635 replies to this topic

[RegimentalPyro]

Those ball mills look identical to the one I have, except that I paid ?20 more for mine as I recall. These look like a bargain.

The ends of the barrels are pressure fitted rather than screw on but that doesn't seem to cause any problems.

I did have some trouble with the barrels slipping unless they were loaded "just so". A couple of wide elastic bands solved that one.

[BurlHorse]

Hey there guys,

Just thought I'd expound on some methodology for figuring out rotational speeds of some of the Parts I mentioned in earlier posts, so here's a little info on critical speeds/optimal speeds etc. for ball mills.

Taking into account that in this instance, I want to use 4"ID for the milling jar. The fittings make the actual Milling Jar OD 4.75" , it has a length of 21 " for reasons that are only critical to me, but the important things are the ID and the OD for figuring everything out. From, Best of AFN 3, an article on the the Magic of grinding things, Authors Initials LES, I am borrowing the chart with figures for Critical/optimal RPMs for various size containers. Accorrding to him, for a 4" ID milling container, utilizing 1/2" media, the Critical/Optimal speed is...142/92 RPM, 5" with 1/2" Media, C/O = 125/81, for a 6" Jar, 1/2" media is 113/73, for a 6" Jar, with 1" media, the C/O = 119/77...........For my calcs below, again the jar is 4" ID with 4 - 3/8" rounded lifter bars in the mill jar positioned, North, East, South and West, PVC, "Welded with PVC Cement to the inside of the Milling Jar......

Quickly, Critical RPM is the speed at which centrifical force pulls the balls to the outside of the milling jar, hence little or no grinding action (Except to the Milling Jar!), Optimal is the speed at which the cascading of the media does the most efficient grinding...........OK, so here we go........

For the math, the best place to start is at the tumbling drum and work backwards. Before we do though, we need to determine circumferences of the items that we will be using (2 x pi x r). The drum, with a 4.75" OD, has a circumference of 14.915 inches. The roller, being 1.5" OD will have a circumference of 4.71 inches. So, with this information we can calculate that the roller has to rotate 3.167 times for the tumbler drum to complete one revolution (14.915/4.71=3.167). So, if the drum needs to revolve at 92 rpm then we just multiply the roller number by 92 (3.167 x 92) to get 290 rpm at the roller. With me so far?

Okay, now that we know that the roller has to spin at 290 rpm to get the tumbler to spin at 92 we can move on. This is the easy part. If we know that the motor spins at 3450 rpm and we need to get it down to 290 at the roller then all you need to do is divide the motor rpm by the desired roller rpm (3450 rpm/290 rpm). The result is 11.98665.

This is our total reduction required 11.99:1. Since I'm using a one inch pulley on the motor, then I only need a 11.99", let's just call it a 12" pulley on the roller.
Now that we have the answer, let's work the problem in the other direction to see if we get the desired tumbler rpm.

3450/12=287.5

287.5*(2*3.14*.75)=1354.125 linear inches per minute

1354.125/(2*3.14*2.375)=90.79 rpm

So, it seems that our result of a 12" pulley is the correct answer (actually it is slightly larger than we really need to get exactly 92 rpm but it is easier to find at the local hardware store) Since we want to stay closer to the optimal speed rather that the critical speed, 2 Rpm slower is better than 2 rpm faster. Since the Motor I am using is a 2 HP motor, I am not worried about the Motor loading the speed down, if you are using fractional horsepower motors, then it may be wise to consider an 11" pulley as loading will have an effect on the RPM, try the equation using the 11" pulley just for kicks and see where you'd land, with it driving 10 Lbs or so of media and whatever else you are grinding, I'm sure it would be in the ball park.......Have fun!


Hope that helps, Stay Green,

Bear

[Rhodri]

Good post Mr Horse - good post!

(Hope you don't mind me calling you Mr Horse? )

Seems like you've got your process down to such a fine degree that Waltham Abbey will be shaking on its 'ghostly foundations'!

[BurlHorse]

I Thank you kind sir, your too kind Just thought that may be of use to some folks....I am working on some seperate formulas, Math, to dual drive from the same motor, the above formula's remain constant, but going from forward to reverse and on four rollers.....well it gets a bit dicey.....Perhaps I need to paint some of those hypnotic spirals on the jars as Bernie did to keep track of what direction is which....

Regards, Stay Green,

Bear

[Jerronimo]

I just received a reply from Don Edwards from tidewel.

