G38.x and acceleration (not stopping)

linuxcnc.org/docs/2.6/html/gcode/gcode.html#sec:G38-probe

I wanted to use G38 to touch the top of the pcb (copper surface) with a drill while just connecting drill and pcb as ends of the probe but when I tried it out (of course without a drill, manually connecting ends of the probe wires) I see that when probe "connects" linuxcnc initiate the stop but the acceleration/deceleration settings prevent the head to "stop" but it moves further until it decelerate to a standstil. No idea how to solve this, however "slow" I setup the feed it still moves "past the probe point". .. my Z acceleration is rather low (1mm/s/s) and I'd like to keep it that way but I'd like linuxcnc to stop "immediately" when probe touches... any way to do it?

linuxcnc.org/docs/2.4/html/gcode_main.ht...38.2:-Straight-Probe

The move stops (within machine acceleration limits) when the programmed point is reached, or when the requested change in the probe input takes place.

It seems you are asking linuxcnc to stop faster than the physical limits of the machine. A slow probe speed should help and you could possibly probe, stop, pause, reverse and use the point where contact is lost to set the height.

Rick G

Hi,
Yes, exactly what I want to stop "faster then set limit" ... is that possible?
If not, any idea how to calculate "stopping length"? So if I have Fx (feed rate of x mm/min), max acceleration of Y mm/sec/sec, what will be the stopping distance?
Will the simple
S = 1/2 a*t*t where t = F/a
(10mm/sec) / (1mm/sec/sec) = 10sec
0.5*1mm/sec/sec*100sec*sec = 50mm
work?
If so I might be able to get a speed that's slow enough but still .. it will always go past the mark, for XY this is ok as I can always design the touch probe to have some space to follow trough, I will lose some mm's on the travel but I will be able to make precision positioning, but for Z I can't do this, I want to touch the copper plate with the drill, it's 0.25mm drill bit, if I plunge it into copper it will break so I need to stop "immediately" ... any ideas how to solve this? if I set acceleration to super high value and set feed to a slow value this works it stops immediately but I can't have machine then work that slow nor have acceleration that high (stopping is not a problem, starting is) so either is there a way to make starting and stopping acceleration value different or is there a way to override acceleration settings for the axes from g-code? then I can set acceleration to infinite, feed to slow, touch off with G38 and then restore original acceleration settings?

Thanks!!

thing is I have a puny machine, simple ER11 chuck, so I need to rehome the Z every time I change a tool, and what I want to do with it is often drilling pcb's ... now for drilling I can even bring the drill to hover above the copper and that's good enough, I still drill more then 1mm deeper then I need to so that's ok, problem is with milling pcb's there I really need to touch it, doing it manually is possible (attm I use a simple buzzer) but would be cool to automate it

Might want to look here...

softsolder.com/2010/04/14/emc2-ugliest-t...-probe-station-ever/

Rick G

Thanks, this shows me how to deal with different length drill bits, great, as for the probe, he plunges his tool into the button quickly and then retracts and go slowly .. if I plunge into anything 0.25mm drill bit will disintegrate ... it does give me an idea.. is there a way to have 2 different probes in the same time? I see only one assignment for the "probe-in" - is there a way to have 2 probes? so I go down fast till I hit probe1 and then stop and go further more crawling till I hit probe2 ? ...

Suppose you had a switch that triggers at exactly .5"
You could set the switch on top of your workpiece and probe with the drill. Down at med speed, back up then slow probe down. (the switch has some overt travel to protect the drill)
The point that the switch triggers would then be .5" above the work. So you have found the top of the work without gouging the work piece.

Rick G

so I make the optical barrier with a hole, let dril go in, measure the distance between the "breakage and work" use that as offset.. should work, not nearly as simple but should work.. thanks!

btw just to confirm, me not finding a way to "override" acceleration for axis is correct, there's no way to do it with linuxcnc?

I ask because in "my cnc world" (3d printing) we use M201 (max acceleration during "work move"), M202 (max acceleration during "idle move"), M204 (default acceleration), GRBL also support "$120, $121, $122" for setting acceleration, never used it so not sure how that works and is is run-time or only configuration..

btw just to confirm, me not finding a way to "override" acceleration for axis is correct, there's no way to do it with linuxcnc?

It has come up before, but i know of no easy solution. I believe the best bet is to set up the machine for the best reliable acceleration and control the probe speed with the F command. The reason for a fast probe then a slow probe is to reduce the time of the probe if it has a long distance to go. If you get the probe close 1st then a slow probe which will stop faster than a fast probe is fine.

Rick G