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View Full Version : An interesting tool idea, only partially mine...



J Tiers
06-12-2012, 01:37 AM
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 :eek: 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.

http://img.photobucket.com/albums/0803/jstanley/electedgefinder1.jpg

View of the tool and work.

http://img.photobucket.com/albums/0803/jstanley/electedgefinder2.jpg

The meters in place, just Fluke 75 or 77 meters, nothing fancy

http://img.photobucket.com/albums/0803/jstanley/electedgefinder3.jpg

armedandsafe
06-12-2012, 02:32 AM
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
06-12-2012, 02:46 AM
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
06-12-2012, 04:32 AM
Similar principle to this perhaps?

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

John Stevenson
06-12-2012, 04:44 AM
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. :rolleyes:

J Tiers
06-12-2012, 09:19 AM
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
06-12-2012, 09:26 AM
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
06-12-2012, 11:05 AM
I love this idea! Maybe it would work better if you used a high frequency AC current instead of DC.

SmoggyTurnip
06-12-2012, 11:14 AM
Sorry I misread - you are using AC - great idea.

flylo
06-12-2012, 12:32 PM
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.

lwalker
06-12-2012, 12:42 PM
Would it be possible to keep the toroid transformer concept, but use DC. In that case, when the tool touches the work, the current through it will cause a pulse through the toroid that can be detected. The edge of the pulse will probably spike fairly high so that would probably allow you to use less current. A clamping circuit and a flip-flop should work to detect the edge of the pulse.

xs hedspace
06-12-2012, 04:12 PM
Did you try a simple ohm meter connected to the work, and the tool, and see if a jump in the resistance occurs, when the tool touches the work??

rmuell01
06-12-2012, 07:48 PM
no idea what I'm talking about:confused: ---- but how bout sonar?

A.K. Boomer
06-12-2012, 07:55 PM
Did you try a simple ohm meter connected to the work, and the tool, and see if a jump in the resistance occurs, when the tool touches the work??


If he didn't I did earlier -- I thought with all the surfaces covered in vactra and on and on I might have a shot at "change"

no go --- and did it on a mill and that even has the quill factor along with most all the others - that's not saying there's not a super sensitive ohms meter that might be able to detect something but mine would not on any level,

I was trying to think this morning if diodes could come into play somehow but again - no go...

I say just turn the music off - turn the machine on, get close with the tool and keep moving in and "listen" for the connection...

lane
06-12-2012, 10:11 PM
I always just used a black magic marker.When you scratch the color with out scratching the part you are as close as any machine is going to work .

J Tiers
06-12-2012, 10:47 PM
I always just used a black magic marker.When you scratch the color with out scratching the part you are as close as any machine is going to work .

That doesn't work too well down inside a bore, behind the edge of a groove..... but works fine on the OD......

kf2qd
06-12-2012, 10:59 PM
Need a high frequency, low power signal. it will be easier to detect.

J Tiers
06-13-2012, 01:09 AM
Did you try a simple ohm meter connected to the work, and the tool, and see if a jump in the resistance occurs, when the tool touches the work??

Didn't have to.... the gentleman who brought up the need to detect contact did try it, and has manufactured a fancy device to detect the drop from 10 milliohms to something less as the tool makes contact....

But that needs some fancy calibration. To avoid that, the original gentleman suggested insulating the tool, which naturally would work fine, but isn't always convenient.

The induced signal is pretty much a "property" of the electrical setup, the only thing that would obstruct it is a true open circuit, which seems to be rare on most machines.

It's hard to get simpler than hanging a toroid over the boring bar or the like..... little or no calibration required.


Need a high frequency, low power signal. it will be easier to detect.

Might be, might be........ 60 hz does pretty well, though.

The Artful Bodger
06-13-2012, 02:01 AM
A Wheatstone Bridge might be worth a look at if you are trying to detect a tiny change in resistance.

I think if you were measuring the resistance between the tool and the workpiece there would be something measurable when the tool is not touching which would drop to something less at the moment of contact.

