Thread: HF start plasma and CNC...
Has anyone here got a cnc table running?
i've just built a small one and ordered a new cutter to fit onto it, due to budget, i couldn't get a non-HF start torch but the one I have coming has a socket for CNC triggering so I presumed it will be ok.
since ordering I have read nothing but bad stories of how HF start plasma cutters wreck pc's and cause all manner of control issues.
i have taken care in building my table to ensure no low level signals are taken outside the control cabinet but have not used screened cable on the stepper motors as I presumed the high current/voltage would stop rf pickup problems there.
I have also got a hand control pendant but apparently they can be a nightmare for interference too.!
this is doing my head in now, has anyone got a CNC table running on an HF start plasma torch and PC????
You will almost certainly need shielded cables on the Steppers and any other Signal cable and make sure your using star grounding.
I have a test setup for testing machines and the minute My Tig with HF pulse fires the PC freaks out, doesn't crash but it stalls n stutters and no way could it run a CNC machine at same time. I'm pretty sure that's HF going done the mains line and a decent filter would help but still shows how HF Pulse can and will affect the machine.!!
Last edited by JAZZCNC; 18-02-2014 at 10:04 PM.
Oh bum, back to the drawing board then, again.
Years ago I built a hf start cnc cutter. I had the problem of the hf start making the computer hang (an old 386 running in dos) . I used a 1: 1 isolation transformer on the pc side. I also ran the plasma torch cable suspended from the roof to keep the cables as far apart as possible. What about one of those chokes they use on the input line on a vsd ? Hope this helps
As Jazz says you need to use shielded cabling with everything. The HF pulse from plasma start will go anywhere and everywhere possible if you dont get the grounding correct.
Ground your shielded cables at one end only, preferably the supply end and get all of your ground cabling back to a single star point on the machine somewhere then use a single large cross sectional area conductor from here to ground. You could even try using an earth electrode outside your workshop hammered into the ground and connecting your star point to this.
Short story but this got a customers machine working years ago doing this. His machine was working fine until he had new building work done. A reinforcing concrete wall was built at the end of his workshop and this had foundations that were alot deeper than the original. This dried out the ground under his workshop and prevented a good earth from his building steel work that his machine star point was connected to. Basically it lowered the water table level. I hammered in a TT rod ouside his workshop and connected the star point to this and the machine work as it should without jumping all over the place when the HF kicked in.
Ok, I'm going to rewire with shielded cables and the torch lead was going to be hanging anyway.
ill also earth the floating head back to the gantry and the gantry back to the frame.
Having been informed about the risks of interference from a plasma cutter affecting the Pc controlling it, I am doing some work prior to the cutter arriving next week. There is an excellent thread here as well..General CNC Plasma And Oxy Fuel Cutting Machines > Grounding CNC Plasma Question
So far, I have fitted screening braid to the stepper motor leads up to the point where they connect to the (now) shielded 4-core cables, this shield is connected at one end only to machine ground. The motor cable shields are connected at one end only, inside the steel control cabinet directly to my cabinet star earth stud.
I have connected the Z-axis to the gantry carriage at the steel terminal box, this point is the run back to the cabinet star point with its own ground cable. Next I have connected the gantry back to the star point with it's own ground cable. The steel cabinet is bolted directly to the chassis so I can't see much point in connecting another ground lead there??
The travel limit, e-stop and probing switch circuits are all relay isolated from the BOB and run on a separate 24v PSU, cables are just standard 2-core flex.
The e-stop relay also kills the torch control signal in case the board locks up with the torch on.
I have fitted several ferrite clamp-ons to supply cables etc.
The case of the PC is connected back to the control star point with a ground lead.
The keyboard and mouse have ferrite clamp-ons fitted at the PC end.
Now for the unknown bits....
The case of the plasma PSU has a bolt lug for grounding, I intend to connect this to a ground spike, the chassis of the machine also, as well as the slat bed with its own ground lead. Is this a good idea or should I just connect the slat bed ??
The torch lead will be suspended from above and not run in the cable-track.
How long can the ground leads be to the earth spike??
Where the machine will likely be going is about 15' from the ground outside, I could drill through the concrete floor but I have no idea how far down actual 'soil' is underneath the shop.
I could possibly drill through the wall and sink the spike there, that would be about 4' away.
Is it best to remove the ground clamp from the plasma and bolt the lead directly to the slat bed or just clamp it on??
Just an update,
in the end I could not get the machine to work with the HF, total nightmare and spent weeks trying.
in the end I gave up and bought a hypertherm powermax 30xp cutter, since bolting that on, it has worked perfectly, I now have the plasma unit in the same socket as the PC and CNC unit, plus I have routed the torch cable along the gantry with the other cables so it's much tidier and it still works.
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