using a vibration / noise sensor as edge finder in a CNC machine
If you're familiar with Arduino you could utilize a PVDF piezoelectric sensor strip to pickup the vibrational patterns of your system.  You can find them here:  https://www.digikey.com/product-detail/en/te-connectivity-measurement-specialties/1002794/MSP1006-ND/279646

Wire the PVDF strip into the analog in port of the Arduino (one side into A0, one side into ground).  Then you could record a normal vibration spectrum when it's not being contacted as well as a vibration spectrum when you know it's being contacted using the int value = analogRead(A0); function and do a comparison.

That being said, if you're getting vibration from the bit touching a surface, you're also getting heat / rubbing / cutting / other bad things.  So, my advice would be 2 part:
 1.  Don't reinvent the wheel, get an edge finder.
 2.  If you're going to reinvent the wheel, do it well so I can buy a PVDF based edge finder and don't have to remove my bit each time.  I expect to see your new edge finding invention listed on the web somewhere for less than 30$. :)  Go to town!