When Sensitivity Means Precision

[Rick Sparber]

Over the past few years I have developed a number of Electronic Edge Finders (EEFs) to be used on both my lathe and mill. All of them tried to have a simple "idiot light" on them that indicated when the cutter (or equivalent) touched a conductive reference surface clamped to the rest of the machine.

This is easy when the change in resistance is greater than 3 ohms. Not too bad when the change is around 0.15 ohms, and down right hard when around 0.02 ohms.

Along the way I found that I needed to develop a milliohm meter so I could precisely set test resistances. Much to my surprise, this meter has turned out to be the best EEF I developed. I've used it for over a year now and it has not failed me yet. This meter can measure down to 0.0001 ohms yet uses only a 10 mA test current.

On my mill I have a change in resistance at touchdown of at least 0.1 ohm so the meter easily detects it. Once in a while the change is only 0.02 ohms but it easily sees that too. You can see my "Resistance Amplifier" at rick DOT sparber DOT org/electronics/ramp DOT pdf. You can read about the theory as it applies to a lathe at rick DOT sparber DOT org/ueef DOT pdf

Note: GoDaddy scans all of my files for tampering at least once a day.

Now, my Resistance Amplifier is designed to precisely measure milli ohms and that is overkill. All we need in order to have an EEF is see changes in resistance. I have been playing with a version that is far simpler which actually can fit inside a Harbor Freight multi meter and should cost under $5. As with the Resistance Amplifier, it would have no on/off switch. I have a bad habit of forgetting to turn battery powered devices off. So instead, removal of the probes turns the device off.

On my mill, I use the Tormach Tooling System. When I want to find zero, I slide a 0.750" diameter length of drill rod in the TTS collet. When zero is compared to running my mechanical rotary edge finder, they agree to within a tenth. So run-out is acceptable. On my lathe, it is best to take a skin cut so the surface is at a constant radius. Then I can set zero to better than a tenth. This EEF is particularly handy when I must touch down a boring bar in the bottom of a hole.

Rick Sparber

Rick DOT Sparber DOT org
KG7MQL

 

[Chip Hacket]

Very nice Rick! I like the circuit and your site in general. That seems to me quite the balancing act getting the desired gain for your OP Amp. I would have expected the need for precision resistors. By the way I just may adopt your mechanical laptop fuse. You should certainly copyleft that!

--Chip

 

[Rick Sparber]

Thanks Chip. The electronics part of my website is really a tiny backwater. The mother load is at rick DOT sparber DOT org/ma DOT htm

One thing that puzzles me about hobby-machinist DOT com, I see a lot of people reading the threads but very few actually responding. Sure would like to hear from more people - both positive and expecially negative comments.

I now have an early draft of the schematic and circuit description for anyone that wants it. A Harbor Freight Multimeter, three resistors, one bypass cap, and a common dual op amp are needed. I prefer not to post the draft to the public yet since it needs to be tried on more than my machine to be sure it can be trusted.

Rick

 

[dbassing]

Hi Rick,
I would be interested in any info regarding your new EEF. I was fortunate to have you help me when I built one of your earlier versions of an Electronic Edge Finder. I learned a lot and you were most generous with your time. To all who don't know about Rick and his really creative work check out his site and definitely try building an EFF. I look forward to trying this new version.
Cheers,
David

 

[Rick Sparber]

I'm sending you the link via private mail.

To anyone interested in building an EEF - although a bit complex, my "Resistance Amplifier" has served me well for 3 years. It will work on a very wide range of machines. My other designs require the user to first measure their machine in order to match it to the correct sensitivity. If my newest version passes the test of others building and using the design, then it will be the best solution of all. It uses 3 resistors, one small capacitor, and half of a common quad op-amp. Couple this to a Harbor Freight Multimeter and you are done.

 

[Lornie McCullough]

Hi Rick..... I don't post here often, but I have been a subscriber to your youtube video's for some time now. Just taking this opportunity to say Hi!

Lornie

 

[Rick Sparber]

Hi Rick..... I don't post here often, but I have been a subscriber to your youtube video's for some time now. Just taking this opportunity to say Hi!

Lornie

Lornie,

Great to hear from you. You have reminded me that it has been a while since I posted a new video. I'll see what I can do.

Thanks,

Rick