On one of the brand-specific sites, a gentleman has been trying to design a suitable "electronic edge finder"....
For whatever reason, he wants to electrically determine when the tool has just touched the work. This obviously can be useful sticking a boring bar down in a hole, etc., particularly if you don't want to remove any material, but just detect contact. The spindle would not be turning.
His solution is a somewhat elaborate circuit which passes a rather large current (1 amp) through the tool, toolpost, carriage, ways, headstock, bearings, spindle, and work, and he is attempting to detect the very small change in voltage when the tool touches the work, and thus adds a slightly lower resistance in parallel with the lathe body.
While attempting to assist him, I came up with an interesting idea, which I have now tested successfully. This started from the idea that 1 amp is rather a lot to pass through ball bearings on a regular basis.... and with the very low resistance of the lathe body, 1A is about the minimum.
The gentleman in question flatly rejected my suggested idea as not likely to work, and my email is having one of its regular hissy fits and is messed up right now, so I thought I'd post it here instead. (I'll have to see if my webmail is still working, it usually is)
I got to thinking and did my usual "turn the problem on its head" re-arrangement of ideas.
What I came up with was to treat the very low resistance path around the tool , lathe body, spindle and work as a single turn secondary on a transformer. Instead of being the main problem, it becomes an essential part of the system.
The idea is to induce a small voltage in that single turn, which can be detected (I used 10 millivolts) and then detect the disappearance of the voltage when the turn is shorted by the contact of tool and workpiece.
it works very very well. The test setup I used obviously could be transformed into a more compact unit, probably battery powered.
And I found that the net current in the shorted condition was small enough that a regular current probe didn't find it. I will need to use a better measuring tool to figure out what it is, but I expect it is quite small. Clearly much less than the resistance detector needs.
What I did for the test was to put a small toroid transformer over a "simulated workpiece", and attach the voltmeter clips to the work and the toolpost.
I adjusted the input to the toroid with a variac, to get 10 mV.
With the voltmeter it was easy to see the abrupt drop to zero as the tool touched the work. It might also be possible to see a small variation as the tool just brushes the work, but I have not tried that yet
The system could be packaged as a pretty compact deal, two probes and a small toroid to hang over the cutting tool, perhaps....
My crazy lashup for testing.... with current probe and second meter to measure the induced current for dangerous levels.
View of the tool and work.
The meters in place, just Fluke 75 or 77 meters, nothing fancy