CNC Plasma

with a Cut 40

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Back at the end of 2014, I bought a cheap Chinese Plasma Cutter, called a Giant Cut40 from a local trader, at £185, it seemed to be a reasonable buy, the trader was local in case of problems, so it was a reasonable safe purchase.

By January 2015 I was decided it was project time and I started to acquire parts to make a CNC frame for the plasma, Nema 23 stepper motors, timing belts and pulleys were purchased via Ebay along with a ball screw and a 4 channel controller.  The software in use at the time on a CNC milling machine was CNCPro an old DOS based program.

The frame slowly started to take shape over the next few months, I used 50 x 25 mm box section.

By this time it was June 2015, so I was six months into the project and to be honest wondering why I was bothering.

At the end of June 2015, I thought the stage had been reached to move the frame from the floor to a bench position and apart from blowing controllers to useless pieces dead electronics every now and then I managed to achieve some good cuts.  I use Qcad under Linux for my drawing package and purchased a copy of Sheetcam to translate across to Gcode.

The following video was June 2016 so still using the dos program CNCPro But it toom me to this stage to stop the controllers from failing on arc start.  A home made filter system of caps Zener diodes and MOV's between the motors and the controller did the job for the most part.

One of the issues was cutting arc start up.  To this point I was dragging the plasma tip across the metal, that was the only way to ensure the arc started.  But I knew I had to improve things, so that meant investing time in better software such as Mach3 even though Mach4 was coming out soon.

I looked at how I could get some form of pilot arc from the cut 40 so I started to experiment, I did try it in the early days with some success but for some reason stopped, now was time to try again.

So I proved a system that would work and improved on it in stages.  It is worth note that in 2016 and 2017 I started to see Cut 50 Plasma cutters with a pilot arc built in, Oh well!

I had a request for the nozzle shroud I was using on the PT31 style torch.  I found a drawing I did, this is as much information as I made about it, suggestion would really be to get a nozzle that is capable of taking the connection for pilot arc, but if you wanted to convert a non pilot arc torch you could try to copy this or make something better.


When I changed over to a SG55A straight torch (see later videos) I also changed the resistors slightly to three 100 Ohm, 100 watt resistors wired in parallel.  I had a few requests for a circuit, so here is something very simple.


The next stage was to probe the top of the work piece for Zero height.  If I had pilot arc I did not need to drag the tip any more?  So a micro-switch was fitted to the 'floating head' design I was using, wired into the probe input of mach3 and some macro code worked out with help from the Mach3 forum and away I went.

It was now December 2016, 2 years from buying the Plasma cutter and nearly two years into the project of CNC'ing the unit.  It has been suggested I should have bought a Hypertherm, in fact I still get those comments, perhaps when I can have a table bigger then the size of a desk I may, but for now :)

The next stage of improvement was to add Torch height Control, THC.  Mach3 has a means of taking signals from some basic units and moving the Z axis up or down to suit, but prices again can be expensive and would cost me more that the Plasma cutter cost in the first place, so a sub project was started, please see a little further down.

Filtering on the CNC Plasma

I have had a few comments about filtering, I guess that as people start to build their own units, like me they have issues with the HF start on the Chinese cut machines getting back into the control electronics and computer.

So here is the circuit of the filter I added to the output from the motor controller to the stepper motors.  The diagram only shows the output for one motor/axis, so if you use 3 axis, you need 3 filters.  This worked on mine, or shall I say, it helped a lot, each machine set-up will be different, so best experiment yourself and see what is best for your own machine.

Here is the Youtube video that accompanies the circuit.

A few repairs and a better camera!

June 2017 and I have just updated the table, for the table it is a major update and potentially throws all my previous cut setting out of the window!  The ball screw and number of steps per mm, meant the maximum the table would run the Y axis at reliably was a maximum speed of 1200 mm per minute. I would cut at 900 mm per minute to make sure it did not miss steps.
So the Y axis was changed over to the same belt system as X axis and I could go to a maximum speed of 7,000 mm per minute. (275 ipm).

So  new learning curve awaits as I figure out Amps verses speed on my machine.

Cut parameters

I have been playing again with settings and  on 1.5mm steel cutting at 27 Amps at 2,200 mm/min.  Also a repair, or more like permanent fix of the Z height probe switch as it kept moving!

So once I sorted that, added an aluminium shield to protect the Y axis ways from the splash-back I had another go with what I call a test piece.  Apart from desperately trying to pick out the tip-ups before they caught on the head, this cut without any errors. Total running time was 18 minutes 15 seconds.

Anyway this was my test piece for tonight, 450mm diameter circle.

I am saving my newer cutting parameters and will update the table when I have played a bit more.

The above mentioned Z Probe switch meant I needed to remove the axis to sort it, so I did a couple of videos on my way of doing the Z zero setting. details of mechanical switch, macros and Sheetcam.

