CNC Plasma cutters, DIY, building info and guide

More updates coming soon, been very busy as usual and my brother took my workstation again ! Not that i am out of computers ( currently i have over 15 of them and several laptops ), but that is a mighty machine when it comes to compiling stuff at short notice and working on 3 monitors at the same time.
There is a nice thread going on about a new component for THC and Plasma cutting in Linuxcnc made by Phillc54, so please do read that as it is very much related to this thread and it will enable even the hobby machines to have all the features of the big machines.
forum.linuxcnc.org/plasma-laser/35449-an...ma-component?start=0
Regards,
Tom

Just uploaded the toma_THC config for version 2.8pre of Linuxcnc at it's original thread, with some more info and updates and much easier to edit.
forum.linuxcnc.org/plasma-laser/34978-th...-with-how-to?start=0

Also working on other versions of THC anything and everything. Need caffeine badly !

Hi.
What about "pancake" DC motors? Are those a good idea with belt drive? The commutator resolution is high, low inertia of rotor and high acceleration. I'm still in the "think about" stage, however I have some of them around. And some reinforced belts somewhere too. Would 1kW be too much or is that a reason to make the construction very rigid? Or should I save them for something different?
Also, how do You keep the control/plasma wires separated on the gantry? Do you use separated segmented cable conduit or can the power wire be reliably hung on some supports on a piece of pipe?
Thanks.

jmr wrote: Hi.

Also, how do You keep the control/plasma wires separated on the gantry? Do you use separated segmented cable conduit or can the power wire be reliably hung on some supports on a piece of pipe?
Thanks.

I just put it all in a drag chain purchased on ebay. This has been fine with both my Everlast 40 amp and more recently on my Thermal Dynamics 120 amp. Both of these have pilot arc (blowback) starting. I'm just in the process of upgrading it because the new torch lead is thicker and I need to run some wires for ohmic sensing. Make sure your table is properly earthed and you put attention into grounding cables.

An associate just purchased a table with a 200 amp Hypertherm air plasma and he has some problems piercing mild steel which causes a fault. They think its getting some interference because the torch and control wiring is too close at the torch end so they are sending some revised parts that separate the wires further apart. He was not sure if its a HF start or not when I spoke to him last night. He does not get the problem on aluminium which they think is because it has better conductivity.

rodw wrote: Both of these have pilot arc (blowback) starting.

Those 2 terms do not go together. If i remember correctly when watching your videos you had a pilot arc one, not a blow back one. I just can not know if it was a HV or HF/HV type pilot arc. Please do correct me if i am wrong.

There is so much confusing and plain wrong informations about pilot arc and everything related to HF HV plasma etc, that i am baffled ! How did that happen? Where was i when that happened? Who allowed that to happen? ......
Oh nevermind, internet is slowly but surely becoming the worst place to get proper informations, just sifting through that makes the head hurt.
Do i sound a bit grumpy ?

Scratch start = the torch has to physically touch the material it is cutting to strike an arc

Blow Back = has a partially moving nozzle that when pressed against the material it shorts to ground and as it is released it strikes and arc.

Pilot arc = has the means to strike an arc all on it's own even when far from material and more importantly it can maintain that arc for some time without touching or moving any torch parts. This makes it very reliable and that is why almost all expensive ones have one of two types of pilot arc, and this is why cheap Chinese ones insist they have pilot arc although most of them do not.
The implications of using a pilot arc are far from simple and it takes quite a bit of explaining so i will try a short version:
there has to be some provision inside such power sources to protect the rectifier inside, the rectifier can not be disconnected and connected again after a successful arc transfer as that would cause much more trouble etc etc.

Then there are 2 types of pilot arc start systems the HV or High Voltage and the HV/HF or High Voltage and High Frequency
The HV uses a transformer to raise the voltage to some 10 to 20 thousand Volts ( this is why the protection above mentioned ) and use that voltage to strike a maintainable arc between the electrode and the nozzle. This is done at 50 or 60 Hz frequency so it is much easier to isolate and causes less interference to other equipment.
The HV/HF is a bit different as it also changes the frequency of that high voltage to several KHz, hence the inherent eagerness to induce itself on anything and everything around it at some fairly long distances.

Now some more info about the interference caused by plasma power sources:
They all make interferences ! ALL! Some more some less, but for us who use them with CNC machines the most important thing regarding the two types we use, namely the HV and the HV/HF types is : NOT IMPORTANT, it absolutely is not important what of the two types you have, they are very reliable and work perfectly well given the proper grounding and shielding.
As a reference of why they are the same in terms of interference, is the fact that the HF is a bit harder to isolate, but the HV makes much stronger interference. That can be easily verified by checking the power usage of the ignition system : The HV type use roughly 80 to 150 Wats of power for the arc while The HF one uses 30 to 60 Wats.

I think this was enough for tonight, thank you all for everything, going to grab some ZZZZZZZZZZ now.

Tommy

I cannot speak for European torches but have used torches from both Hypertherm and Thermal Dynamics (now ESAB). Both of these manufactures describe their AIR plasma torches/systems as being blow back torches for arc starting. When using either manufactures torch you never touch the torch to the work piece when starting the arc. Both manufactures connect the nozzle or tip to the the work piece potential through internal wiring and electronics while the electrode ( negative side of power supply) is touching the nozzle both electrically and mechanically. Then the air pressure is applied and the electrode is moved from the nozzle causing an arc that is transferred to the work piece when the internal connection to the nozzle is disconnects. The actual mechanics of how this happens is not the same for both because of IP issues. Now both manufactures do still make industrial plasma systems that use HV or HV/HF arc starting. The operation of both manufactures torches is fully described in the user manuals which are available online from both manufactures. This goes for production systems for at least the last 10 years (mine is older). I think that system (blow back) from both manufactures are the gold standard for small shop/hobbyist users because of the low EMI. I can safely say that without resorting to elaborate grounding measures I have never had an interference issue caused by arc starting/cutting (I wish I could say the same about my TIG welder).

John