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valveitude 22nd April 2005 06:36 PM

DIY Powdered Iron Tranny Core
 
Hello Everybody,

I am new to this forum, and was going to wait until I was further along with this project before I posted anything on it, but with all the activity around tranny winding lately, I decided to go ahead and put my ideas out here.
I have been intrigued with the idea of a powdered core audio tranny for some time. The cost of producing a core suitable for audio seems to be prohibitive, since I’m not aware of any. That doesn’t mean they don’t exist somewhere, I just haven’t seen them. Either way they’re not readily available to the diy’er.

I hope to change this injustice. :D

The first step was to find a suitable material. It has to be iron, not iron ferrite (the material most “iron powder” cores are made of), it has to be readily available, and most importantly (to me anyway), it has to be cheap. The answer to this problem came from a surprising place…your local brake shop. Brakes are made from a material called “grey iron”, and every time a brake drum or rotor is turned, a pile of granular iron drops in a tray. I searched the net to find out the composition of grey iron to see what was added to the raw iron, and the main additive is…wait for it…SILICON. It seems that silicon not only helps or tranny to avoid saturation, it helps are breaks from warping from the heat. Now, I don’t know the exact composition of the iron, but it no doubt will work. And the price ? How about FREE!! Its just waste to the shop owner.

Before I go any further, a word about safety. Iron dust is not to be breathed, even our un-processed bucket of iron has dust in it. ALWAYS wear a dust mask if your playing with it.

The next thing is to process our bucket of dirty brake turnings into our iron powder. I first tried wacking the stuffing out of it with a hammer and cast iron pan. A weekend of effort netted me about pound of dust fine enough for my purpose. This will not do. The solution, I believe, is a simple device called a “ball mill”. Basically it is a small drum filled with steel balls that spins on rollers (I hope to have mine finished this weekend). As it spins around the balls smash the grains to powder. Before it goes in the drum (or frying pan if your more inclined to beat it into submission), I heat it up in my metal furnace ( I have a hobby aluminum foundry) to a cherry red heat, and while hot, pour it into a metal pot. As the iron hits the air, it cools rapidly, hardening it. This does 3 things, first it burns any residual oils off the iron, second, it makes the iron quite brittle for easy smashing, and third it puts a very thin layer of oxide on the surface as an insulator. Once pulverized I’ll re-heat it in the furnace to oxidize the broken edges, but this time I’ll let it cool slowly to anneal the metal back to its natural “soft” state.

Once processed, the powder will be mixed with an epoxy binder, and rammed under pressure into a break-away mold. These details will be worked out when I get there.

Comments and suggestions would be greatly appreciated.

PRR 22nd April 2005 07:45 PM

> powdered core audio tranny for some time. The cost of producing a core suitable for audio seems to be prohibitive, since I’m not aware of any.

They have silver-wire transformers. I don't think the reason for no iron-powder is "cost".

It probably does not work well.

Consider. Use solid (sheet) iron, you get high inductance. Break the solid iron with an air-gap, you get lower inductance. In applications where the iron is soaked in DC, a small air-gap improves AC performance.

Your plan seems to have a ga-zillion air-gaps in it. Inductance could be rediculously low.

There are ferrite cores for audio. I remember when you could not get 1% of the inductance of iron with ferrite, so it was only used at high (radio) frequency. Apparently there has been great progress in the ferrite racket. Certanly switcher power supplies have pushed 20KHz core performance.

> a surprising place... your local brake shop.

Brake rotors are iron, yes. But the rotors are contaminated with brake pad lining. This used to be asbestos (a potent carcinogen) and Bakelite/phenolic resin. Asbestos pads are nearly gone from the market, but brake shops may have decades of old asbestos dust everywhere, and it isn't clear that the asbestos-replacement materials are entirely benign to the body.

As you found, processing iron shavings into fine iron dust is VERY hard work. You can find pre-made iron dust in several places. The inside of drum brakes accumulates a nasty mix of brake-pad dust and iron-drum dust. The rear drums on modern cars hardly wear enough to be worth the collection, but big trucks still use big hard-worked drum brakes and must generate a pound of dust between reshoeings. Magnetic separation would get the iron out of the pad-dust. There is also fine iron in engine oil, though with modern oils and low-wear engines, you might have to process a lot of oil for a little iron.

"Iron powder" on Google

If you manage to collect enough iron dust, be aware it can be flammable. Not easy to ignite, but very hard to extinguish. Mixed with an oxidizer, iron dust is "Thermite", which can weld railroad tracks or burn a hole in a safe. Iron has to be super-fine to sustain combustion in plain air, but if you carry the experiment far enough you could get in trouble.

> SILICON. It seems that silicon not only helps or tranny to avoid saturation, it helps are breaks from warping from the heat.

