PTL negative error

[woffler]

I keep getting this PTL measurment is negative error and then it sends mach into a e stop .

Can some one tell me what this is?

Using current mach lock down version current MSM version ,In Master tool mode with probe tool probe configurred .

[DaveCVI]

Hi,

When MSM Measures a tool, it cant' measure a TLO (as an "offset" is not a physical value), it has to measure the Physical Tool Length (PTL) and then calculate the TLO value (which is mode dependent) from the PTL value.

The PTL calculation that MSM does always uses machine coordinates:

PTL = Z level where the probe event or touch occurs
- stored Z Top of the TCP touch plate
- effective probe radius.

At each PTL measurement, MSM sanity checks the result - and it faults if the PTL < 0 - as clearly a tool can not really have a negative physical length.

So when this happens, the question becomes what could cause it?

Here are some things to check:

1) Are you measuring the probe tool's PTL? if so, check the tool table... is the diameter of the probe tool tip set correctly? Since a probe tool has a spherical end, the diameter is also the tip radius value. an extreme example for illustration a probe tip value of 10" will result in a bad negative PTL value when the Tip radius amount is subtracted out....

non-probe tools use a forced radius value = 0 in the calculation as a cylindrical cutting tool is treated as a 0 radius spherical end.

1a) is the probe tool a real probe tool? I.e. for this discussion, it has a spherical tip?
(There was one fellow that tried to use a piece of bar stock as the probe tool and that violates the assumptions MSM makes about the "probe tool" - to MSM all probe tools have a spherical tip.)


2) has the Z location of the TCP TP moved physically?
If the stored value for the TCP MCz is not the actual physical Z value of the top of the plate, there will be an error and it could result in the PTL negative test tripping.

3) Has the Z home switches moved or become non-repeatable?
The Z home switch anchors the machine coordinate grid in physical space. If the home switch is flaky, and not triggering at the same spot each time the machine is homed, this will show up as variation in the PTL value (just as if the TCP TP were moving up and down a little bit, or as if the stored TCP TP MCz was altering by a little bit each time the machine is homed).

If these don't point you to the issue, I can tell you how to get a debug log from MSM which will give us a trace of the exact sequence of actions during the measurement - then we can evaluate the specific numbers in the calculation when you do the measurement.

Dave

[woffler]

Thanks for getting back to me Dave ,i will check all these out as soon as i can .
I will be gone Wendsday but will get on it first thing Thursday morning and report back .

[woffler]

Hi dave , i just got back and ran some test's everything was fine but for some reason the gauge line and touch plate height where off.
I suspect that the home switch moved maybe i bumped it or something or it could be a proximity switch going bad on me .
I tested the repeatability of the switch it is fine for now if it happens again i will replace the switch!

Thanks for the help and the fine product

[DaveCVI]

yep, makes sense - the stored TC TP location is relative the the home sw location as of when the value was stored, so if the home switch got bumped, the stored value would then be off.

It actually would have been a worse problem if the switch got moved the other way as the PTL would have gone overly positive instead of negative - but MSM can't detect that condition. The too large PTL would shift all the TLO values and you can go nuts trying to figure out why all the part surfaces were shifted in Z....

Glad to know it's straightened out for you.

Dave