GRBL as cnc plasma?

Discussion about the arduino based g-code interpreter, grbl
tm america
Posts: 84
Joined: Tue Feb 04, 2014 5:58 pm

Re: GRBL as cnc plasma?

Post by tm america » Tue Jul 08, 2014 12:30 am

on my other machines i use mach 3 as the driver software. there is a torch height controller . it reads the voltage between the torch tip and the piece of metal as it cuts.It then sends a signal back to the controller to one wire if the torch needs to go up or a different wire if it needs to go down. There are three different types of torch homing. First is ohmic which is a wire attached to the tip that senses ohms when the tip touches. Seccond is a touch probe the third is a floating head with micro switch type .the ohmic types are not reliable if the metal is dirty or rusted it can come down and not make continuity. seccond type with a touch probe is not good either as often times there is no metal where the probe would touch cuz that part has already been cut out. the third is ussually the best the torch comes down till it the tip touches the metal it continues down till the head pushes up opening the micro switch then is sends a signal to the controller from there the torch lifts to the proper pierce height..it then triggers the torch once the torch gets full arc it sends a signal to the controller telling it that it is ok to move . From this point the torch height controller starts to control the height..Mind you this is the simplified version. As it also has to be able to sense when it runs across a part that is already cut or the torch will dive and crash into the work piece. It also has to know when it is going into a tight turn as the machine slows down and the metal is already blow away and would cause a torch dive..Worn tips affect this to as the torch voltage goes up making the machine think it needs to lower the torch focing it to crash into the work.. I use both the floating head and omhic type on my machines at the same time as an extra safety.. if the floating head malfunctioned the ohmic tip would act as a end stop and shut things down

Nigel K Tolley
Posts: 226
Joined: Sat Feb 08, 2014 4:06 pm

Re: GRBL as cnc plasma?

Post by Nigel K Tolley » Sun Jul 27, 2014 9:21 am

But when I'm cutting free hand, none of this is an issue because the plasma torch auto adjusts the current. It has to, there's just no way a person could make a half decent cut otherwise.

I figure that will make the system more effective than a 'dumb' CNC system will be.

When hand cutting, you just hold the torch close and pull the trigger. Air turns on, arc fires & away you go. So I think decent results will be possible with just a simple wait command.

tm america
Posts: 84
Joined: Tue Feb 04, 2014 5:58 pm

Re: GRBL as cnc plasma?

Post by tm america » Mon Jul 28, 2014 6:31 pm

I have never seen a plasma cutter that auto adjusts the torch voltage..Some units auto select input voltage so you can run them on 110 or 220 volts without any changes.They have hi-def systems that are tied into the cnc system .They control the pilot arc voltage,cut height,air pressure,cut speed,and cut height,These are all done with the cnc controller ..If you start your torch at proper cut height with cnc plasma table the metal will blow back and clog the tip .Not to mention the other 1000 things that give you a not so nice cut ...When hand cutting you usually start the torch on an angle to start piercing the metal then turn the torch to the correct angle ..This keeps the metal from blowing back into the torch..But cnc plasma is a totally different story..Everything able cnc plasma cutting is different than hand cutting.The torches are different ,the consumables are different .The plasma cutter itself is different.. Most hand cutters use hi frequency to start the arc. On a cnc machine that would give you a horrible interference that would give you all kind of clitches..Cnc plasma cutters usually use A retractable electrode that is pushed away from the tip causing a small arc in the tip to start the arc..It works like a set of points in an old gas motor..This doesn't cause interference .The consumables are different in that hand cutting tips usually have a stand off built into them this allows the tip to be dragged on the cut material and still maintain proper cut height..On cnc cutters they can not have this as the cut height is around .0625 so they need to be very close to the material .If they have a stand off built in it would crash into the material and either drag the cut piece or blow slag back into the tip ..

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