Maybe I should do a check myself!:)


[2 minutes later] I did a quick check using a regular digital multimeter set on the lowest resistance range. One probe on a chuck jaw and the other on the tool post. It report 0.7 Ohms and when I moved the carriage so that the tool contacted the chuck it reported 0.3 Ohms.

Obviously the value of the readings are not important just the sudden change when the tool makes contact.

darryl
06-13-2012, 03:55 AM
The detection circuit, whatever mode it uses, will probably be sensitive to the changes in resistance through the parts of the lathe. I can see this happening anytime a slide is moved, which is what's going to happen as you bring the tool closer to the work piece. I think it might be prudent to rely on something other than the change in resistance as the tool makes contact.

Here's another idea- if one accepts the idea of clipping on some wire leads, then why not leave the torroid off the lathe entirely, but run one loop of wire through it and put alligator clips on it. With those attached to the work piece and the tool, the frequency could be varied such that the conduction path through the lathe parts doesn't load the torroid much. Shorting the clips would make the biggest difference to the current in the torroid since the shorted turn of copper wire would definitely reflect back as a short. That copper wire loop could be a heavy gauge, say #12 or so, and no longer than required to make the clip connections. It could also be more than one turn- that would be something else to experiment with.

Comes to mind you could make the torroid circuit self-oscillating, in which case there would be a very noticeable change in frequency when a turn is shorted. You would feed a speaker from a secondary winding and just listen for the change. This way you wouldn't be distracted by having to focus on a meter.

A benefit is that the torroid doesn't have to 'fit' anywhere. Now I'm down to putting the whole thing in a project box, complete with a speaker and battery, with just an on-off switch, possibly a single control to set the operating point, and the pair of wires coming out with clips on them. And now I can get rid of that word 'torroid', and instead call it a transformer core, which is what it is. It could be of any type, including torroidal. Could be an E core or whatever. It's just one component of the oscillator circuit, and it could actually be quite small.

The Artful Bodger
06-13-2012, 05:54 AM
The detection circuit, whatever mode it uses, will probably be sensitive to the changes in resistance through the parts of the lathe. I can see this happening anytime a slide is moved, which is what's going to happen as you bring the tool closer to the work piece. I think it might be prudent to rely on something other than the change in resistance as the tool makes contact.



My tests using a meter that shows resistance to one decimal place shows 0.1 ohm or less variation with moving the compound or cross slide or moving the carriage. There is a larger variation of 0.3 or more when turning the spindle.

Obviously if one wanted to perservere with this project a simple insulator somewhere in the circuit would make the results more obvious. This insulator could be as simple as a coat of paint or a layer of oxide between components.

However, simple resistance meter between the tool piece and the lathe spindle gives an obvious indication without further complexities.

mike4
06-13-2012, 06:03 AM
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.
I second the LED idea as it is simpler andeasy to use.

J Tiers
06-13-2012, 09:17 AM
However, simple resistance meter between the tool piece and the lathe spindle gives an obvious indication without further complexities.

You MAY find that to be a bit less universally true than you think.... but it very much depends on the machine. The man who originated the discussion has looked into it a good deal more than you seem to have, and found that "a simple ohmmeter" was not sufficient.

The difference may depend on whether you have sleeve (plain, journal) bearings or rolling element bearings. The latter tend to maintain a pretty good metal-to-metal contact, and short the ohmmeter pretty effectively.

I don't have a dog in the fight, in fact I am not aware of a fight, so whatever..... I only found a simple improvement to the complex device the originator was using.

BillTodd
06-13-2012, 09:37 AM
1A DC through a bearing is asking for trouble (long term) probably better to use RF at much lower current (wonder what impedance there is twixt tool and part ???)

Bill

Bob D.
06-13-2012, 10:29 AM
Would it be impractical to just insulate the boring bar from its holder? I have some teflon sheet material and I *think* one layer around the bar would insulate it and not affect the holder's ability to keep the tool rigidly in place.
Then a plain old ohmmeter would do the job.

