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PostPosted: Mon Jun 14, 2010 2:41 am 
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OK.. as I promised a while ago, I thought it was about time for the DIY plasma Cutter to have its own posts!

I must point out that the plans shown here are for educational purposes only. The use of HV A/C and HV D/C can kill instantly and should only be attempted by person knowledgeable of it. The plasma used in this equipment is anywhere from 25-50K degrees F and can inflict severe burns. Plasma cutters also shoot molten metal (NOT SPARKS) and present a severe fire danger. Proceed to read these with extreme caution and awareness of your surroundings.

Now on with the fun!!! :yay:

I started off searching for plans on the internet after hearing rumors of a working DIY plasma cutter. I found a crude hand drawn version from a YouTUBE video and though I'd try it. After gathering the parts and piecing it together, you can imagine my surprise when it wouldn't even fire off! Here is that initial drawing.

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So after a bit more searching... isn't the web wonderful?.. I found a set of plans that looked a bit more complicated but more promising. The end result was the same. The failures had a benefit though... as I was going through each of these, I was learning more and more about plasma and how to generate it and how to use it. Here is that drawing.

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So after multiple failures, I was getting desperate. While searching for the plans for the DIY PC, I stumbled across someone selling "plans" for a working PC. There were even slick videos of him and his creation cutting all kinds of things. So after thinking about it and not wanting to give up on the idea of my own PC and at $10 for the plans, how could I miss!

So I sent him my $$$ and received my plans. 30 minutes later I was in the garage tracing out wires and arranging components. You can imagine my utter devastation when I hit the switch for the first time and got... NADA! Out of 100's of attempts, I could get it to start and cut maybe 3 times. and those cuts were short. Here are what some of the first cuts looked like.

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Now I was pissed... after months of trying and finally conceding that I needed true "plans" to build this thing... they were useless! Someone was going to hear about this!!!

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PostPosted: Mon Jun 14, 2010 2:41 am 
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OK... so I contacted the "designer" and told him of my issues with his "plans" and asked for some help determining the issues. I was told that since I didn't use the exact components he did it wouldn't work. Now it's been a few years since Electronic school, but I still remember how to make a full wave bridge rectifier from diodes and the difference between a 12v AC rectified to 12vd DC supply instead of my computer power supply with 12v output is what?? Nothing.. thats right! Below is what he told me I had to have!

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So after going round and round with him uselessly via email, I decided to try and determine where it was going wong and correct it. After all.. I could get it to fire off if I started the arc manually.. it just was impossible to keep it going long enough to make "cuts" of any measure.

Does everyone know how a plasma cutter works? For the new guys in the back... A plasma cutter is a HUGE short circuit. I'll spare you the intimate details (go here for some basics on it) but it's akin to arc welding but you blow the arc into the metal and that plasma melts the metal, the air blows it away and then the process starts over again.

To start the arc you need to ionize the air, basically making it conductive, so that the capacitor bank can discharge across it and generate up to 45000 degrees F to melt any conductive material put against it.

My problem was that the arc was shorting into the DC of the capacitors and not across the tips as it should. No matter how I advanced or delayed it... isolated it or not.. I could not get it to work. My only success was when I disconnected the tip wire and held it in front of it. It would then "jump" out of the tip and start cutting. But plasma cutting involves the arc going out quite often, so this lead to some frustrating times!

In a fit of madness, I tore apart the whole system and started over.. adding only one system at a time. It was while talking with my partner from Battlebots that we hit the mother lode. If it was arcing to ground... lets move the place where the arc starts! The rest is history! We took the High Voltage wire from the coil and ran it to the tip instead of the rectifier. I touched the tip and a fraction of a second later it was spitting plasma and cutting like a champ! That was it! A single wire change and we were cutting!

So here is the schematic. Changed to reflect the wiring that actually works. You'll notice that the only change to the entire setup is in the lower left corner!

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That's it! That's all it took to turn my pile of parts into a working Plasma Cutter.

Now I had to make it pretty!

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Don't ever become a pessimist. A pessimist is correct oftener than an optimist, but an optimist has more fun, and neither can stop the march of events.-Robert A. Heinlein

Add yourself to the Locost Builders Map!

Jigsaws, Hacksaws... There is a better way! Want to get creative with metal? Want a Plasma Cutter but can't justify spending the $? Check out a DIY Plasma Cutter! A true LOCOST way!


Last edited by botbasher on Mon Jun 14, 2010 3:12 am, edited 1 time in total.

