Some MSM manual references:
Edge width: 7.1.3.3.5 & 7.1.5.11 & 7.1.6
Calibration diameter: 7.1.3.1.2 & 7.1.4.10
I just added the following info to the How to post:
[Info added 8-14-2010]
The basic mach probing sequence:
1) G31 starts, mach starts moving machine
2) Probe input triggers; mach records the position of the probe when it was triggered
3) Mach decelerates the probe and movement stops. Note that the stopping position is not the trigger position. The difference is “overrun”. A bit of Overrun is expected and good – as it keeps the probe in the triggered state.
4) Mach returns from internal G31 execution to program that issued the G31 (for the MSM probing operations, this will be a return to the code in the MSM probing library)
5) Script looks at probe signal –
a. If probe is not active, we ran out of distance w/o triggering the probe
b. If probe is active, we found something and were triggered.
6) Probing Lib gets the trigger position and does what it needs (For example sets axis zero to the face we found).
7) Library backs off by the overrun amount (this lets the probe become un-triggered)
08) Library back off the user defined clearance amount.
Noise on the probe input line will make this sequence not work correctly –
Example: A short noise pulse at step 2 can stop the G31 motion; but at step 5 if probe will not be in the triggered state when checked. MSM thinks nothing was found and so puts the probe back to the starting position.
Adding some Debounce may help:
Mach's debounce is really a software fix for a hardware problem.
The ideal solution is to remove the noise from the system and use a debounce of 0.
Many people either can't or don't want to expend the effort required to accomplish that.
The pragmatic approach is to lower the debug number until the problem appears, then raise it back up a little.
That leaves the issue of "How do I tell if my required debounce setting is too high"?
"Debouce" is the amount of time a signal must be continually "on" before mach will consider it as just having turned on.
Each number in the debounce DRO represents about 40usec of time (25kHz Mach: 1/25KHz = 40uSec)
A debounce of 40uSec* 4000 = 160mSec. That is a long time for a computer, it's even pretty noticeable to a human.
At 4000, the probe signal has to be on for a minimum of 160ms before mach will see it and stop the probe motion.
How fast are you probing? Let's just pick a number for illustration:
Say you are probing at 50ipm; during the debounce interval the probe will have moved (50ipm * 160msec ) / 60 sec/min = 0.133 inches....
So the probe will travel 0.133 in past where the edge is before mach will think the probe is triggered - and then it has to decelerate and stop.....
Consider - how much tip travel does the probe have? 0.133 inches over travel could be hard on the probe - depends on the probe design.
Same scenario with 200 debounce: 50ipm * (200*40usec) / 60 sec/min = 0.0067 inch over travel
Personally, I prefer to see a debounce of no more than a few hundred. but the real answer is whatever combination works that makes the machine run reliably.
Play with the numbers and decide what you are comfortable with.
[end added info 8-14-2010]
Dave