What it is the optimun speed for a ball mill ?

[whitewolf_573]

I have done a ball mill , recenly finished , but the motor has too rpms , is too fast for use in a ball mill (the pvc cylinder that i use , left of the shafts).

 

With the motor i will use a 40 mm pulley , because i have a 50 mm driven pulley, in one of my two shafts. In the other side of my shafts there are two 40 mm pulleys to transmit the movement from one shaft to the another one.

 

So , if the motor what i want to buy is about 600 rpms ,with the first reduction , i have 480 rpms, and there is another reduction , as the shafts are 10 mm of diameter , and my pvc cylinder 100 mm diameter , the final speed is 48.

 

It is about 50 rpms a good speed ?

 

A lot of people use 60 rpm or some more , if 50 is not enought i will change my driven pulley with another one of 40 mm or less to increase the speed.

 

Sorry for my bad English

[Frozentech]

I have done a ball mill , recenly finished , but the motor has too rpms , is too fast for use in a ball mill (the pvc cylinder that i use , left of the shafts).

 

With the motor i will use a 40 mm pulley , because i have a 50 mm driven pulley, in one of my two shafts. In the other side of my shafts there are two 40 mm pulleys to transmit the movement from one shaft to the another one.

 

So , if the motor what i want to buy is about 600 rpms ,with the first reduction , i have 480 rpms, and there is another reduction , as the shafts are 10 mm of diameter , and my pvc cylinder 100 mm diameter , the final speed is 48.

 

It is about 50 rpms a good speed ?

 

A lot of people use 60 rpm or some more , if 50 is not enought i will change my driven pulley with another one of 40 mm or less to increase the speed.

 

Sorry for my bad English

Calculation of optimum speed depends on knowing the media diameter and jar I.D. also.

For instance, if your jar had in inside diameter of 90 mm and your milling media was 12.7 mm diameter lead balls, the optimum rotation would be 98 RPM.

 

Optimum RPM= .65 x Critical speed (cascading action of the media stops)

 

Critical speed = 265.45/sqrt (Jar I.D. - Media O.D.)

 

with dimensions in inches.

[whitewolf_573]

Finaly my motor runs at 140 rpms (my motor has two velocities , and in one fails , and stops ,becuase gets hot a cause of that the ventilator don`t work , but in the height velocity , 2800 rpm , works propely)

 

Using pulleys i divide by 2 the velocity to 1400 rpm , but the container works at 140 rpms , because there is a reduction of 1/10 (the container spins one time each ten turns of the shafts) in the velocity.

 

The motor has been working for 30 minutes , and works good , and it is not hot ( The ventilator , in this velocity , works )

[FrankRizzo]

Congratulations on your ball mill!

 

You'll want to further reduce your motor speed by 3:1 to mill more efficiently. Right now, your RPM is just slightly under the speed where the media won't do any work. The optimum speed for your jar to turn is around 90 RPM like Frozentech said.

[techtwit]

I think there are other variables that enter the milling process that have as much or more effect on overall milling efficiency as drum speed. Consider:

 

Take an 8' length of 1X4, support it at each end and try to drive a large nail into it (approx. midway between the supports) with a 13oz. hammer. Now take that same 1X4, place it on terra firma with the ground in firm contact directly under the spot where you'll drive the same size nail and whack said nail with a 20oz. hammer. The results apply equally to ball mills.

 

Try this for a tumbler: Using 3/8" rebar and 10 ga. remesh, cast a cylindrical reinforced concrete ring about 14" high, 20" dia., over and around a core made from the bottom 10"- 12" of a plastic 5 gal.(US) paint bucket. This makes a very massive, non-springy tumbler with approx. 4"-5" thick walls and bottom. Cast bolts into the concrete to fasten a wooden (say 1-1/2" plywood) lid and cradle it using casters mounted on a sturdy steel or wood frame. It'd be a good idea to give it about a 30-40 degree back tilt. One or both of the bottom rollers could be powered by a 1/3 hp.– 1/2 hp. motor via belts and pulleys to yield a drum speed of 60 rpm. or less, per Frozentech's helpful optimum speed formula. Alternatively, a V-belt groove could be cast into the circumference of the drum, or, a flat belt, such as an automotive serpentine belt or a clothes dryer belt , could be used without a groove. Doing this would avoid the need for drive roller/s — using instead only easily obtained casters.

 

For media try this: Using a small muffin pan as a mould and wheel weights as alloy, cast as many "hammers" as desired. Their truncated cone shape will give them both a rolling and tumbling action. Wheel weights can usually be gotten for little-to-no cost from garages and tire dealerships, consist primarily of lead with some antimony for hardness, and if the "hammers" are water quenched as soon as they solidify in the muffin pan mould, they'll be considerably harder than 100% soft lead. One could try coating them with a thick layer of epoxy paint to prevent contamination.

 

This is an untried idea meant only to suggest a good direction to experiment in. The basic principle is that when it comes to any kind of pounding action, the more mass and the less vibration, bounce and spring, the better.

[Frozentech]

Wow, that would make some monster mill.

 

If you really want to learn every thing you will ever need to know about milling theory and practice, invest in a copy of Ball Milling Theory and Practice for the Amateur Pyrotechnician, by Lloyd Sponenburgh.

 

BTW, as far as rigidity of the mill jar, I have worked around some of the largest SAG (semi autogenous grinding) mills in the world, used to reduce lead and zinc ore to fine powder. They use heavy rubber liners (reminded me of chunks of truck tire about 100 lb each ) to cover the entire inside of the mill, with shot put and larger sized media balls of chrome manganese steel. Proper milling in a ball mill doesn't take place by *hammering*, rather by shearing action of a continuous cascade of media, imagine a perfect curling wave at the Banzai Pipeline, that never ends, with the powder and media being rolled endlessly.