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PostPosted: Mon Jun 14, 2010 2:42 am 
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Now.. I guess I should show you how it started off and what it eventually turned into.

As I was "basically" following his plans to start with, I simply laid everything on a board... wired as needed.. and worried about "pretty" later.

Here is what my initial board looked like:
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Once I got everything to where I was sure it was working, then I could concentrate on making it pretty... what's that? How did I do it for $100? Well, kinda the same way Uncle Chapman built his car for $250 pounds.... just plain dumb luck and a bit of outside the box thinking!!

Read on for more details....

A lot of the stuff I used came from my stockpile at home or my other favorite place to live.. the Surplus Store!! The thing that really started me off on this kick was one visit to one of my favorite local stores, I found a bag (literally a bag) of caps. 400v 330uF strobe caps. Really what that means to this is that they are designed for repetitive violent discharges.. perfect for a Plasma Cutter. The price... $10 for 20... I bought all he had! Still have enough for a second set-up!

The valve.. normally a sprinkler valve, can be had for below $20 at Home Depot. I happen to have it it from my days of Spud Guns! The fact that it's 24v is a plus as I down rated all the initial components to a common voltage to eliminate the various voltages (120v, 12v, 24v) that are in his design.

The contactor was a steal (near literally) from another surplus house.. It's 24v and 4 pole 40A with a 4 pole Auxiliary on top. I got the whole thing, after the kid behind the counter couldn't figure out what it was, for $20!! It's normally a $500 contactor and that's without the $150 Aux switch. What does that mean to this though? Well, instead of having all those relays switching air, and the HV and etc... I can have a single component that controls it all. If your building one, I highly recommend pairing down the components and making life simpler!!

Since I work in Commercial Kitchens, I ended up with a spare booster heater element years ago... don't know why I kept it, but glad I did! It turns 130 degree water into 190 degree water in minutes. This particular one is 9kW at 208v. It has a resistance of 5.4 ohms. It gets WAY too hot to leave exposed. I got a 5 gal bucket from Home Depot with a lid. Cut a hole in the lid and filled it with water and stuck the element in. It's a temp solution. I'm looking at various other mfgs to find a element that will work for our needs. Once I have that, I'll update the parts list.

The biggest cost was uncontrollable. I had to have high amp Rectifiers. Ebay supplied those for $30. When your looking for these, make sure they are rectifier paks and not SCRs!! Won't work the same despite looking identical! If you can find a FULL WAVE RECTIFIER, then go for it, but it's not necessary. You can get two HALF WAVE BRIDGE RECTIFIERS and pair them up. Just make sure they are 100% identical or you'll have an imbalance that could destroy either one.

The piece de resistance was the case! While at my fav store again, I knocked over a stack of boxes and under them was a pair of HF radio boxes. I knew it was perfect so I picked it up for $20!

What does it look like you ask? Read on...

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Don't ever become a pessimist. A pessimist is correct oftener than an optimist, but an optimist has more fun, and neither can stop the march of events.-Robert A. Heinlein

Add yourself to the Locost Builders Map!

Jigsaws, Hacksaws... There is a better way! Want to get creative with metal? Want a Plasma Cutter but can't justify spending the $? Check out a DIY Plasma Cutter! A true LOCOST way!


Last edited by botbasher on Mon Jun 14, 2010 7:06 pm, edited 1 time in total.

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PostPosted: Mon Jun 14, 2010 3:14 am 
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So without further adieu... I present the finished DIY Plasma Cutter!!

Image
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The torch tip was a eBay special I picked up a few years back when I saw the auction ending and couldn't resist the $15+5 shipping for it! It was a foolish bet, but it paid off!

I have since upgraded it to a ESAB unit. A PT-31 (now using the long tips) to be exact, and I highly suggest you don't skimp here if at all possible. It's worth a few extra bucks for the accuracy of the cuts and quality of the handle!

The indicators on the cover have never been finished (maybe after the cars done) but it's operation is so simple they are not really necessary!

The tip is the heart of this beast and tends to generate the most issues besides the supply voltages.. let me cover both...

I ran the HV Arc Start line inside the protective sheath, into the handle and simply drilled a little hole on top to exit. It then wraps around the head to where I have it zip tied in place. I wrapped it around a piece of SS TIG filler wire which I bent around the tip. This works AWESOME! The only thing to watch for is shorting out the HV Arc Start. You'll notice that it's UNDER the head? Because when it's on the side and you use a piece of steel as a guide, it shorts out and you can't start the cut! Yeah... took me a little wile to figure out just WTF was going on there! After I moved it under the head (simply rotated it) I haven't had any more issues!

