Sciencemadness Discussion Board

Aquiring grinding material for a ball mill

RogueRose - 31-5-2020 at 05:13

I'm working on a few different ball mill's and have found that the grinding media is going to be some of the most expensive part of the project so I was wondering about using some unconventional material for this and was wondering if anyone has any suggestions.

There is obviously ball bearings from normal hardened steel to stainless to chrome finished. Then there's sling shot ammo which is available from .2" up to 3/4" and I'm guessing it's not as round or as hard as ball bearings but plenty round enough. I'm wondering if the hardness is an issue and I'd be fine with hardening them myself by heating and quenching.

Then there's marbles that they sell for sling shot ammo as well and some for paintball ammo, so you can buy it by the lb or kg in all different sizes.

just found that rebar is incredibly cheap for 20ft lengths and is available in lots of sizes from 1/8 to over 1". I have a chop saw that I can use an abrasive disc with and I can cut little "bars" maybe 1.5x the diameter. I figure these would mix well with bearings, but might work well alone and maybe instead of bearings I could cut shorter pieces to replace the bearings. Cutting the steel is also a great way to get iron filings by using a plastic covered magnet to catch all the grindings. I know the edges will be sharp and that might be a problem with a plastic milling container, so I'm planning on using the bench grinder to knock the edge off quickly. I'd harden these as well.

I was considering lead balls, I have plenty of lead but I'm worried about abrasion and the lead mixing with the material and wonder if I can harden it by adding a little Sb, Al, Mg, Ti, Zn, Cu, Sn or whatever will give me a much harder alloy - I'm guessing there has to be some kind of lead alloy that is pretty hard and if not, I'd be interested to try making one...

Can anyone think of other good sources? I can get stainless steel bolts, with various thread types (machine and/or wood) and partly smooth shafted, in all kinds of sizes, for VERY cheap as they are pulled from equipment and can't be reused for structural/pressure sensitive applications. I'm wondering if these would be better than rebar and how the threads would effect the milling.


Ubya - 31-5-2020 at 06:21

well first of all, what do you want to mill?
most grinding media is not good for everything.
for example if you are milling aluminium it's safer if you use softer media like lead balls (hardened steel could spark), sometimes ceramic ball also are used.
if you need to grind stone, throw whatever you have in it, big heavy steel nuts? steel cylinders out of rebars etc.

about lead, maybe my logic is wrong, but i think that it's better if you use soft lead, as instead of abrading it will just deform under the impacts.

Herr Haber - 31-5-2020 at 08:02

I would recommend against mixing different hardness media such as ball bearings and rebar. The softer media will get milled by the other.

RogueRose - 31-5-2020 at 16:26

Quote: Originally posted by Ubya  
well first of all, what do you want to mill?
most grinding media is not good for everything.
for example if you are milling aluminium it's safer if you use softer media like lead balls (hardened steel could spark), sometimes ceramic ball also are used.
if you need to grind stone, throw whatever you have in it, big heavy steel nuts? steel cylinders out of rebars etc.

about lead, maybe my logic is wrong, but i think that it's better if you use soft lead, as instead of abrading it will just deform under the impacts.


I'm planning on grinding a wide range of things, hence the many different media's and containers. I want to be able to mill whatever I need to when I need it, which is what makes it an expensive proposition to get the media to do it, and is why I'm looking for a range of material type and sizes.

I have milled aluminum with steel bearings and have never had a problem and I don't think I'd use anything but spherical on pyrophoric materials, or maybe nice rounded rods mixed in.

I was wondering if it is better to have a mix of media sizes like 12mm, 8mm, 6mm & 4mm - all mixed in together, so the smaller pieces can fit in the cracks between the larger ones.

I'd really like to get some ceramic balls but IDK if they are worth it as compared to marbles as I never really understood how hard glass can be - I've seen some marbles not shatter when being shot at over 400fps against a brick wall, though maybe at a 10-15 degree angle.

RogueRose - 31-5-2020 at 16:29

Quote: Originally posted by Herr Haber  
I would recommend against mixing different hardness media such as ball bearings and rebar. The softer media will get milled by the other.


That is a good point. I did plan on heat hardening the rebar but I'm not sure how well it will harden.

Fulmen - 31-5-2020 at 23:43

Hardening rebar: It might be possible, but it's a crap shoot at best. There is no standard for the composition, the specifications are limited to their physical properties. But some of the steel used should perform OK, so it's not impossible.
Machine bolts would probably be a safer choice. Like rebar they have no standard for the composition, but anything above grade 5.6 should be heat treatable for sure. The shape will probably cause problems, the threads and sharp corners will most likely suffer excessive wear and product contamination. On the other hand it's magnetic so it shouldn't be hard to separate out.

Using a mix of sizes is quite common. Logic suggests it will be more efficient for very fine materials as you will get more impacts but with less force. It's also unavoidable in the long run due to media wear.