Actually, Silicon in iron makes it easier to cast.

> anneal the metal back to its natural “soft” state.

Transformer steel works best in the direction it is rolled in. I'm not sure that cast steel would have the same magnetic properties.

> mixed with an epoxy binder

I'm trying to picture this. With fine iron, and most epoxy resins, the thick epoxy would keep the iron apart, giving a mass that is say 50%+ epoxy, not iron. By simple conglomerate geometry, I think you want a super-thin binder.

valveitude 22nd April 2005 08:53 PM

"They have silver-wire transformers. I don't think the reason for no iron-powder is "cost".

It probably does not work well."

The cost would be the complications of casting larger cores.

"Consider. Use solid (sheet) iron, you get high inductance. Break the solid iron with an air-gap, you get lower inductance. In applications where the iron is soaked in DC, a small air-gap improves AC performance.

Your plan seems to have a ga-zillion air-gaps in it. Inductance could be rediculously low"

Actually its called a "distributed air-gap"..its an advantage. Properly done, the casting would have about 90-95 % the mass of solid metal, ergo, 90-95% of the permeability. I intend to build an SE amp with parallel 813's. The Idle will be about 200ma I need an air gap.

"Brake rotors are iron, yes. But the rotors are contaminated with brake pad lining. This used to be asbestos (a potent carcinogen) and Bakelite/phenolic resin. Asbestos pads are nearly gone from the market, but brake shops may have decades of old asbestos dust everywhere, and it isn't clear that the asbestos-replacement materials are entirely benign to the body. "

See the saftey precaution mentioned in original post "ALWAYS wear a dust mask". That said, after riddling the iron through a screen outdoors in the breeze the powder contaminants were effectively removed. As I mentioned in the first post , the rest are burned off in the kiln.

"Actually, Silicon in iron makes it easier to cast"

Actually, according to ALL the metalurgy sites I visited, the use of silicon was to increase its wear/heat resistance.

"Transformer steel works best in the direction it is rolled in. I'm not sure that cast steel would have the same magnetic properties."

It won't, it will be better. Because of the random placing of the particles it will have an amourphous mag pattern.

"I'm trying to picture this. With fine iron, and most epoxy resins, the thick epoxy would keep the iron apart, giving a mass that is say 50%+ epoxy, not iron. By simple conglomerate geometry, I think you want a super-thin binder."

My first crude attempt with household epoxy and poorly rendered powder and no real pressure yielded about 75% mass, the 25% waste was due mostly to not enough pressure, as I used 5% by weight of epoxy. True enough thinner is better, however with proper mulling thick works fine. It's more a question of proportion by weight than anything else. You want just enough binder to hold it together, no more. As I said in my original post I will work out the exact method later.

Thanks for the reply.

Yvesm 22nd April 2005 09:15 PM

Quote:

Actually its called a "distributed air-gap"..its an advantage. Properly done, the casting would have about 90-95 % the mass of solid metal, ergo, 90-95% of the permeability. I intend to build an SE amp with parallel 813's. The Idle will be about 200ma I need an air gap.
The main principles you exposed fly 100,000 feet above my head.
I just want to point that, using "standard laminations" the gap represents 1 to 5 per 1000 of the total magnetic lenght (not 5 to 10 per 100).
This reduces the permeability by a factor of about 20.

I really don't know if my remark is pertinent in this context, just a remainder.

Yves.

Sch3mat1c 22nd April 2005 09:30 PM

You're both wrong, silicon is added to keep it from turning brittle! :D The carbon content (mostly present as graphite flakes and pearlite) is what provides the lower melting point and great fluidity.
Silicon does increase temperature and wear resistances a little though.

Dumping the stuff hot through air is going to burn *everything*, depending on just how fine the stuff is. Magnetite is still an okay material, but it's not known for a high permeability. (But hey, you can always bond it with some clay and make your own ferrites.)

Aside from dense epoxy bonding, which evidently does work well enough? You could try sintering it, heat powder to about 2000øF in a refractory mold under hydrogen atmosphere.

Tim

Sheldon 22nd April 2005 09:49 PM

Quote:

Originally posted by Sch3mat1c

You could try sintering it, heat powder to about 2000øF in a refractory mold under hydrogen atmosphere.
Tim

LOL!! Now that's what I call a committed diy'er (or perhaps should be committed:D).

Sheldon

valveitude 22nd April 2005 10:49 PM

Yvesm said

"I just want to point that, using "standard laminations" the gap represents 1 to 5 per 1000 of the total magnetic lenght (not 5 to 10 per 100).
This reduces the permeability by a factor of about 20."

True..but a powder core has the gap distributed. Think of how the bricks lay in a wall, for every two bricks laying end to end with morter in between, there is a brick on top bridging the gap. Thats not to say there isn't a loss, its just not a linear loss like with laminations. I wont know what the total effect will be untill I actually have a core to test.