What am I missing on this? Seems toroids and high currents are way overthinking this problem...:confused:

J Tiers
06-13-2012, 11:48 PM
What am I missing on this? Seems toroids and high currents are way overthinking this problem...:confused:

Ah, well a quick review of the already posted info will cure that problem....... I recommend it......

Toroids *AND* high currents are definitely NOT on the menu...... at least not together

But, for now I am perfectly happy to have folks lose interest in this....;)

lwalker
06-18-2012, 01:21 PM
Was the person whose idea sparked yours Rick Sparber? Reason I ask is that this thread (and his), along with me poking around the innards of a Control Gaging electronic grinding monitor I won at auction have me wanting to reinvigorate an idea I had some time ago: measuring parts while they are still in motion.

If it's not Rick, do you mind sharing where you first saw the idea?

J Tiers
06-18-2012, 09:53 PM
Was the person whose idea sparked yours Rick Sparber? Reason I ask is that this thread (and his), along with me poking around the innards of a Control Gaging electronic grinding monitor I won at auction have me wanting to reinvigorate an idea I had some time ago: measuring parts while they are still in motion.

If it's not Rick, do you mind sharing where you first saw the idea?

The thread got me thinking, but for sure nobody else had the idea, in fact it got pretty roundly dumped on until it was proven to work, and was still drawing "deprecation" recently......

So, no I did NOT "get the idea" from Rick S, I suggested the idea TO him.

Boostinjdm
06-19-2012, 12:41 AM
I think insulating the boring bar would be the easiest way out. I have a reducing sleeve in my quick change tool block. Swap that metal sleeve out for something non-conductive and you'd have the problem licked.

It would only be a very minor inconvenience to set up all boring bars with sleeves.

BillTodd
06-26-2012, 02:21 PM
This guy seems to have cracked the problem in a nice simple manner :)



http://rick.sparber.org/sceef.pdf

J Tiers
06-26-2012, 11:30 PM
This guy seems to have cracked the problem in a nice simple manner :)



http://rick.sparber.org/sceef.pdf

Check posts 27 and 28.....................

PStechPaul
12-12-2013, 09:46 PM
I found this while searching for something else. One way to detect the tool contact would be to have a small lamp in series with the primary turns of the toroid and the AC supply. The toroid will present a high impedance with the secondary open, but will drop when the tool completes the secondary circuit, and the lamp should light. Perhaps use a 120V 3W (25 mA) lamp as used in night lights, or an LED with a 25k series resistor (5 mA). There will be some magnetizing current with the secondary open, but it should increase enough to detect a change in brightness, or perhaps almost fully off and on.

I just tried this with a toroid that is about that size, and at 120 VAC it draws 12.5 mA open, and about 80 mA with a shorted turn. I think it is about 400:1 (0.3 V/t), so the secondary would be 0.08*400 = 32 amps but I read 18 amps. With about 10 volts, I get 1.8 mA open and 4.5 mA shorted, with about 1 amp flowing through the short.

It would be nice to be able to clamp the current source onto the workpiece, so the setup will not be disturbed. I tried using a clamp-on probe with 1000:1 ratio and driving the coil, but it doesn't have enough iron to produce significant current in the shorted turn. However, it should be possible to cut the toroid core and wind the primaries on both halves, and then make a sort-of clothespin clamp to connect it to the work.

boslab
12-12-2013, 10:42 PM
What happens if your machining plastic?, suppose you could go capacitive or inductive
Mark

dp
12-13-2013, 01:34 AM
Toroid cores under 1" diameter that are saturable using simple op amps can be used as phase detectors to produce an analog output that is proportional to the quality of the contact. The contact would be in series with a secondary winding on the toroid to change the saturation points. These same reactors were frequently used in electronic compasses and surveying magnetic dip angle. I have a schematic around here somewhere I can dig out. I haven't messed with those since the mid 1980s, but I created a device that used RC servos to create a geomagnetic tracking device (and an autopilot). Now that I think about it, that might be an interesting Arduino project for detecting microcurrent, earthquakes, all manner of interesting things.