cnc plasma build questions


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    Default cnc plasma build questions

    Hi I am building up a cnc plasma and had a few quetions regarding the build. I was wondering if the gantry should be belt driven on x and y or direct drive, rack and pinion.
    Thanks

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    Default Re: cnc plasma build questions

    Depends on the table length and the weight of your gantry . There is a reason most tables you see use R & P especially longer than 4 ft. There have been builds with all kinds of drive systems: Rack and pinion, endless loop belt. fixed end belt, leadscrew. chain drive and a couple using small cable and spools. Its all about ratios, belt flex, backlash and acceleration VS Speed. Plasma needs speeds but usable acceleration up to the projected cutting speeds. Using belts you have to deal with the final drive pulley has to be larger than a pinion gear for the belt to stay on the pulley. That changes your drive ratio (lowers torque (acceleration) and adds speed you can't use.



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    Default Re: cnc plasma build questions

    Thanks for the reply, i was looking at some 207 oz nema 23 honestly i cant figure out if they are 207 or 270oz.



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    I'm with Torchhead, rack and gear is the way to go if your longest axis is over 4' or if you ever plan on extending that axis. You'll get plenty of accuracy for a plasma with alot less headache. Keep in mind that wide gantries may require the use of a rack, gear, driver, and motor on each side of the gantry. You'll have to come up with a happy medium of weight versus rigidity on the gantry as well. Lighter is better but sometimes rigidity is sacrificial in that adventure.



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    Default Re: cnc plasma build questions

    Ok, i figured rack and pinion was going to be best, right now i can not afford the cand cnc thc so mwhats your thought on the cheaper options out there such as Compact THC 150 ( Proma-Elektronika Plasma Torch Height Control ) | eBay

    BTW is there a need for pinion belt reduction?
    Thanks



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    Default Re: cnc plasma build questions

    I also forgot to mention i have a high frequency esab pcm 875 and was wondering on your thoughts about shielding electronics from hf, I have a ground rod right were my table will be which i think will fix the electrical noise.
    Thanks



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    Default Re: cnc plasma build questions

    ANyone?



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    Default Re: cnc plasma build questions

    Quote Originally Posted by bridgeport13b View Post
    I also forgot to mention i have a high frequency esab pcm 875 and was wondering on your thoughts about shielding electronics from hf, I have a ground rod right were my table will be which i think will fix the electrical noise.
    Thanks
    Based on your manual plasma solution, I am guessing that your target cut thickness will be less than 0.750. To address the hi-frequency question, the ground rod needs to be less than 20' from the table for starters. After that, it needs to be an effective ground. What does an effective ground mean? It means that the resistance of the ground rod should be 3 ohms or less. There are a couple of ways to measure that. One is a rather expensive piece of equipment (I forget the name) that, at last check, costs roughly $5K. The other way to check the resistance of the ground rod involves the use of a 100 watt incandescent light bulb, a lightbulb pigtail socket, a power cord and a multimeter. The light bulb needs to be a real 100 watt bulb (no watt saver/energy saver bulbs need apply).
    A quick description of the test setup is:all connections to the ground rod are removed. 120vac hot connected to one side of the light bulb. The other side of the light bulb socket is connected to the ground rod. Once it is powered up, measure the AC voltage between the ground rod and the AC neutral on the power cord. That AC voltage reading is equal to the resistance of the ground rod. The math only works for a 100 watt light bulb.
    Walmart still stocks 100 Watt toughlight light bulbs, so they are cheap and easy to obtain. Failing that, a 180 Ohm 150 watt resistor will work, but those are a bit harder to come by.
    Now, about the ground rod. Size doesn't matter. 3 foot, 100 foot. it doesn't matter. Whatever gets you under a 3 Ohm resistance. That being said, I have never met a 20 foot ground rod that when installed, didn't give me less that 3 Ohms resistance.
    Next is the connection to the table. The table is the central grounding point. So from the ground rod to the table - 1 connection. This should be the only connection to the ground rod. Everything else attaches to the table. The PCM 875 work lead, the gantry ground connection, any connections to the rails etc ... all attach to the table. If your smart. you'll weld a piece of 1/4" or 3/8" steel to the side of the table with a number of 1/2" holes drilled in it so you can bolt up lugs from the various things that need to be attached to ground.
    Lastly, Power Company Ground never gets attached to the table.
    HF shielding: I'm not sure what torch came with your PCM 875, but for a mechanized application the factory torch I believe is a PT21 torch. I'm not even sure that torch is even supported anymore. The trick to shielding the PT21 was to put a braided shield only half way along the length of the torch and ground only 1 end of that shield.
    Make certain the spark gap on the PCM 875 is at 0.040, no wider. You can go as low as 0.036 (the wider the spark gap, the stronger the HF. The stronger the HF, the greater the chance of HF errors at your control. )



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