Sch3mat1c said

"Dumping the stuff hot through air is going to burn *everything*, depending on just how fine the stuff is."

How fine, and, how hot. I'm "only" heating it to 1200-1400 deg. The idea here it not to burn the metal, rather to temper it "full hard" so that its brittle enough to pulverize, and to generate a thin oxide layer to act as an electrical insulator from one particle to the next. If you have a piece of "hot rolled" steel around, the black layer on the surface is from the steel coming into contact with the air while its still red hot coming out of the rollers. This layer is a lousy conducter. Just ask a welder..he has to grind a spot down to metal to put his ground clamp on.

"Aside from dense epoxy bonding, which evidently does work well enough? You could try sintering it, heat powder to about 2000øF in a refractory mold under hydrogen atmosphere"

Aside from the fact I don't have the resources needed to sinter, sintering would provide an uninsulated path for eddy currents. One of the main advantages of a powder core, is that each particle acts as an individual lamination (ideally anyway), hence very little loss due to eddy currents.

Epoxy and oxidizing weren't my ideas. This is how industry does it when working with real iron as opposed to ferrite. The difference is, the big boys oxidize with a acid bath. I tried this with vinegar as a mild acid, it worked, but what a mess :dead:

dnsey 22nd April 2005 10:58 PM

Please be aware that an ordinary dustmask offers virtually no protection against asbestos dust.
If experimenting with this idea, be sure to obtain and use proper respiratory protection, and dispose of the waste safely.
Even 'asbestos free' brake linings can contain a proportion af dangerous fibres, so take no risks!

valveitude 23rd April 2005 12:28 AM

Well.. Its looking like my original impulse to hold off posting on this topic until I was at the point of testing the electrical/magnetic properties of an actual core was the correct one.
I understand that you guys hear a lot of off the wall design ideas from nubes. I also realize that the people here don't know me from Adam, that I have been a component level service tech for 25+ years, that I have been a hobby machinist for the better part of 10 years, ect.,ect.. I don't profess to be a MIT research scientist, but I have had an idea or two that has panned out (along with several that didn't). Will this one be worth the effort ? I don't know yet, but clearly I believe its worth a shot.
What I didn't expect was the chorus of negativity, especially when you consider the potential it has. From reading the responces you would think ...A)There can't be a break man alive over the age of thirty, what with all the dire consequences of being around break shavings 40 hours a week B)If a powdered iron core for the 5hz-20khz range was such a good idea Radio Shack would have 'em already.
I didn't come up with my methods out of thin air. I spent a great deal of time reading up on powdered cores and how they are manufactured. Basically I took industries approach, and figured out how to replicate it on a much smaller scale..what I'm trying to say is the basic approach is sound. Will I pull it off ? Time will tell. Will it be worth it if I do? Boy Howdy!!
I was hoping to get suggestions on how to improve my approach, to generate just a little excitement about the possibility of making a core to match the tranny, instead of winding a tranny to match the core. To be able to make any style of core you want, not having to pull apart a tranny to get the lams, or meet the minimum order requirements for an industry that wants to sell a ton of lams not the few pounds you need.

I'm still hopeful that there is some imaginative soul(s) out there with a inventive spirit that would like to chip in with an idea or two.

smoking-amp 23rd April 2005 01:29 AM

Powdered iron cores of various shapes and materials are readily available and cheap:
http://www.amidoncorp.com/aai_cost_ironpowderpower.htm
or www.micrometals.com (they even give out samples)

I'm sure you can find some real cheap cores on Ebay too if you look for a while. Problem is, powdered iron material is typically very low permeability, ranges from Mu relative of 3 to 100. Also, any magnetic material formed from powder will be high loss. This is because the magnetic field is highly concentrated where the particles touch, but low in the bulk of the material. This causes saturation losses to set in early. Somewhat better are 20% iron/ 80% nickel powdered cores (powdered permalloy) for Mu up to 200.

Ferrite material has much better magnetic properties (Mu 2000, 5000 even 10,000, TDK, and very low loss and good initial permiability at low fields) and you can definitely find E cores for switch mode power supplies on Ebay cheap. (E cores are easy to gap for SE use and easy to wind too) Only drawback is lower (about 1/4) maximum saturation field. Fair Rite Products sells a 6 inch OD ferrite core. Stack 2 or 3 of them up and you can handle 100 watt audio. Toroids are a major pain to wind however without automatic equipment.

In bygone days, a lot of audio xfmrs for telephone equipment was made using magnetic wire (oxidized coating) to form the cores. Old ignition coils too. Hard to find decent material now though. Could look for the old magnetic recording wire spools maybe.

Don


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