Plasma Questions

Rick G

I checked the voltages a three different cutting heights .06", .07", .08". I did 6 tests at each height using the following MDI program:

G91
G01 Y8.0 F220
G90

I used a piece of fairly clean 14ga scrap as a test piece. The consumables (electrode, nozzle, shield) were all cleaned with a Scotch bright pad before starting. I estimate the electrode to be about at 50% of life. As usual for straight cuts the quality was all excellent. Light dross on first .375" of cut at most. Work clamp connection cleaned to bright finish before connection.

Now remember this is using the old Torchmate AVHC to read the arc voltage and control the torch height, a precision piece of equipment this isn't. So what happened.

.06" 75V
.07" 80V
.08" 86V

11V / .02" = 550V/inch. * .005 = 2.75V for a .005" change in cut height. So the slope of the arc voltage curve at the working height is about 1/2 what I previously estimated. Now this is not good news because this means we are working over a smaller span with a very noisy input signal. I watched the arc voltage with my old analog o'scope when setting up the voltage divider/protection circuit to use with the THCad board and was amazed at how noisy it was.

John

John,

If you go back to this link I posted earlier in this topic, you will find Rick G's motivation for his experimental THC.

forum.linuxcnc.org/38-general-linuxcnc-q...tions?start=10#84529

The limitation in cutting corrugated iron is how fast the Zaxis can move. If the material height is changing 40mm every 40mm in travel laterally, than the xy speeds must be low enough to allow the Z to keep up.

Its good you made progress. I've seen it suggested to run the THC output though a filter in HAL to help clean up noise.

Also PCW recommensd that the /32 option is selected on the THCAD board.

rodw wrote: Your welcome. I've tried to avoid it too becasue I have a background in the printing industry where postscript rules supreme but we have smart software to do the dirty work whereas with HTML you eventually have to drop to source code.

(Off Topic)

A friend of mine writes raw PostScript in a text editor any time he wants a diagram. I think he also codes loops and conditionals.
Clearly, though, he is a lunatic. Writing raw SVG is so much easier.

Hello

I dumped the probing code out of the Animatics Smart Motor on my present Z axis and cobbled together this pseudo code for probing material top surface with a conductive probe. I am looking for suggestions on the best way to do this. Should I use an O code subroutine or HAL or HAL user Python code?

Things that I don't how to do for a more or less direct conversion of code are:
1. Make a velocity commanded move on an axis. Can this be done from HAL, G code or Python?
2. Call a specific piece of HAL code from inside G code program.
3. Call a specific piece of Python user code from inside G code program.

Pointers to examples of doing the above will be very helpful.

John

I can't really answer your questions but I've reviewed your code.

Float Switch as a Limit Switch
I think there is an error in logic treating the float switch as a limit switch. If there is no material under the torch, nothing will be triggered and the axis will crash. You really need a separate limit switch for the lower end of travel. A lot of examples X axis mechanisms I looked at did not have a lower limit switch but I added one to my machine. (Due to space constraints, I used a mechanical micro switch instead of a proximity switch) I figured it would have a low duty cycle so should never wear out.

If your float switch and upper limit switch share the same output, there is no excuse for this when you are using a Mesa card which has inputs to burn.

Float Switch as Back up if Ohmic Circuit Fails
When probing, the float switch should be treated as a valid input as it should serve as a backup if the ohmic sensing circuit fails so it is definitely not an error if triggered while probing.

Gotos are considered bad form
I personally don't like using goto statements but they can be efficient for error handling but nested if statements also do the job without adding excessive code. I hope I've cleaned them out correctly.

Implement timeouts using current time
With time outs, somewhere you have to read the current time as you enter a routine for the first time and then check if the time exceeds this time plus desired delay. Normally in C, I would use a static variable that holds its value between calls. IN HAL, it seems global variables are used, so I've added two variables. One to hold the time and the other a flag that is true on the first call to the procedure. I did this as I treated the code as if it was an interrupt service routine in case this procedure actually gets called multiple times during execution from the servo thread. Don't forget to reset the flag on exit from the procedure.

So with those comments, here is what I came up with.

I also forgot to mention that I altered your do... while loops to a simple while statement. I figured it would be more efficient to check all the conditions before wasting time moving anything.