Touch sensor for lathe and mill setup

 Some time ago I learned about a machining web site created by Rick Sparber:  right here.  He has a number of interesting articles about DIY machine accessories and improvements, but one in particular caught my attention -- a simple touch sensor that can be used to set up a metal lathe or mill for machining metal.  

The design can detect a small change in already-low resistance.  The basic idea is to measure the resistance between the cutting tool and workpiece being machined.  When the cutting tool is NOT in contact with the work, the current path is through the machine -- the spindle bearings being the major source of resistance compared to the body of the machine.  When the tool comes in contact with the work, that is a lower-resistance path -- and that is the basis of the touch detector.

 It looked pretty good, but being an electronics kind of fellow, I thought there might be some room for improvement.  The idea was to change the design to allow 4-terminal or Kelvin sensing.  Rick's design uses the same wires to force current through the lathe/mill AND sense the voltage change when the tool touches the workpiece.  This means that the design is sensitive to contact resistance at the tool and workholder ends.  The 4-terminal approach avoids this problem by separating the force and sense connections.  A good article regarding Kelvin sensing can be found here .  I hasten to add that Rick has some more-refined designs that DO implement 4-terminal sensing, and interested readers should look into them, particularly if wanting a good milliohmmeter or CNC-compatible touch sensor.  But for various reasons I think my design is a worthy alternative to his simplest design, and I offer it here.

My design schematic:

The design is quite similar to Rick's, including the automatic power-up scheme implemented by Q1.  The major difference is how the inputs to U1-1 and U1-2 are connected.  They are routed to separate sense lines (although my design does permit simpler 2-wire use if that works OK on a particular machine).  Rick's original design used a couple of spade connectors and super-magnets to attach the touch sensor to the cutting tool and workholder, but that's not compatible with the 4-terminal approach.  One of his other touch sensor designs uses two miniature battery charger clips with the sense connections brought in via an insulated contact, and that's what I'm using with my design.  I drilled a hole in the jaw of each clip large enough to accommodate a 1/4" nylon screw, then chucked the screw in my lathe and drilled and tapped a #4-40 hole down the center of it.  A #4-40 brass screw was threaded into the nylon screw, then the nylon screw was attached to the jaw with a nylon nut.  The outside end of the brass screw has a nut to attach a spade connector for the sense line.  If it's not clear from my explanation, the brass screw head forms a Sense contact.  Experimenting with different-sized end mills suggested that it would work better if I added a washer underneath the brass screw, so I also did that.

I haven't had a chance to debug the design yet, but once that's done my plan is to make it open source.  More on that later....