Machsupport Forum

Mach Discussion => General Mach Discussion => Topic started by: fastimes on February 07, 2007, 09:35:38 PM

Title: thc help
Post by: fastimes on February 07, 2007, 09:35:38 PM
we are running mach3 1.84 version on a plasma/router table using a mp1000 thc.  i understand a little of the thc function on thin metal, but if my feedrate in my plasma manufactures chart says a specific height with a certain amperage, tip volts, speed, and pierce height, will cut x inch metal, then why should i run thc if i could just create the right height in my gcode.  i wouldn't think anything over 3/16 wouldn't move that much.  with that being said, can i turn thc off and still run parts.  it seems when we do that our torch will starting moving before it fires.  also, what exactly does the thc dro mean.  i see it moving up or down in numbers but what is it doing for the torch.  table is grounded with ground rod also.  also would it be recommended for us to download the newest version of mach3.  i read something on here with someone saying to change from 45khz to 25khz with a similar prob.  is this the kernel speed.  if so ours is at 35khz.  would this help.  hope this isn't another one of these "we've already answered this one" topics.  please help.
Title: Re: thc help
Post by: Brian Barker on February 08, 2007, 09:37:03 AM
I have a THC on the Plasma that I run and you are right! I never use it on the thick stuff. I built my own THC and added code  so I could turn it off and run the head with an Up/Dn button. You will need to contact the one that made the unit that you are running (sorry I have never use that one) and see how to trun it off. Also ask them if there is a way that you can move the Z during the cut :)

your should be okay at 35 KHz, you just need to set the THE corection speed DRO to a lower value. Hope that helps
Title: Re: thc help
Post by: fastimes on February 08, 2007, 04:09:37 PM
what about our version of mach.  should we update to the new one or stay with what we have.  thanks for the info.
Title: Re: thc help
Post by: Brian Barker on February 11, 2007, 09:11:01 AM
I would update the software :)
Title: Re: thc help
Post by: xjdubber on February 11, 2007, 08:39:35 PM
I would still use the thc on 3/16 it will still warp up and down a bit. Getting the best cut quality out of a plasma is dependent on speed, matching your amperage to the material right (use an amperage rating that list the metal your cutting as a mid range) most important is your torch to work distance. If you want to achieve a straight cut (which you really cant) you have to have your torch to work distance as close as possible and consistant. The flame of a plasma torch is like the shape of the flame of a lighter this means that if you have the flame in the right position you will create a concave shape cut, which will be the straightest you can get.

Does anyone here have a plasma that will touch of on the material and set its right hight? Id be intrested to see how you did it!
Title: Re: thc help
Post by: Jeff_Birt on March 04, 2007, 10:35:14 AM
The touch off control works through the plasma power supply.  The power supply applies a small voltage to the tip and the CNC control drives the torch down slowly until contact is made between the plate and torch.  The power supply senses the current flow (voltage drop) when contact is made and sends an output to the control.  Well, now it occurs to me that the above is how it used to work on ThermalDynamics power supplies I think C&G may have had this built into their THC and used the arc voltage lead as the sense lead for touch off.  I may still have some schematics around here, somewhere.

At any rate, touch off can have its problems, if electrical contact is not made the Z-Axis can keep driving down until the torch or lift station break.  This used to be a problem on C&G lift stations but I have not worked on them for 7-8 years so they could have changed them.
Title: Re: thc help
Post by: xjdubber on March 04, 2007, 02:27:22 PM
Havent seen any touch off's work that way. I think you might confuse pilot arc with touch off. Ohmic sensing is usually done trough the cnc control and really has nothing to do with the plasma unit. And many commercial cnc plasma units have a secondary switch as a fail safe, or for cutting brushed stainless that has a plastic layer on it.
Title: Re: thc help
Post by: Jeff_Birt on March 04, 2007, 08:06:53 PM
Well, yes I am both a bit rusty and confused.  The touch off is a function of the THC working back through the voltage sensing lead.  The power supply would supply an arc established line (current sensing) and the voltage sensing lead to the THC.  And now that you mention it, xjdubber, I seem to remember that at least one lift station had a sort of release on the downward torch travel that would not allow the torch body to be driven into the work (and it also relased a micro switch as a signal to the control). 
Title: Re: thc help
Post by: xjdubber on March 04, 2007, 10:08:36 PM
Actually the arc voltage is only being used to determine height when cutting and is just the voltage the plasma unit is cutting on. the first "touch off" on the metal is a function usualy of the cnc controller.
Title: Re: thc help
Post by: Jeff_Birt on March 05, 2007, 12:47:30 PM
I guess you and I have experiance on diffrent equipment.  I just checked the maintance manual on C&G's (with which I am most familiar) website and it does appear that all functions of the lift station are controlled by their THC.  I don't want to keep beating a dead horse.  Thanks for setting me striaght that touch off is not a function of the plasma power supply.