If not, the idea was mine....

Having seen the carnage caused by the failure of a Z-probing routine for tool height a couple of times now, I was thinking if there was a probe plate on the market that was spring loaded - heavy springs will be needed, had the top surface as the normal probe plate and then as the springs compressed due to a failure of some sort, the extra movement triggered an e-stop?

Having heavy springs will make it appear as a solid to the probe routine, but still be able to compress and hit the e-stop.

Just wondering 

A few years back I designed a low profile tool setter which was intended as an open source project for all. There must have been something there that I had not realized because I was approached with a request to sell the design. Well I sold it (so cannot disclose any further details than have already been posted) but, to this day, I have never seen it on sale.
The related thread is here http://www.machsupport.com/forum/index.php/topic,21525.0.html and post #54 shows the overrun ring which triggered the EStop in case of emergency.
I use the prototype many times a day and it has never overrun so I consider the G31 to be 99.9999% reliable. Just why it behaves for some and not for others is probably one of those great mysteries that may never be solved.

If I built one, then there has got to be a fail-safe probe plate available on the market somewhere.  

Tweakie.

Hmm, seems they shelved it - i cant find it if it is out there

Some use a small cylinder with a couple of prox switches and a plate mounted to the end of the shaft. 1st switch seen as probe input, 2nd switch seen as estop. One end mounts to table/bed so that shaft is pushing in direction of spindle. Something like these. The right single acting cylinder (spring extend) may not even require air to work for this.

https://www.automationdirect.com/adc/Shopping/Catalog/Pneumatic_Components/Pneumatic_Air_Cylinders/Metric_Compact_Air_Cylinders_(H-Series)/H12M020MD-M

https://www.automationdirect.com/adc/Shopping/Catalog/Pneumatic_Components/Pneumatic_Cylinder_Position_Switches/4mm_Square_T-Slot_Cylinder_Switches

Thats quite a neat idea, i have no idea how accurate the trigger repeatability is for the proxy though - may not be good enough. I have a load of that stuff here so might be easy to test.

Decent switches will have extremely accurate repeatability. The rate the probe is moving at each time the switch is being made is likely the most important variable (slower = higher accuracy).

I have been doing a lot of probing on the lathe using G31. Have yet to have a problem and have tested it over and over for touching off tools, finding points etc.
BUT
In my probing for the lathe screen I manually jog to say about .1" of the surface and then probe at a SLOW federate. Repeatability is +- 0.0002" ( frankly I find it faster than the usual method of a double probe ......fast probe rate and then a slow rate ). At the slow probe rate. the amount of probe overtravel is less than most could even measure
( probe overtravel is calculable ).

To each their owne ..........

RICH

Solid advise Rich. 

Yes a lot do use it and one of the fails i had was my fault although due to an unexpected issue - i was probing with a tool length offset applied and the routine did not like it so plunged the head into the bed at rapid rate

The other time was an engraving tool with a 0.2mm tip - might have been dirt or something but it drove that tool into the aluminium bed nicely

Both would have been prevented with a "safety" touch-plate so it does seem a good idea.

I have no idea how it works on a lathe though - do you have insulated tooling or still use a plate somehow or are you talking about an electronic probe itself???

what code are you using there is one out there that does crash if the home and work is under 25mm diffrences