BE AWARE: if you are playing with it and do not give the HV anywhere to ground to... it will bite your butt! Ask me how I know! That is why it's in the sheath and doesn't exit till the very top!!

ImageImage

OK, Power Supply concerns..... VERY VALID CONCERN... but simple to dispel.

Yes... I left out the isolation transformer. I have played with and built Tesla Coils (1.5 million Volts - 34" arcs) so I am fully prepared for voltage concerns. Leaving out the iso didn't worry me. I was intending on putting it in, but I found something odd. It makes sense now... but only after you see it happening.

I have actually touched the work... while cutting... without any ill effect. Do I suggest you do this... NO WAY.. not without an isolation transformer... but it got me thinking and looking at just what was going on.

You are converting AC to DC with the Bridge Rectifiers, right? In full wave, there is no direct path to the mains if all the components are working.. Electronics 101. I have measured the frequency of the work clamp (positive) in reference to earth ground.. I get 0 Hz. I have measured the tip (negative) to ground.. yep 0 Hz.. there is no measurable AC voltage at any point past the Rectifier. DC doesn't jump and needs a very conductive path (sweaty wet hands...) to move.

I have touched the clamp to earth ground.. NADA! The tip, unless arcing, is internally isolated by the design of the head, but when disassembled.. it won't short to ground either! Does this mean it's safe to lick the ends... I would advise against it. My rule of thumb here has been to treat it as I treat a arc welder. Yes.. the tips are live and can zap you, but won't unless you do something abnormal.

It's your decision as to how you build it.. do I suggest you forgo a portion of the safety in this circuit.. NOPE! Can it work and be acceptably safe without it.. I believe so.. but you make your own choice!

Well, that's it for my lecture... I present this to you in the spirit of LOCOST!!! If you do build one.. be safe and smart about it. I kid you not.. the caps can store several kilo-joules worth of energy.. plenty to blow a hand off without even fully discharging! And they will self generate... recharge themselves without anything attached to them!!! BE SMART AND BE SAFE!!! If you don't know HV equipment.. find a friend that does before you start! I can't stress that enough!!

Any questions or help required to figure something out.. Just let me know! Happy to help!

KS

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Don't ever become a pessimist. A pessimist is correct oftener than an optimist, but an optimist has more fun, and neither can stop the march of events.-Robert A. Heinlein

Add yourself to the Locost Builders Map!

Jigsaws, Hacksaws... There is a better way! Want to get creative with metal? Want a Plasma Cutter but can't justify spending the $? Check out a DIY Plasma Cutter! A true LOCOST way!


Last edited by botbasher on Mon May 14, 2012 1:27 pm, edited 3 times in total.

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PostPosted: Mon Jun 14, 2010 9:00 am 
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Cool. Although the schematics don't mean too much to me (which means I definitely shouldn't attempt to make my own!) Any pictures of the final machine?


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PostPosted: Mon Jun 14, 2010 9:44 am 
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gtivr4 wrote:
Cool. Although the schematics don't mean too much to me (which means I definitely shouldn't attempt to make my own!) Any pictures of the final machine?
Final chapters coming.. it was just too late last night to keep posting!! :yawn:

KS

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Don't ever become a pessimist. A pessimist is correct oftener than an optimist, but an optimist has more fun, and neither can stop the march of events.-Robert A. Heinlein

Add yourself to the Locost Builders Map!

Jigsaws, Hacksaws... There is a better way! Want to get creative with metal? Want a Plasma Cutter but can't justify spending the $? Check out a DIY Plasma Cutter! A true LOCOST way!


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PostPosted: Mon Jun 14, 2010 9:55 am 
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Very interesting...Thanks for sharing!! :cheers:

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PostPosted: Mon Jun 14, 2010 10:28 am 
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Very very nice!


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PostPosted: Mon Jun 14, 2010 9:30 pm 
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Here are a couple of pics of the quality and ability of it while cutting through 3/8 plate steel! :D

Image

Image

That long arc on the right is what is in the pic above and I managed it in two cuts (had to stop to put out my shoe!).

The water was very warm after I finished, so I know that this is about the limit of it's ability!

KS

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Don't ever become a pessimist. A pessimist is correct oftener than an optimist, but an optimist has more fun, and neither can stop the march of events.-Robert A. Heinlein

Add yourself to the Locost Builders Map!

