Works when doing it manually- but how well does that work when nobody is home?

The reason stock size matters a lot is because the feed rate prior to starting to cut is much faster than the feed rate can be once it engages the stock it is cutting. You can approacjh at 2 inches per second but you can't cut steel that fast..

 

But you don't always know precisely where the edge of the material is, this is what these touch sensors do.
Max.

We used a standard edge finder, the type that slides sideways when the edge is picked up. like this -


And only when doing some kind of rework on an existing part. When starting on a new part stock was left over size and cut on all edges to a finished size, the vises were a known position on the table surface.

 

Yes they are cheap and work pretty good, the industrial probe units such as Renishaw etc, have software that read & automatically register the 3 axis work positions into the system for machining.
Max..

 

You can approacjh at 2 inches per second but you can't cut steel that fast..

Why do you think that? 2 inches per second is only 10 SFM a very doable light cut on a real CNC machine.
https://www.google.com/search?clien...&ved=0ahUKEwiOrY2N7uHtAhWSxFkKHTXmBCYQ4dUDCAw

 

Yes they are cheap and work pretty good, the industrial probe units such as Renishaw etc, have software that read & automatically register the 3 axis work positions into the system for machining.
Max..

We used the touch sensors more as a coordinate measuring thing instead of a edge finder.

 

The ones I am familiar with used them for integration into the program in order for operator free operation of the CNC machine.
In some cases a robot would load the part and start the CNC machining, which would probe and machine, then another robot unloaded the part and placed it on a pallet etc.
Max.

 

OK, SB, If you say so. All of my milling machine operation has been on manual machines, since I was doing the prototypes. How many actual EEs can set up and run a mill and produce accurate prototype pieces?? And at all times the feed rate had a big effect on the surface finish. Thus my comment about feed rates.
Your experience evidently has been different.

 

@MaxHeadRoom - I need to be able to detect when the tool in the spindle makes contact with certain features in the machine to complete the circuit. Similar to probing in a CNC machine.
Thanks for the suggestions so far. This will keep me busy for a couple of days trying out a few things.

Using a tool in the spindle is not a very satisfactory way of probing.
What are the 'features' referred to? are they metalic?
What method did you intend in order to arrive at a position?
Could you use the simple touch off sensors used by the DIY CNC crowd?
A little more about the intended purpose and end result is really needed to give any definitive answer.
As mentioned before, the motor frame is not a good idea for the circuit path IMO.
Max.

 

In some cases a robot would load the part and start the CNC machining, which would probe and machine,

Again, many different ways of doing things. The CNC's that they used for production type jobs, like you seem to be talking about, the job was only located on the very first part. All of the robot controlled loading and unloading had raw material loaded on what we called "pallets" that got located in a fixture clamped to the table.
Similar to but not the same brand as these - https://piersonworkholding.com/mini-pallet-system/ Can't remember the name of the ones they had.

 

All of my milling machine operation has been on manual machines,

A real dedicated CNC machining center is no way like a manual mill, other than it removes metal. Something like a Bridgeport can't even come close to taking the type of cut or the speed of cut that a CNC does.