My probing routine errors out before reaching destination

I'm using G38.2 and moving to an uncompensated Z position of 20 mm, which should be just above my toolsetter. But if I start the move up high (e.g. 150 mm), it will lower the spindle toward Z20, but stop after less than 20 mm of travel, indicating that the “move finished without making contact.”

The docs say “If the probe is not tripped before the destination is reached G38.2 and G38.4 will signal an error.” But it’s not reaching the destination. G38.3 does the same thing, but without the error.

You would normally specify a destination distance. eg: 

In this case if it gets to Z -50, and its not been tripped, it will error.

rodw wrote:

You would normally specify a destination distance. eg: 

G38.2 Z -50

In this case if it gets to Z -50, and its not been tripped, it will error.

 

The docs say “The axis words together define the destination point that the probe will move towards.” and that’s the behavior I want. I set absolute mode (G90), cancel tool length compensation (G49) and do a slow move toward the lowest reasonable spindle position. But it’s not doing that. It’s not doing a relative move, either, as I’m specifying Z20, but it’s moving downwards (negative Z), and it’s not moving 20 mm, it’s moving more like 18.

Its too long since I played with probing to remember as QTplasmac does it internally.
I'm pretty sure its always been relative moves.
The Tormach docs state no negative moves are used which probably answers your question
tormach.com/straight-probe-g38-x
I'm guessing only the absolute value of the relative probe move is used.
 

How are you issuing the gcode command? MDI, Ngc script, python code?

I've attached the program. I issue `O<pz> call` in MDI mode.