An interesting tool idea, only partially mine...

[J Tiers]

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

[armedandsafe]

If you are just detecting touch and the machine isn't turning, why not just clamp one connector to the work and the other to the tool and close the gap until continuity is achieved? No need to drive current through the entire machine frame.

An LED lamp in series with the battery and the leads/clamps. If the workpiece is too large to clamp to, pick up one of the jaws.

Pops

Edit: I see from a second look at your setup that you are basically doing that, so I don't see the need for the induced current.

[darryl]

One to the work and one to the tool- would make sense except the path is already more or less a short. Tool, holder, top slide, cross slide, carriage, bed, headstock, bearings, spindle, chuck, work piece. Nowhere in that path is the current-conducting path broken. That's where the problem lies. The solution as JT has suggested is to make use of that current path- when the tool touches the work, the loop is completed- in other words you go from an open loop to a shorted loop, and that's what's being detected.

I would have thought to detect it by sensing the change in inductance in the torroid itself, but instead the voltage on the ends of the loop is being measured, and when that voltage drops it's an indication that the tool has touched the work piece.

[oldtiffie]

Similar principle to this perhaps?

https://www.machineryhouse.com.au/M690

[John Stevenson]

Similar principle to this perhaps?

https://www.machineryhouse.com.au/M690

No. all those edge finders require some part of the holder to be insulated.
They just work on a battery and bulb system where the switch is a wire or in this case part of the holder touching to work.

In Jerry's case he's working on a system that is already a dead short in that the tool is always connected to the work via the metallic path through the top slide, carriage, bed, headstock etc.

Interesting concept.

Would not work on a Bridgeport due to the many layers of licorice forming an insulator.

[J Tiers]

Similar principle to this perhaps?

The idea is not just to find the edge, any old way, but to detect when the actual tool that is in use touches the work so as to set a zero for further work. The internal groove cutting tool touching the bottom of the existing internal groove, perhaps.... in the case where you want to take very little off, but really need to take off that very little. Prevents "instant" overshooting of the depth that might occur in the process of just touching off if trying to find the surface with the machine turning.

Those LED edge finders obviously won't work on an internal groove, and they require very good pre-measurement of the cutter diameter and runout if you want to use them to close tolerance. Otherwise you know where you are with the detector, but not with the actual tool.

And, with the induction unit, one does not have to do any tool changing, just use the tool you already have, so the amount of trouble needed to do multiple parts in a row, or multiple surfaces on a single part, is minimized.

I would have thought to detect it by sensing the change in inductance in the torroid itself, but instead the voltage on the ends of the loop is being measured, and when that voltage drops it's an indication that the tool has touched the work piece.

One could detect the change in "Q" of a tuned circuit including the toroid, or the increase in current as the secondary was shorted.

I also thought of using teh tool/machine/work loop as a tuned circuit and detecting the change that way, but a little calculation indicated that the inductance probably was rather small, and this simpler version came to mind as an obvious way to create an easily detectable "signal" with minimal hardware and hassle..

[hornluv]

In addition, if the purpose of this is to find where a boring bar touches inside a hole, using an electronic edge finder isn't going to tell you where that tool tip is.

edit: J Tiers beat me to it.

[SmoggyTurnip]

I love this idea! Maybe it would work better if you used a high frequency AC current instead of DC.

[SmoggyTurnip]

Sorry I misread - you are using AC - great idea.

[flylo]

Probably a stupid question but why wouldn't a simple ohm meter with 2 magnets work?
Sorry for some reason I thout it was insulated until it touched the edge.