Jigsaws, Hacksaws... There is a better way! Want to get creative with metal? Want a Plasma Cutter but can't justify spending the $? Check out a DIY Plasma Cutter! A true LOCOST way!


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PostPosted: Tue Jun 15, 2010 12:57 am 
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KB58 wrote:
Without an isolation transformer, this is like giving instructions on how to make nitroglycerin. "As long as you're careful, it's fine." Yes, but make one mistake... Worse, what if someone who's barefoot comes in while you're working and touches the far end of the welding table? Unless it's just you in the garage, I have great reservations about the unit as-is. And yes, I am an electrical engineer.
I did state clearly and repeatedly that I don't have and iso and do not suggest anyone follow that lead. USE AN ISOLATION TRANSFORMER!! The plans that were supplied to me include it and were forwarded unedited save the single wire change. Your concern is respected and duly noted.

KS

p.s. and yes... I do know how to make nitroglycerin and about half a dozen other kitchen sink explosives. It's not that hard or that dangerous!

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Don't ever become a pessimist. A pessimist is correct oftener than an optimist, but an optimist has more fun, and neither can stop the march of events.-Robert A. Heinlein

Add yourself to the Locost Builders Map!

Jigsaws, Hacksaws... There is a better way! Want to get creative with metal? Want a Plasma Cutter but can't justify spending the $? Check out a DIY Plasma Cutter! A true LOCOST way!


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PostPosted: Tue Jun 15, 2010 10:51 am 
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Can you post bigger schematics? Some of those are barely readable


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PostPosted: Mon Jun 21, 2010 3:28 am 
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McCheese wrote:
Can you post bigger schematics? Some of those are barely readable
Some of them are originals and some are pdf clips.. which one are you asking about?

KS

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PostPosted: Mon Jun 21, 2010 3:41 am 
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After cutting a bunch of parts for a side project to help pay for some car parts, I started to wonder about a phenomenon I started to see with the Plasma Cutter.

When cutting a part out (thickness of material seems irrelevant) the slag tends to collect more on the side of the piece with the ground clamp (actually positive) attached to it. Basically, if I was cutting a circle out of the center of a square piece of steel, the slag would collect on the outside of the circle (where the clamp is attached) and only barely on the cutout circle. Why is that?

I have taken to trying to make the cutoff portion of the part the thing I am keeping and noticed that my clean-up time with each part went way down after doing so.

I'm thinking that it involves the basic electrical principal of electrons flowing from (-) to (+) (depending on your teacher) and depositing them on the (+) side as they are blow out, but since the entire piece is at relatively the same potential until the very last mm is cut I'm at a lost to explain why it's happening.

Has anyone else experienced this and is there anything you did that changes it? Can anyone explain what causes it?

Inquiring minds want to know!

KS

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Don't ever become a pessimist. A pessimist is correct oftener than an optimist, but an optimist has more fun, and neither can stop the march of events.-Robert A. Heinlein

Add yourself to the Locost Builders Map!

Jigsaws, Hacksaws... There is a better way! Want to get creative with metal? Want a Plasma Cutter but can't justify spending the $? Check out a DIY Plasma Cutter! A true LOCOST way!


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PostPosted: Thu Jun 24, 2010 2:04 pm 
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The reason that the slag is staying attached to the larger piece may be that the piece you are cutting off tends to ge hotter and slag is attaching to the cooler part. You might want to play with gas flows and see if the quality of your cut improves.

ALSO - if you are using air as your plasma gas make sure to grind any cut surfaces you wish to weld to. The nitrogen in the air forms compunds (nitrates or nitrides, its been a while...) that will create porosities in the welds and substantially weaken them. Not a big problem, just clean up the surface before welding.


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PostPosted: Thu Jun 24, 2010 3:47 pm 
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kf2qd wrote:
The reason that the slag is staying attached to the larger piece may be that the piece you are cutting off tends to ge hotter and slag is attaching to the cooler part....
I thought of that after I posted that example initially and tried a few different options, but all with the same result.

While cutting a sheet of stainless for a heater box I tested these ideas out. No matter the size of the piece (i.e. both would experience the same temps) the slag always deposits opposite the clamp (+).

Air pressure does change the resultant amount of slag, its always there though.

It chips off very easily, my interest in it is limiting the time needed to dress up cut parts.

Thanks,

KS

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Add yourself to the Locost Builders Map!

Jigsaws, Hacksaws... There is a better way! Want to get creative with metal? Want a Plasma Cutter but can't justify spending the $? Check out a DIY Plasma Cutter! A true LOCOST way!


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