CNC plasma table with height control...?

[Sperstad]

Hello people.
I`m considering building me a little and simpel CNC plasma cutter table, but there are some things that I need to find out first. I se that many tables have "height control" function installed. I know what it does in general. But can anyone explain in detail how this work? And have anyone here buildt a plama cutter with height control?

[Torchhead]

height control: Sometimes called Torch Height Control (THC), Automatic Torch Height Control, (ATHC) or Digital Torch Height Control (DTHC)

Since plasma cutting is done with the tip rasied off the material by a small amount and done at high speed you need a way to maintain the perfect gap (often 1.5mm) as you buzz along at 2500mm to 5000mm per minute. If the material is not perfectly flat or warps a small amount, the tip will either be too close or too far. If it touches the material it will either burn out the tip or shorten the life of the consumables drastically. If it's too far away the kerf widens, the flare gets worse and the slag increases. At some point you lose arc.

You can maintain the gap by building a floating spring loaded head and use a tip that is made for dragging on the surface (Drag Tip). That will work for simple cuts and bigger pieces. It tends to hang up and cause problems on more detailed cuts. Everyone using a drag tip yerns for a true THC.

A true THC measures the gap voltage and works on the principle that plasma cutting is a constant current process so that the voltage varies with the gap. By measuring that and providing a "servo" feedback to a Z motor you can maintain constant height. The devil is in the details. The amount of voltage change is small to hold tight gaps (+ - 1.5 VDC or about 1% of the cut voltage) and there is a lot of noise. The other challenge is to control other critical parts of the process. The pierce cycle should go like this:

Move to XY location to pierce the lead-in start point
Move down and sense the top of the material (because the material can warp)
Move the head up an amount of apprx 2X the normal cut height
Fire the torch
Delay using a signal that detects a valid Arc has been established
Release movement and move the Z rapidly into the cut down to the normal cut height
Delay by 1/2 to 1 sec turning over control to the THC to stop the tip from diving into the material from the voltage spike from the pierce and down move.
Maintain the gap within .5mm while cutting.
At the end of cut, turn off torch and pause about 1/2 sec to let things stabalize.
Raise the torch up to rapid height and do it again for the next cut.

As you can see the process is more complex that doing a simple cut with a router.

There are two approaches to doing THC with CNC. One that has been used on lower end systems for years is a "Stand Alone" THC. It works independantly of the control software and has to do all or most of the moves listed above on it's own. Because the electronics are complex the prices are high starting at 2499.00 and going as high as $10,000 USD.

The other approach is to have THC as part of the control software and like you do with other cutting operations let the computer control XY and Z movements. That approach ties the THC to the toolpath and allows things like anti-dive when the machine slows down on a tight corner cut. In the past integrated Control and THC software only came on very high end ($$$$) systems.

There are now cost effective solutions for both approaches.

If you want to keep your costs down but still have a good THC look at the THC300 at CampbellDesigns.com. It's not Digital but there are hundreds in service out there running with MACH.

The inclusion of SHeetCAM as your toolpath (CAM) program gives you precise control over the plasma cut process and it works directly with MACH3. Mach is one of the few pieces of control software that has THC logic in it's DNA and it will also be used for controlling virtually anyother type of CNC operation. It's low cost and very well supported.


tomCAUDLE
www.CandCNC.com
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[Sperstad]

Thanks allot for all the information you gave, this was very usefull! I`ve looked at the "THC300", and it looks like a very interesting system. It has all the options i need, and it`s cheap also. I have now downloaded the manual on the "THC300", and I will read it soon.

But have anyone here made a THC system from scratch?

[Torchhead]

Originally Posted by Sperstad . But have anyone here made a THC system from scratch?

Yeah.......me. Designed and built the first THC300 in my electronics shop and ran it for 6 months making inprovements and learning how to make quality cuts using an old HF start Miller 500. That was over 4 years ago.

Tom Caudle
www.CandCNC.com