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From where I might be able to source some 1300 micron (1.3mm) mild steel sheet?

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Michael Gilligan26/10/2021 19:30:33
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If we knew what you were trying to make, John … People might be able to offer useful comment, based on their experience.

But as it’s all confidential : “Tricky stuff, eh?” is probably a good summary.

MichaelG.

Dave S26/10/2021 19:51:09
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I’m not sure I agree with “Tricky Stuff”.

Possibly “narrow specialism, niche application stuff”

Magnetic properties of materials is a specific area of materials science, and a well studied one at that.

The information is available, it’s just not actually that critical to most applications.

Dave

John Smith 4728/10/2021 01:23:11
271 forum posts
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Posted by Dave S on 26/10/2021 19:51:09:

I’m not sure I agree with “Tricky Stuff”.

Possibly “narrow specialism, niche application stuff”

Magnetic properties of materials is a specific area of materials science, and a well studied one at that.

The information is available, it’s just not actually that critical to most applications.

Dave

You could probably say the exact same thing about doing surgery on the human brain or about sending a rocket to the moon. In fact.. just a wild guess, Dave, but... maybe you do! ;^P

J

John Haine28/10/2021 10:14:36
4259 forum posts
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Posted by John Smith 47 on 26/10/2021 18:40:31:

.....

In my experience everyone thinks that they understand magnetics but is an extremely hard subject.

It turns out that in practice (magnetic) "Permeability" is almost irrelevant and the only number that really matters for magnetic latches using permanent magnets is the (magnetic) "Saturation Flux Density".

Nobody will tell you this, but my understanding is that (roughly speaking) Permeability is how easily a material can start to be magnetised by a weak magnetic field, but Saturation Flux Density is how much you can magnetise a material... and it's the latter that matters most in practice!

....

Hmmm. How close you can get to saturation depends on the flux density produced by a given magnetic field, and so on the permeability. So for the unmagnetized polepiece of a latch you want a material that has high permeability but also a reasonable saturation flux density, but low remanence so it doesn't itself get magnetised. There's a distinction between field and flux, which roughly speaking correspond to voltage and current. But in the end mild steel is probably a reasonable compromise and for most simple magnetic devices is as good as you need.

**LINK**

John Smith 4730/10/2021 02:05:14
271 forum posts
11 photos
Posted by John Haine on 28/10/2021 10:14:36:
Posted by John Smith 47 on 26/10/2021 18:40:31:

.....

In my experience everyone thinks that they understand magnetics but is an extremely hard subject.

It turns out that in practice (magnetic) "Permeability" is almost irrelevant and the only number that really matters for magnetic latches using permanent magnets is the (magnetic) "Saturation Flux Density".

Nobody will tell you this, but my understanding is that (roughly speaking) Permeability is how easily a material can start to be magnetised by a weak magnetic field, but Saturation Flux Density is how much you can magnetise a material... and it's the latter that matters most in practice!

....

Hmmm. How close you can get to saturation depends on the flux density produced by a given magnetic field, and so on the permeability. So for the unmagnetized polepiece of a latch you want a material that has high permeability but also a reasonable saturation flux density, but low remanence so it doesn't itself get magnetised. There's a distinction between field and flux, which roughly speaking correspond to voltage and current. But in the end mild steel is probably a reasonable compromise and for most simple magnetic devices is as good as you need.

**LINK**


Permeability vs Flux Saturation Density

I notice your link fails to even mention Mild Steel.

Well, when I first researched this topic, I thought sounded like it would be all about (magnetic) Permeability. And the Permeability numbers certainly vary dramatically, as your link demonstrates.

If you also think Carbon Steel is close enough to Mild Steel, think again because Ferritic Stainless Steel has a much higher Relative Permeability. "100" plays "1000 – 1800"...

i.e. If it was all about permeability Ferritic Stainless Still would have at least x10 the magnetic pull compared to Carbon Steel (and you might thought Mild Steel). But we know that if anything it's the other way around!

[See how "tricky" this topic is!]

The fact is that in practice, when using standard modern high-strength neodymium magnets, it turns out that the saturation flux density is vastly more important than Permeability. For example in my experiments both in FEMM and in the real world, a Ferritic Stainless Steel will have c. 30% to 40% LESS magnetic pull than Mild Steel has.

And in this regard the best ferromagnetic steels available don't perform all that much better than mild steel. From memory the uplift in magnetic pull was only something like +30% or +40% (??) and meanwhile the costs started to escalate.

"magnetic field" and "magnetic flux"
TBH, I never did feel that I 100% understood the difference between "magnetic field" and "magnetic flux". I think magnetic field can be thought of as the strength (i.e. total energy) of a magnet, whereas the flux is how concentrated those lines of force are within a specific area.

Remanence
My thinking is that I don't care about remanence AT ALL. The latches are not being turned on/off electrically. In fact they are using permanent magnets and simply use physical separation. i.e. The are very similar to how a magnetic kitchen/wardrobe door closure 'sandwich' works.

i.e. When the door is open the magnetic sandwich is far enough from the steel that they close on to as to not have any tendency to close the door. i.e. If the steels within the 'sandwich' stay magnetised that isn't a problem. Likewise if the steel that meets them stays magnetised (by them) that sounds fine to me too. So unless any remanence is fighting the magnet, I think I can forget about it.

One thing I have noticed is that even though when you separate the steel from the magnets, the steel do not stay significantly magnetised, if you leave the magnets in place for a few seconds the magnetic pull SEEMS to gradually get quite a lot stronger in a matter of say a few seconds. This may be some kind off illusion I'm not sure, but I as far as I can tell it doesn't appear to be caused by the magnets slowly moving closer to each other. Either way if the magnetic pull is good enough immediately and if it gets say twice as strong in a few seconds that's fine by me.

But the main point is that my magnetic assemblies work really well which is all I really need to know!

John Sykes 118/11/2021 09:46:04
10 forum posts
1 photos

The "pull" of the magnet is actually calculated from the incremental change in stored energy in the air gap as the pole piece (or whatever) is moved towards or away from the magnet. Many non-linearities will render precise calculations of absolute magnetic flux somewhat futile, but the general principle is there, and tells is that the pole piece should have a high saturation figure and should be adequately thick to handle the total flux generated by the magnet (if the objective is to obtain the maximum force). You can observe this phenomena using a magnetic dial-guage holder, first on thick cast iron, and then on a thin plate, where the force is much reduced. The surface roughness / flatness also comes into play when considering the maximum "pul-off" force, as any residual air gap when the parts are touching will impact on the "closed loop" flux density.

BTW, 1.3mm is very close to 16 AWG !

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