Missing eddy currents in copper?

[pete]

I have a set of little flywheels that have a magnet to break the motion of the system. I have one aluminum and several copper ones all water jet cut roughly to size. The copper and aluminum disks I have are all the same dimensions but bought from three different sources.
Yesterday I machined the Aluminum disc and three of the copper discs to the correct specs on the lathe.

I checked the amount of drag caused by these discs some time ago and found the copper was far more effective than the aluminum. Today I needed to check the operating distance of the magnet so repeated this test on the newly machined parts and was shocked to find the three copper discs provided so little drag it bearly registered. The aluminum and the other copper ones worked very well. The drag the magnet courses on the other copper wheels is considerable so the difference is huge, one copper disc brings the whole system to a halt immediately but with the other discs I machined yesterday I can barely feel any drag at all, at first I was unsure there was any until I spun it up faster then I could just about feel some slight resistance.

I thought the strength of the eddy currents produced was dictated by the conductivity of the metal used so I am now confused as to what happened here. As I said before the copper discs are from three different sources so maybe different coppers. Does anyone know what has gone wrong and if it's the copper which coppers are best for use in this job?

 

[Rive]

The ... disks I have are all the same dimensions

If this includes the thickness then the most likely culprit is the material composition. You might happen to get some brass or bronze instead of pure (electrolytic) copper.
random list of copper alloys composition and conductivity

 

[pete]

I guess, I don't know about brass but bronze maybe. The supplier could have made a mistake when he sent these out. The color is identical though with that pink hue, as are the small spots of black discoloration on both discs. I don't know any sure way to test.
What your saying is that if it's copper then is behavior or ability to create drag would be equal or close enough across the different types of copper sheet I could have?

 

[anorlunda]

What your saying is that if it's copper then is behavior or ability to create drag would be equal or close enough across the different types of copper sheet I could have?

I think you missed @Rive 's major point about the thickness rather than the materials. How thin did you machine those disks?

 

[pete]

I didn't change the thickness, there all 6mm - 1/4 inch thick.

 

[CWatters]

I wondered if machining had annealed the metal changing the conductivity but a paper I found suggested this might only change the resistance by a few percent.

Can you post photos of machined and non-machined discs?

 

[Rive]

What your saying is that if it's copper then is behavior or ability to create drag would be equal or close enough across the different types of copper sheet I could have?

Yes. With the same geometry what matters is the conductivity, and as long as it is 'copper' it will be ~ the same. See the second table from the link from comment #2
Just a few percent difference in conductivity should not affect the eddy currents // drag.

 

[JBA]

If the black spots are on only the two discs that are not performing as expected then impurities in their copper material could be an issue. Maybe by measuring the resistance across the diameter of those discs compared to that of a performing disc could give some indication of a high level of impurities.

 

[pete]

The discs are 100mm diameter by 6mm thick, The machining was just a relatively small counter boar in the center, 32mm diameter with 3.1mm depth. The majority of the surface is untouched by the tool. I'll try measuring the resistance with a multimeter but I'm thinking that they must have sent me some alloy by mistake, it really looks identical to the copper though.

 

[Baluncore]

Is it possible you have changed the orientation of the braking magnet(s) or the magnetic path ?

Since skin depth is a function of frequency, I would expect braking to be a function or RPM, thickness and conductivity.
We have no idea of the RPM you are making your tests.

 

[pete]

No need to worry about the mechanism. By hand, if you pass a magnet across the copper ones they move across the table. If you pass a magnet across the unknown ones you can bearly feel any resistance. One is clearly creating drag and the other not.

 

[DaveE]

I'll try measuring the resistance with a multimeter...

Probably a waste of effort. A normal multimeter (even a 4-wire measurement) can't measure resistance as low as what you will have here. You could make this measurement with a high current power source, an ammeter and a voltmeter though.
With a normal multimeter you will actually be measuring the resistance of your test leads contact at the sample, etc.

 

[Baluncore]

One is clearly creating drag and the other not.

Then it is simply a matter of material conductivity.

 

[Dadface]

Small amounts of impurity can reduce the conductivity of copper substantially. That's why copper use for circuitry is purified by electrolysis.

 

[JBA]

By going to the below supplier site you will see the two copper grade designations you should probably be using when ordering your plate.

http://www.farmerscopper.com/ofhc-plate-sheet.html?_vsrefdom=www.google.com