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grey
05-12-2003, 08:54 AM
I have built a 2d cnc gantry for a plasma cutter. I could buy a $600 palasma cutter but wondered if I could just buy the nozzle and electrodes as spares, buy a 30A bridge rectifier to turn 240V A/C into 240V DC. I have a cnc mill so manufacturing a machine torch out of plastic would be fairly straight forward. Most of the cutters seem to work in the 80-220vDC range. Then all I need to do is create the spark and turn on the air. Some manufacturers use high frequency starting, (dont want to do this) and others create the pilot spark to the shield can, then use a relay to send the curcuit to the stock. Has anyone done something similar or could they provide some guidance on the potentially dangerous event. Would it be easier just to buy a machine torch and design some simple driver electricals?

alumtuna
05-12-2003, 11:49 AM
Hello Grey,

I could understand why somebody would want to build a CNC gantry system. The cost savings could end up saving the home machinist several $$$. But for a plasma cutter or welder i would definitely go out and just buy the one you want. From my understanding the welding/cutting equipment industry margins have been beaten up to a point that it make no cents to go that route. IMHO

PS GJ on the gantry system.

ibewgypsie
05-12-2003, 02:19 PM
The ignitor in a plasma torch starts the arc. It will not start without it. It holds hi-voltage until a arc is established then switches over to a lower voltage. This is teh way my miller worked anyways.
I would opt to purchase one, then change out the tip. I don't think you will be happy with a 600 dollar one either, they have the air compressor inside them also. Won't start, won't burn in my experience with them. Dragon cutter? is one that comes to mind.
I had the same ideal at one time, then got over it. Why reinvent the wheel unless you have lots of time and money to invest.
I swapped my plasma c for a old hot rod.

lvanduyn
05-12-2003, 04:05 PM
Hello Grey
There are a number of problems with your idea. You could, as you say, just rectify the AC into DC and connect one side to the electrode and the other to the work. Now you have both both the work and electrode at a hundred plus volts above ground. This is fine if you don't plan on living to a ripe old age. Second big problem is that a plasma machine is a constant current device. The voltage has to vary to maintain the set current level. A torch is not anywhere as simple as you descibe. Mine has a solenoid operated air valve and moving electrode and a cup which is where the arc is initiated. These few points probably are part of the reason that a plasma machine has a couple of large circuit boards, a few transformers, relays, current sensors, and odd assorted parts. You will be much further ahead to purchase a machine..

Thrud
05-12-2003, 04:12 PM
grey:
Bite the bullet and buy a commercial unit - it is far more complex than you think. It took Hobart/Miller/Lincoln/et al decades of development to get it to the good operational characteristics they have today. Don't bother re-inventing the wheel - square wheels are not an improvement! http://bbs.homeshopmachinist.net//biggrin.gif

grey
05-13-2003, 02:57 AM
Thanks everyone, so 4 votes for biting the bullet to nill. Not even the electric company suggested a way of creating cheap plasma. I guess the $600 unit has it then. I was hoping to assemble a cutter for about $50 using existing electrodes, diffusers and shields, the bridge rectifier was less than $10, it looked so easy, but dangerous. The CNC table cost less than $400 to build, so spending $600 was a fairly large investment considering that I know these entry level models are usually a bit deficient. I have a new cheap mig welder that can create sparks 40% of the time, welds 5% of the time on a good day, this was not a good investment. I would hate to spend another $600 and get a unit that clutters up precious workshop space. I'll post a picture of it when it's finished.

Thrud
05-13-2003, 04:03 AM
grey:
Once you use a Plasma cutter, you won't want to a gas torch ever again. A good investment if you weld.

yf
05-14-2003, 05:10 AM
Why don't you try out the gantry with an O/A torch?
A machine guided torch makes a real nice cut.

You can add a solenoid valve to open the cutting O2.
Or a solenoid to squeeze the lever.

grey
05-14-2003, 08:55 AM
Havn't looked at O/A, just presumed that the cost would be higher than Plasma, the Plasma set although probably crap is only $600. I require a no skill solution to cutting, the last time I tried O/A I emptied the bottles and created iron scupltures and spent the next 3 weeks darning the holes in my overalls. I like the thought of having another form of matter living in the workshop, seems kinda homely.

mikem
05-14-2003, 09:46 AM
About 6 months ago, I got a Hypertherm plasma cutter after posting questions here about the best ones to buy. It is a pretty slick machine, but it will only cut up 1/4" thick material. It cost about $650 used--about two years old and in nice shape. Do a search on plasma cutters on Yahoo--you'll get some really good hits. I think one is called "Plasmacam". It had some comparisons(performance reviews) of different cutters when used with CNC tables.

Do you have any pictures of your CNC table? How big is it? Did you use someone else's plan? Use new or used parts? Did you have to refinance your house to pay for it? Sorry for so many questions http://bbs.homeshopmachinist.net//smile.gif

mikem
05-14-2003, 10:04 AM
http://www.plasmacam.com/plcut.htm is the website for the plasmacam plasma cutter comparison chart. The whole website is great.

WJHartson
05-14-2003, 10:14 AM
I did some research when I was thinking about building a CNC plasma cutting operation. The Hypertherm came out on top. They make the best torch, Miller uses it also. There can be a problem with interferance and interface with plasma cutters and computers. Hypertherm has covered all of those problem areas.

Joe

grey
05-14-2003, 10:47 AM
mikem, I think it cost about $400 to build, the steppers were used, the frame was new unistrut, new 20dia bright bar, new 10mm acme lead, size is 1.2m x 1m (I have a space problem in the workshop and needed something that would park vertically when not in use. Absoultely no need to mortgage the house, very easy to build a machine for less than $500. For $1000 you could design a very serious unit fit for industrial use. I'll post some pictures when I buy the torch. The design was in-house, accuracy is not great +/- 0.25mm but good enough for cutting. Max speed is about 400mm/min, but I am improving this. I have a large (but not robust 800x500x180mm) CNC m/c that I used to m/c the end fitings. I also have plans to extend the table and swap the head to a router, so that I can machine even larger molds that I can't fit on my CNC mill without segmenting it.

BTW can your hypertherm cut vertically and is 400mm/min fast enough? This would really help my space problem.

I am also working on a home grown CNC tube bender as an additional project.

mikem
05-14-2003, 02:59 PM
Haven't tried to cut vertically but I think that should work. The cutting speed is hard to judge. Somebody else will know more about this than me.

What do you think is the cause of your limit of accuracy?--Mike.

Thrud
05-14-2003, 03:52 PM
grey:
Cutting speed depends on capacity. For high speed cutting get a unit capable of cutting 3/4"or thicker material. A big torch is pretty zippy with 1/4" and thinner Steel and Aluminum. I would also look for an invertor type unit as they have more stable current/voltage and are more energy efficient.

grey
05-15-2003, 09:12 AM
mikem,
the limit of the accuracy is down to the following 5 things
1 me
b $400
f me
6 was designed in a hurry, fed up sub-contracting cutting work. Should have spent more time in the design.

The accuracy will be fairly easy to improve, when I need to adapt it as a bigger router, due the modular design of the machine, the steppers, are capable of .02mm, the rest is m/c slop.