Idea for a $10 plasma torch height control- Thoughts?

[sbalder]

After having built and restored several CNC machines I have acquired three 4X8 Spadix adhesive tables that I am making into plasma cutters. This is my first experience with plasma cutting so of course I'm digging through the web for information before digging my credit card out. I have an idea for a very simple and dirt cheap torch height control application. Here goes:

Required Items- Plasma cutter, XY cnc table, computer, cnc software, standard radio control servo, various electrical parts.

The assembly- Plasma torch is mounted on a small slide where height can be adjusted mechanically and the slide is in it's middle position when the servo is neutral (90 degrees). The torch is manually set at the correct piece height for the material to be cut.

The circuit- There is a potentiometer in each RC servo which is connected to the output shaft and used for positioning feedback. This potentiometer input is disconnected and the input terminal is wired to an SPDT relay. The relay is controlled by the computer. In the "off" position the existing potentiometer input is used to hold mechanical neutral. In the "on" position the potentiometer input is the torch voltage, divided down with resistors. The resistors can be precision adjustable units to allow for fine voltage setting

Cutting logic-

-CNC software moves the plasma head to the correct X-Y starting position.
-CNC software turns the torch on and holds for a fixed time per the part program to allow the arc to pierce.
-CNC software turns the torch height control (THC) relay on.
-The THC senses the arc voltage and moves the torch up and down to maintain target voltage.
-CNC software moves the X-Y position to make the cut per the part program.
-CNC software commands the THC relay off; the servo returns to the pierce height.
-CNC software turns the plasma torch off.

Many of us could build this with the stuff lying around in our junk drawers.

I'd appreciate feedback and thoughts on this idea.

[Khalid]

Great Great Great idea... could u plz put some drawings, circuitboard or picture..This will b ur contribution to lot of us:-)

[sbalder]

I've been thinking about this idea since I posted it before lunch and have a few ideas refined already. The machines have a linear bearing slide setup on the Z-axis that is driven by an air piston. I came across a high power (by rc model standards) servo used for model sailboats ( http://www3.towerhobbies.com/cgi-bin...?&I=LXGZU3&P=7 ). Unlike conventional servos which rotate about 90 degrees maximum, this one makes 3 1/2 turns. My plan is to mount a small precision gear rack on the z-axis slide and drive it up and down with a small spur gear on the servo output shaft. A half-inch diameter gear should give 5+ inches of travel. Alternatively I could use a large spur gear on a standard servo; a 3" gear would give a couple of inches of travel

My other inspiration since posting is that it is really is not necessary to hack up a servo. Without a pulse signal, servos will not hold neutral. A simple servo tester is available to purchase (http://www3.towerhobbies.com/cgi-bin...&I=LXVK79&P=ML ) that I can use to set neutral and interface the divided arc voltage into the potentiometer.

I'll see if I can get a schematic together for posting. As I read more about the torch height controls on the market I see no reason why this wouldn't work!

[sbalder]

Here's a rough conceptual schematic of what I am talking about.

[Khalid]

Dear Sbalder,

Plz do not let this thread die... plz keep on posting ur progress... many of us watching this thread... I hope ur inspiration may results in low price THC ,, thats what we DIY's wants... I hope u will keep this thing on....
Regards

[Khalid]

Though i dont have plasma router yet.. however i m gona deep in soon.... and i need ur experience what u doing on ur machine...
good luck

[sbalder]

Thanks Khalid. I really have nothing to lose in trying this. The cost of the parts are cheap and I can use them for my RC hobby projects if the whole thing turns out to be a bad idea. Also, I am not really making more than bolt-on modifications to the z-axis, so I can go back to the fixed-height setup presently on the machine if this does not work out.

I have a Hardinge lathe project to finish up and then I plan to get into working on these plasma tables with the hope of burning steel on all three by the end of this year.

[vcooney]

sbalder

3 plasma tables...planing on doing some serious cutting?
I'll be watching this thread...your idea on the THC is my kind of price.
I'm still building my machine...waiting for my son to get his furniture out of my shop, no room to do anything.
Vince

[sbalder]

Vince,

What started as a simple idea to buy a small mill to make custom parts for my model helicopters has turned into growing part time business restoring and selling cnc machinery. The only problem is I can't always control when my orphan project machines arrive. In this case, these 3 tables were too good of a deal to let pass me by. The only problem is I can't park a car in my garage at this point!

[Torchhead]

Let me know how it works out. The plasma voltage is negative and has lots of high freq noise. Grounds in the system can cause unwanted feedback. Arc Voltage has to be maintained within about 1.5 VDC to get anything that approaches a decent cut. To save tips you need to pierce high (about double the normal cut height) then plunge to the intitial cut height. Since you are dealing with distances in the .040 to .187 range, any distortion in the material will throw things off and cause bad pierces and/or ruined tips. You need some way to sense the top of material before each pierce since the THC won't be talking to the toolpath software.

Be careful, you are dealing with lethal voltages and equipment that can severaly harm or maim . There are other gotcha's but if I told you all of them it will take all of the fun and learning out of the project.

Tom Caudle
www.CandCNC.com

[sbalder]

Be careful, you are dealing with lethal voltages and equipment that can severaly harm or maim . There are other gotcha's but if I told you all of them it will take all of the fun and learning out of the project

Thanks for the insight Tom. Anyone who thinks they have an original innovative idea faces one of 2 possibilities- either their idea is truly original and innovative or someone else had the same idea and is lying face down in their workshop and you'll never have the chance to hear about!

The torch pierce height changing from one cut to another is not of that great a concern, and I could add a height sensor. The voltage spikes and fluctuations could more likely cause me fits.

Is there a reference anywhere on the web for pierce heights, amperage, speed, and torch height for different materials?

[sbalder]

So I decided over the weekend that other that the possibility of electrocution that Tom mentioned I have nothing to loose. If the electronic feedback idea does not work I still will have electronic manual control over the torch height. You could even call it a "semi" automatic height control because I can set a pierce height and a cutting height. Also I decided to make these water tables so I am hoping this will help with controlling warpage.

One more "crazy" idea for the water tables. It seems there are several different approaches to standoffs for supporting the workpiece. My idea is to use aluminum soda cans as standoffs. They are free, have consistent size and strength, and offer a lot of support with minimal contact area.

I'll continue to post in this thread as the projects progress.