The beachtumblers he sells have a rpm of 50rpm, a bit slow I think.
I'm planing to use solid brass barstock 15mm for my ballmill, is it better to make my own on 80 rpm or buy one from him at a very reasonable price?

[The_Djinn]

Jeronimo,

Whether you buy one or make one you will probably end up having to play with pulley sizes to achieve the correct or desired RPM.

There is an alternative method as getting pulleys the correct size etc can be a problem. You can use a smaller diameter jar which will increase the RPM or a larger jar which will lower the RPM.

You could purchase the tumbler in question and make up a polypropylene or PVC tube jar that has a smaller diameter than the one that comes with it.. hence an increased RPM.

Think about what is going to be the easier route for you to take.
At the end of the day you can still use the jar that comes with the tumble but you will have to increase the time you spend milling.

Mark

[PanMaster]

Thats not true Djinn . A larger barrel means a slow rpm, thus a slower time to process a load of powder. A larger barrel also means a lot more powder inside, so more can be made in the same time, making the big barrel as fast as the small one
I have found my beach tumbler rotates more rapidly when the barrel it is heavier.
You could also add more lead media to decrease the time taken. The trouble is it is very difficult to judge how well ground powder is when it all looks the same, microscope perhaps?

[phildunford]

I've always used the rule - 1/3 fill the barrel with compo and 1/2 fill it with media to work pretty well...

(to clarify, the amount of media should half fill the barrel on it's own and the compo should 1/3 fill on it's own)

[willd]

a small barrel means a fast rpm as as the numer of turns of the bar to turn the circumference of a large barel is much more than a small barrel as the cicumference is smaller

[Loci]

http://www.pyrosupplies.com/ has some larger Ball Mills, although based in the US, a couple weeks of saving and I might get one

Fraser

[Prodigy]

Well I've ordered one, supposed to arrive here on Monday/Tuesday. I'll report in on the quality after some testing

[The_Djinn]

Loci,

Just remember to select 220v power if you order off that web site and you will notice it pumps the price up by around $120.00... ?60-?70, praticaly what you can by a mill for here in the UK.

Mark

[BurlHorse]

Thats not true Djinn  . A larger barrel means a slow rpm, thus a slower time to process a load of powder. A larger barrel also means a lot more powder inside, so more can be made in the same time, making the big barrel as fast as the small one 
I have found my beach tumbler rotates more rapidly when the barrel it is heavier.
You could also add more lead media to decrease the time taken. The trouble is it is very difficult to judge how well ground powder is when it all looks the same, microscope perhaps?

Pan,

The Djinn is correct, and a microscope will prove that a mill operating at optimal speed will grind more efficiently with the resulting product being ten fold finer than running an improperly loaded mill for days.
Yes, you can make more powder in a larger milling jar , but at the end of the day or in the scenario you suggest, end of dayS, your not talking about milling efficiently. I do not know what rpm's you are using, but I do know that at any RPM the media heats up and the longer you mill, the hotter it gets. My 6" Milling jar is quite warm to the touch after about 4 hours milling time, after an hour of cool down before I approach it, and I am running it at the optimal speed for 1" hardened lead media. So to suggest that you can add more media and "Grind" it for an eternity, in an already inefficient arrangement is just a bit off when you consider all the facts.

I have used inefficient mills and they get way too warm for my personal taste....Were talking about friction here, and the slower you go, the media just slides in the milling jar, which builds up heat from friction, lifter bars or not. Conversely, the faster you go, upwards from the optimal speed, the balls centrifically lock to the outside of the jar....and really do nothing except build up heat and tear up the milling jar. While I suppose you could do what you are suggesting, If you insist on doing so, pre-processing the Seperate chemicals in a coffee grinder or the likes, may speed up the process so your mill does not have to go for days.....Even so, I'd keep the mill a long way from anyone, unplug from the far end and let it all cool for a few hours before you approach it......if it lasts that long.......

Rock tumblers are made to do just that, T-U-M-B-L-E rocks.

Regards, Stay Green,

Bear

[adamw]

Please dont let this post put anyone off using a 'rock tumbler' as a ball mill. Personally I find nothing wrong with them for milling small quantities. I use rubber barrels and have never noticed any heat even after 6+ hours milling.

Also, if we set about on a quest for the most efficient ball mill, the money and time spent would probably outweigh that lost due to a simple tumbler's 'inefficency'.

[Phoenix]

Don't forget that whilst a larger jar and larger charge is nice, it also makes the unlikely event of an accident much more serious. That's how I justify my puny little jar