China HF-start plasma CNC design!

[MachinistSF]

Hi,

I just cut the first pieces with my new DIY CNC plasma today!!

It uses 3 steppers with chain drive for all axis. Z is a floating design with limit switch
for detecting zero at plate surface with G28.1 Z. Movement is 70cm x 70cm.
Torch is a PT-31 machine torch. Steppers are controlled with chinese 3-axis
TB6560 card - replaced 12V fan with 24V one and removed the "current
reduction when not moving" feature.

I initially had problems with RF interference from the Chinese CUT-40 HF-start
plasma, but I managed to solve the problem cheaply. I added a ground wire to
the copper electrode sticking out from the plasma torch - this way it creates always
a "pilot arc" (at full current) and initiates the cutting reliably without HF interference.
Without this it would not ignite reliably and would emit HF interference, causing
limit switches to trip, USB mouse at PC to stop working etc.. None of that now,
the immediate arc to groundwire will shunt the RFI, I'd say.. Based on some test
cuts, it works very nicely. I figured out that the electrode is touching ground through
the workpiece anyway, so it won't hurt connecting ground to the electrode always - and
this way it ignites reliably. It will also send a nice plasma flame to air if it is far away from plate!

I'll send some photos later. Now the main point is that this way one can use the
cheapest of the cheap Chinese CUT-XX plasmas for CNC, to save amateur costs!!
I haven't seen this anywhere else so I might have invented a new thing here!

[MachinistSF]

Here are some photos.

First one is an overall photo. The control electronics box is not shown. Plasmacutter is with the unit in a shelf. Ground is connected to clamp and the support "mesh" for part. It is so close to ground so it won't take much space when not used - I can put it under table.

Second is of the torch holder with z movement. There is also z touch sensor - the movement up and down is with chain (hanging from chain), and when z touches chain becomes loose and the lever to spring loaded microswitch opens. I think this is a novel design, I have not copied it from anywhere.

Third is the torch tip. There is a ground connection to tip with alligator clip. This modification produces a "pilot" arc from the torch. Without this it does not work ok with cnc use as ignition is unreliable.

[MachinistSF]

Here's a video!

http://www.youtube.com/watch?v=-VcNw8eoFVo

[MachinistSF]

It can be seen from video that it doesn't yet have THC.. I plan to install one based on a panel display with modbus communication. I'll connect a voltage divider to the display 0-10V input, and MACH3 modbus protocol support will directly read the voltage from the display (scaled based on voltage divider) via rs-485 bus from PC. Then the MACH3 plugin for THC will handle the Z control based on this voltage. But, it is not really very high on my list of things to do as it works quite nicely without THC on thick plates at least.

[MachinistSF]

The moving mechanics and motors are connected to safety ground, using a star ground scheme.

The plasma cutter ground is connected only to ground clamp (to be connected to plate to be cut), the support mesh for part to be cut, and the tip of the torch.

This arrangement is done so there is no plasma current flow to safety ground. These two grounds are mechanically separated by 6mm plastic plates, separating the base of the machine from the aluminium bodied moving mechanics. I think this is essential safety feature - otherwise there could be a ground loop burning the safety ground wiring with 40A plasma current. Just like with welding - you never connect safety ground to welder ground, so welding current can't flow through safety ground wiring (melting and burning it) if you forget to connect ground clamp of welder to a safety grounded item to be welded.

The wiring to control electronics is long enough that I can roll the plasma cutter mechanics outside.. That way there is not dust problem at all, as I can close the door when I plasma-cut. Of course, it limits me so I can't plasma-cut if it rains outside but I can live with that.

[Dan13]

This is cool! Thanks for showing.

Dan

[ThorstenU]

Great Ideas.
Thanks
Thorsten

[gcafonso]

Hello, very nice project, will test into my CNC and CUT40 cutter.  Question: connecting the tip to earth wouldn't destroy the tip if you trigger the torch away from the plate? I think that making the way you did, the plasma beam will cut the tip itself? Thank you for sharing your project!

[MachinistSF]

Hello, very nice project, will test into my CNC and CUT40 cutter.  Question: connecting the tip to earth wouldn't destroy the tip if you trigger the torch away from the plate? I think that making the way you did, the plasma beam will cut the tip itself? Thank you for sharing your project!

The tip will propably be destroyed if you trigger the torch away from plate FOR A LONG TIME. Short times didn't matter, I did try that first to see if it would work at all. In the normal use, the arc transfers from the tip ground to the plate (to be cut) ground almost immediately. After this, only part of the current goes to the wire connected to the tip - and the resistance of the thin wire is much higher than the actual ground resistance to the plate (to be cut).

[MachinistSF]

Hello, very nice project, will test into my CNC and CUT40 cutter.  Question: connecting the tip to earth wouldn't destroy the tip if you trigger the torch away from the plate? I think that making the way you did, the plasma beam will cut the tip itself? Thank you for sharing your project!

The tip will propably be destroyed if you trigger the torch away from plate FOR A LONG TIME. Short times didn't matter, I did try that first to see if it would work at all. In the normal use, the arc transfers from the tip ground to the plate (to be cut) ground almost immediately. After this, only part of the current goes to the wire connected to the tip - and the resistance of the thin wire is much higher than the actual ground resistance to the plate (to be cut).

Just adding.. Also in normal drag cut you are grounding the tip to the piece to be cut..