Here is a link to the M800 file as mentioned in the video M800

Not updated for a while, here is a simple video, just to show it is still here and working.

Just playing, it may give ideas for the festive season, you can draw and cut similar items.

I had a few questions about the Pilot arc system I used, so the best way I could think of answering is with a video or two!

January 2018

I find when the time comes to change or get a new batch of consumables I have issues, I have been finding the quality of fit, manufacturing etc. to be different from one batch to another.  So I am trying a different style torch, the SG-55.  It is physically bigger and meant for more amps typically the 50 to 60 plus range, but I find you can get 40 Amp consumables for it.

The above video is the first explanation and the next one or possibly more will be about use.  First I have to make a mount to fit the SG-55 to the Z-axis.

I eventually realised that the handle part of the Sg-55A torch is 28mm diameter and that is the same as some standard plumbing fittings, so from the local hardware store I picked up some plastic pipe clips the sort that wrap all around the pipe.  Took the securing hole out to match a M6 screw thread and I could mount directly to the from of the Z axis.  This made up for the extra weight and then some as I could do without the steel mount for the PT-31 torch.  Wired in the Pilot wire back to the resistors and ready for it's first go:

This was very encouraging, so time to make something else a simple 'key' holder and time for a second go just to make sure it was not a fluke:

So happy with the new torch, seems to perform better, I believe there should be less variance in the parts, but time will tell, it does mean the custom end to provide Pilot arc is not used so the air passing the nozzle should be more uniform and keep things a bit cooler, perhaps helping shift the dross?

Next up, was to help someone having difficulty with 1 mm aluminium, so a response via email, but testing it myself should help more.  I went to 25 Amps at 5 metres per minute, just under 200 inch per minute, think I could go a bit faster, but ran out of aluminium for the time being.

So that brings the page up to date for a little while.

Oh, I am trying to cut sheet brass 1.6mm thick, a simple clock.  I basically used same speeds as I would for steel, but the cut edge not as sharp, more of a melted edge and it seems the melted metal can solidify causing a kerf/dross line on the bottom edge of the cut that is not as easy to remove.


Next is a general video, in this I used a free to download 3D Puzzle file from the internet.  Dragonfly,   I guess it was originally done for 2m ply wood or similar, but here we go in 1.5mm steel.  I had a few issues where I placed the starts in Sheetcam in the wrong place, it allowed the cut piece to drop out before the cut had actually finished, so the plasma was trying to fire into nothing, so the HF would stutter and try to keep the arc going.

I also forgot to make sure the earth clip was on the table from the 20 Ohm pilot arc resistors.  Think memory must have been suffering a bit!

I am always looking to see if things can be improved, I will try things out and see how my ideas work.  In another video I was cutting 8mm steel, which to be honest was a bit adventurous for this machine, but one thing I noticed was the dross/blowback when piercing could get stuck on the tip or nozzle, so a change to the nozzle by removing the castellations was tried. 
By removing the end, and not going into the jet/arc hole I was able to shave 0.5 mm off the tip.  Now while this may not sound like much, it will, I hope allow me to have a bit more clearance at pierce height, as the new height will be, say, 2 mm rather than 1.5 but the hole is still at the same height. 

Also if I continue to use 1mm cutting height then the hole has come closer to the work from 1.5 to 1.0 mm.  All by removing the end bit of metal from the tip.

Been a while but here is a bit on Home switches, soft limits and G28.
I tried to answer a few questions on the Internet, this made me think and look into using my last input pin for home switches, plus I had to change some button settings on the Mach3 screen so here is a video about that.

I mentioned in the video I would have the Visual basic script on here so here it is:
And here in just text format if required:

Code("M8")              ' Optional to turn on my table if not on!
  While (IsMoving())    ' Wait for movement to complete
  Sleep(100)            ' Sleep, so other threads can run while we wait
DoOEMButton(1023)       ' Reference Y
DoOEMButton(1022)       ' Reference X
DoOEMButton(1024)       ' Reference Z
  While (IsMoving())    ' Wait for movement to complete
  Sleep(100)            ' Sleep, so other threads can run while we wait
DoOEMButton(104)        ' Go to Safe Z 

Some catch up to do on the web site, so here we go.

A general video called Hunting Cats, just to show I was still in existence

Then finally we get back onto a bit more information, going off general questions received on You-tube and via email
more information was required about HF start and the method I used to generate a pilot arc with the resistors.  So another video was born:

A general video about HF interference and some of the things I have done to help:

Finally two videos where I talk about using cut rules in Sheetcam and also an attempt at some basic electronics help on resistors, Wattage, Ohms law etc.  All trying to answer a few questions picked up recently.

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Be safe. 

I have been getting lots of spam so emails are stopped for a while.