Lead: This is the preferred media for milling black powder. If you consult the pyrotechnics community I'm sure you can get some data on wear rates. And any bullet caster should be familiar with hardening lead. The standard alloying elements are tin and antimony (with a tad of arsenic), and the best source would be old lead types from linotype printing. Another decent source is the old "clamp-on" wheel weights, these contain at least some antimony (the newer glue-on style are usually pure lead or zinc). Adding 1-2% tin should improve the alloy further.

Dr.Bob - 1-6-2020 at 07:49

I have some ceramic ball mill media if you are interested. Pieces are cylinders about 3/4" dia by 3/4" long. Might have some more sizes somewhere.

Mateo_swe - 2-6-2020 at 11:20

Im no expert but have been interested in building a ball mill so i read a bit on it.
If ball milling something that could ignite, lead balls is the only thing one should use.
At least that is what i have read.
Maybe some other soft metals could be used but be careful so you dont set your house on fire.

I have a question for the more experienced in ball-milling,
If one mill something to very fine particle sizes, could not a dust explosion happen in the ballmill?
If yes, how one prevent this?
Put argon, nitrogen or some other noble gas in the ballmill?

Refinery - 2-6-2020 at 11:33

Get 8.8 nuts, heat them to cherry red and quench in 10% NaOH. They will get beyond HRC60 and grind everything you feed at them.

Bulk steel can be surface hardened by placing them in metal can(paint can for ex) and filling it with 70:30 C:CaCO3 powder mix and heating it up to 800-1000C for minimum of 2 hours and quenching them in NaCl or lye.

All media should be case sensitive.

Al foil is good feedstock for powderizing aluminum, blend it first with blender. Add few %/w of C to reduce oxidation of Al powder. Even better if you can get airtight milling cylinder. Al will quickly consume oxygen and create inert atmosphere. Otherwise any non-reactive inerting gas could be used, but on small scale dust explosion usually does not cause major hazard. Though, if inert is used, it must be carried upon recovering the mill contents, otherwise the exposed powder can react with air oxygen and due to huge surface area there might be opportunity for heat generation up to ignition. Filling a bag with inert gas and opening the canister inside and pouring the powder into final container should be viable.

Fulmen - 2-6-2020 at 11:48

I see no need for something as aggressive as 10% NaOH, plain water or brine works just fine. And nut's really aren't the best choice, the hole makes them lighter and therefore less efficient. But considering the amount of work invested perhaps it's a fair trade off.

Case hardening would work in theory, but provides limited wear resistance. Two hours probably won't give a layer thicker than 0.2mm, so you would have to re-carburize the media repeatedly.

Refinery - 2-6-2020 at 11:57

https://www.totalmateria.com/page.aspx?ID=CheckArticle&s...

The effect is more pronounced than you think. Plain carbon is slow, but energizer provides an increased rate. There is a cycle of C, CO and CO2 going on which energizer activates.

https://www.engineeringenotes.com/metallurgy/steel/hardenabi...

https://paaba.net/Projects/ZWeekendMet.htm

Carbon steel can be quenched to an extent with lye quench. Mild steel contains about 0.1% C, +- few thous tolerance. This can reach well over 40HRC. The original superquench was 10% lye, but commercial version had to be reduced due to regulations - as you know - to soapy mess, which does the opposite. Cryogenic lye solution can hit miracle out of structural steel and I've personally tested this with controlled sample.

Nuts are otc and very cheap, even though not ideal. I would use bulk bearing balls though, because they are cheap as well. And slingshot ammo are not bad idea on their own, I've used them myself.

[Edited on 2-6-2020 by Refinery]

Fulmen - 3-6-2020 at 00:06

I haven't played around with superquenches, and for something like 8.8 it shouldn't be required. These can be air quenched to a decent hardness, and more isn't always better. A rapid quench produces huge internal stresses, and these don't just magically disappear with a quick annealing. The last thing you want is for the media to be brittle. Then there's the safety aspect...

As for carburizing speeds, I agree my numbers are on the low side. But even with "energizers" I haven't seen (nital etch) anything close to the numbers you quote. I'm sure one can get there, but it will probably require a lot of experimentation that isn't worth wile if you're just want some decent milling media.
And honestly, carburizing is pretty primitive when compared to modern steel. I use it once in a blue moon for non-critical use if I don't have any suitable steel, but that's about it. You can't make complicated parts due to distortion, nor can you machine after hardening. And I sure as heck can't measure the hardness or estimate the net strength of the parts.

Refinery - 3-6-2020 at 00:24

Carburizing is certainly an useful option for example gun parts. (Open bolt) gun lifetime can be significantly increased by surface hardening all the parts. I consider a few thousand rounds a reasonable lifetime for an improvised device. Acquiring and machining special steel is expensive and requires more robust equipment, hence. For locking lugs, more special alloys should be used, though.

I find it reaching to link this to ball milling, because proper media is easily available even in the more developing parts of the world.

eesakiwi - 8-6-2020 at 14:10

I would visit some machine shops, CNC lathe turning etc, and see if you can get some used Tungsten carbide turning tips.
Heavy as, sharp, smallish, wont wear down.

[Edited on 8-6-2020 by eesakiwi]