuxCNC it IS_THE_MOTION_CONTROLLER. This puts some added requirements on the hardware because LinuxCNC MUST service the Servo thread 1000 times a second and if it doesn't due to PC hardware latency, bad things can happen because the motion controller is late to the party on the next call. So while we use external hardware (like the Mesa 7i76e ethernet card I use) the step generation is pretty dumb as an external stepgen is told to generate a certain frequency until its told to change the frequency or stop. LinuxCNC can command those changes every millisecond. This is way cool but it does come at a cost. And that is you need to run a real time version of the Linux operating system, (Usually the PREMPT_RT kernel) But the good guys at LinuxCNC make available an ISO file that installs the right OS, the right kernel and the LinuxCNC program itself!

    The typical Mach 3/4 external THC has 2 signals. One that is enabled when the Z axis needs to go up and the other when the Z axis goes down. So Mach sees these signals and sends a fixed height adjustment either up or down until the external THC says its within range, much like moving with a MPG dial. This bit/bang approach is a fairly crude approach. And it usually involves a dead band either side of the commanded voltage where no adjustments are made. Further more, sampling torch voltage, disabling the THC function when the motion controller is unable to meet the commanded velocity (eg. when approaching a corner) or crosses a void (more frequent than you think if you cut a hole and the slug drops away), just can't be done becasue the external THC does not know what the commanded speed is.

    So compare that with what my internal THC does. First we read the torch arc voltage using a USD $69 voltage to frequency converter. The frequency is connected to the spindle encoder (which we don't need) and we decode it in software to a voltage. On my machine, 1.5 seconds after we start cutting, LinuxCNC waits until the current velocity is witihin 90% ofthe commanded voltage and samples the torch voltage in real time and then enables THC operation. It does not use a bit/bang approach, but instead uses a high performance PID based algorithm to control the torch height and makes adjustments 1000 times a second. We don't bother about defining a dead band as PID doesn't need it. Adjustments are done by applying an "external offset" that the trajectory planner knows nothing about (eg external to the trajectory planner). So now we are cutting, we check the current velocity against the programmed cut speed every millisecond and if it is say 90% below what it should be, we disable the THC until we are up to speed again (so called velocity anti-dive or corner lock). The next feature I need to finish implementing is to look for a void. We do this by monitoring the rate of change in torch voltage (in volts per second) and if it exceeds a threshold (which is several orders of magnitude higher than what is seen due to material warpage when cutting), we can disable the THC until its safe to enable it again.

    One final feature I have attempted to code is arc sensing using some advanced trig suggested by an engineer in the LinuxCNC movement. My hope was we would be able to calculate the arc radius in real time so that we could apply a limit to the velocity based on some algorithm similar to Hypertherm's true hole technology (which I think is dependent on the Post processor.) I tried one approach and it did not get a result but there is another method we want to try. But time is an issue for me.

    So thats the kind of thing you can achieve with an internal THC that has 100% complete access to the trajectory planner and a few hundred hours of research and coding. Compare that with the dumb bit bang approach most external THC's adopt and you tell me how they can be a quality THC?

    Rod Webster
    www.vehiclemods.net.au


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    Default Re: CNC Plasma Table Questions - Suppliers

    @Joe. For the record, a Mesa 7i76e is USD $199 and the THCAD card is USD $69. The 7i76e has 32 inputs, 16 outputs, 2 MPG encoder input, spindle control and encoder and 5 stepgens capable of running at up to 10 MhZ and a few analog inputs. We are talking serious commercial quality hardware designed to use 24 volt circuits which is more immune to noise. I'm at a loss that somebody would choose an ESS board over this device.. You can buy from mesa direct but they are not really geared up to service the hobby market, so I got mine from BigJohnT here https://mesaus.com/

    @damo, plasma and cheap do not go hand in hand!

    Oh and I forgot, When Razz was there, I left a bit of 12mm steel on the table which jammed one side of the gantry so we racked it by about 30mm before I could stop it. This became a great example of LinuxCNC doing its automatic gantry squaring when we homed it.

    Rod Webster
    www.vehiclemods.net.au


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    Default Re: CNC Plasma Table Questions - Suppliers

    Rodders, Thank you for your time today - I have to say your ability is impressive. The e-mail you sent me did not have the attachment to it. As for the metal cutting, I can lay my hands on a 150w CO2 laser with a 1300mm x 2500mm table in top condition. I know this will not do metal, but what if I convert it to a dual purpose by adding a Fibre laser head for thin guage metals up to 3mm thick? Is it possible without breaking the bank? or do I go fishing for the rest of my life?????



  7. #19
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    Default Re: CNC Plasma Table Questions - Suppliers

    Sorry mate, I resent it. reply to my email if its missing. Sorry but I have no idea about laser or how to control them. I just go to Laser Central, C-tech or Rockpress depending on the job if I need laser cutting.

    Rod Webster
    www.vehiclemods.net.au


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