Lowrider2 Plasma Build

LowRider CNC Your Builds
4

Since I’m not building a typical Marlin/GRBL contolled lowrider I’ve been having to do a lot of research on my own and asking questions in other forums to figure out how to do this. Most of the hardware selection was inspired by @uthayne’s build because he has proven it to work well for his lowrider plasma in his build thread. I’m going to do a series of write ups in this thread to document everything I’m doing for my own future reference. Maybe it’ll help someone else in the future as well.

Electronics

For my build I’ll be using the following electronics:

  1. On old retired Dell desktop pc I found in a closet.

    I’ve installed the Linux Mint 19 operating system, a realtime kernel, and compiled LinuxCNC v2.9 to run the cnc. More on that in a later post.

  2. Mesa 7i96 ethernet breakout board

    This card connects to the computer through a standard ethernet cable instead of an ancient parallel port. It’s also much less dependant on low computer latency than standard breakout boards. For these two reasons it makes it a lot easier to find a suitable computers to run LinuxCNC.

    The 7i96 has breakouts to wire up to 5 stepper drivers. It has 11 general purpose isolated inputs that I’ll use for my homing switches, estop, etc. It has 6 isolated outputs that can be used to turn on relays to spindels or in my case a plasma cutter. It also has an expansion port to add more IO if I need to later.

    Finally it has encoder input for reading things like spindel speed, or in my case calculating the voltage the plasma torch is putting out.

    Setup Notes:

    A. Set jumper W5 DOWN and jumper W6 UP so the card will use the IP address from the EEPROM 10.10.10.10. Use this rather than the default 192.168.1.121 because you don’t want the 7i96 realtime connection IP address to overlap with the local network address range.

    B. Jumpers W2, W3, and W4 are for setting up the encoders. Set W2 to the right hand position to put encoder A in differential mode for better noise immunity. Set W3 to the left hand position to put encoder B in singled ended TTL mode so that it’s not floating when left unconnected. W4 controls the encoder index signal, it does not matter what mode this in.
    (Jumper orientation is in reference to the 7i96 with its ethernet port facing left)

    http://www.mesanet.com/pdf/parallel/7i96man.pdf
    http://store.mesanet.com/index.php?route=product/product&product_id=311

  3. Mesa THCAD-10 voltage to frequency converter for torch height control (THC)

    Clean plasma cuts require that the torch tip be a specific and steady distance from the metal it is cutting. When cutting metal with fire the metal heats up and deforms and moves during a cut, like a LOT. It turns out that the voltage put out by the plasma’s cutting arc is proportional to the distance from material to the torch. The THCAD card takes input voltage from the torch and safely converts it to an encoder pulsing signal that the cnc controller can then accurately extrapolate the distance between the torch tip and material and make Z corrections on the fly during the cut.

    There are 3 versions of the THCAD, the -05, -10, -300, these correspond to the card’s voltage reading resolution. All three versions can withstand 500v overload indefinitely, but they’ll only be able to report back to their maximum range (5, 10, or 300v). Typical plasma arc voltage is under 150 volts. So if you choose the -300 version you can only utilize half of its voltage reading range. But with an appropriate voltage divider you can utilize closer to the full range of the -05 or -10 versions. All this means is that the more of the reading range you can use, the more accurate the controller’s calculated voltage will be, and the better the THC will be. The plasma machine I’m using has voltage dividers built in for this so I’ve chosen the THCAD-10.

    Setup Notes:
    A. Set jumper W1 up for unipolar mode to read voltage from 0 to 10 volts

    B. Set the output frequency divider jumper to F/32 for counting pulses in the software

    C. Note the factory calibration values on the sticker on the back of the card BEFORE installing in control panel

    http://www.mesanet.com/pdf/analog/thcadman.pdf
    http://store.mesanet.com/index.php?route=product/product&product_id=127
    For future reference I made a calculator to convert the sticker’s calibration values to the coefficients that LinuxCNC expects: https://jscalc.io/calc/NTr5QDX6WgMThBVb

  4. TB6600 stepper drivers

    These drivers have little switches on the side to select microstepping and current. Note that the default current for mpcnc and lowrider steppers was carefully choosen by Ryan and is around 0.75 amps. The more current applied to the motors the hotter they will get. The PLA mounts will start to deform around 55c. That said I’m going to live dangerously and start out at 1.5amps, keep a close eye out for melting plastic, and see how it goes.

    Also, the default microstepping for mpcnc/lowriders is 16.

  5. 24v/15a power supply to power the steppers/stepper drivers and limit switches
    As I understand it 24v should give me a little more juice to the steppers and a little more noise immunity for the sensors. I could be way off base here.

  6. 5v/3a power supply to power the Mesa cards
    The 7i96 needs 5v ONLY!!!

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