[techtwit]

What you say about milling actually being a shearing process is intrigingjava script:emoticon(''). That might answer one question I've had about what happens when a thick cushy bed of pulverized material builds up to soften the impact of the media as they fall back into the bottom of the drum, namely, it doesn't matter. Are you saying that the drum bottom is not where the actual milling is taking place? Then again, when a particle (be it micron sized or boulder sized) is "crushed" into smaller particles, what is really happening? i.e. when something fractures, what process has occured? Shear failure? Tensile failure? And does Sponenburgh address this?

 

If we really understand the actual "mechanism/s" of milling then it's easier to go directly to a design that will achieve the intended purpose, rather than chasing down blind alleys and rabbit trails. This is what good engineering is all about.

 

Any further input you can offer from industrial milling operations would sure be helpful. Scaling things down to suit our particular needs shouldn't be a problem. provided we understand the basic principles involved. Thanks.

[Frozentech]

What you say about milling actually being a shearing process is intrigingjava script:emoticon(''). That might answer one question I've had about what happens when a thick cushy bed of pulverized material builds up to soften the impact of the media as they fall back into the bottom of the drum, namely, it doesn't matter. Are you saying that the drum bottom is not where the actual milling is taking place? Then again, when a particle (be it micron sized or boulder sized) is "crushed" into smaller particles, what is really happening? i.e. when something fractures, what process has occured? Shear failure? Tensile failure? And does Sponenburgh address this?

 

If we really understand the actual "mechanism/s" of milling then it's easier to go directly to a design that will achieve the intended purpose, rather than chasing down blind alleys and rabbit trails. This is what good engineering is all about.

 

Any further input you can offer from industrial milling operations would sure be helpful. Scaling things down to suit our particular needs shouldn't be a problem. provided we understand the basic principles involved. Thanks.

Well, as it explains quite well in Lloyd's book, the active grinding area runs from near the top of the 'wave' of cascading media almost to the bottom of the jar. Imagine in your earlier scenario, each ball of media gets one 'strike' against the comp at the bottom of the jar. In an ideally charged jar, at the optimum RPM, there are multiple strikes by each ball, against each other, with the comp of course pinched between, all the way down the slope of the cascade. It's a bit hard without pictures, but the balls rubbing against each other continuously is where the work is done, not them slamming the bottom of the jar. For brittle metals, a stamping action is effective though.

[techtwit]

So, now that we know where and how the milling action actually occurs, I guess the next issue is to determine the optimum diameter/length ratio for the drum. And speaking of ratios, what about media diameter/drum diameter?

 

I can imagine that Sponenburgh deals almost exclusively with industrial applications where economics, mainly required run time, is the primary concern. We don't have to place as much importance on that one parameter, but can throw other factors into the hopper as well, such as ease and cost of construction, what can be readily obtained, etc. This is one reason why I can't recommend reinforced concrete, aka ferro-cement, too much. It's a wonder material. If ease of portability isn't a concern then it has few or no disadvantages. At least for one-off construction — much like fiberglass in that regard. Fiberglass — Hmm, Ease of Portability — hmm, hmmm.

 

Getting back to rigidity. I can think of no ordinary cutting, grinding, shearing, extruding, fracturing, or other shape/form altering process in metal, wood, or plastic where flex and vibration is an advantage. If it's present, it's only because it has to be tolerated in the cases where economics or light weight take precedence. As you've described ball milling, there's no apparent disadvantage to allowing flex and vibration, yet there might be an advantage/s to eliminating it nevertheless, provided there's no disadvantage to the resultant massiveness.

Oh — a quick apology for my US-provincial reference to a "muffin pan": Shouldn't have assumed folks outside these shores would generally know what such an item is or looks like.

Thanks.

[FrankRizzo]

So, now that we know where and how the milling action actually occurs, I guess the next issue is to determine the optimum diameter/length ratio for the drum. And speaking of ratios, what about media diameter/drum diameter?

 

I can imagine that Sponenburgh deals almost exclusively with industrial applications where economics, mainly required run time, is the primary concern. We don't have to place as much importance on that one parameter, but can throw other factors into the hopper as well, such as ease and cost of construction, what can be readily obtained, etc. This is one reason why I can't recommend reinforced concrete, aka ferro-cement, too much. It's a wonder material. If ease of portability isn't a concern then it has few or no disadvantages. At least for one-off construction — much like fiberglass in that regard. Fiberglass — Hmm, Ease of Portability — hmm, hmmm.

 

Getting back to rigidity. I can think of no ordinary cutting, grinding, shearing, extruding, fracturing, or other shape/form altering process in metal, wood, or plastic where flex and vibration is an advantage. If it's present, it's only because it has to be tolerated in the cases where economics or light weight take precedence. As you've described ball milling, there's no apparent disadvantage to allowing flex and vibration, yet there might be an advantage/s to eliminating it nevertheless, provided there's no disadvantage to the resultant massiveness.

Oh — a quick apology for my US-provincial reference to a "muffin pan": Shouldn't have assumed folks outside these shores would generally know what such an item is or looks like.

Thanks.

tech,

 

You *really* need to read Lloyd's book; you're not giving him any credit at all. The fellow has spent a significant portion of his career doing mechanical engineering work for Santore & Sons in Florida. He gives all the theory you need to understand what's going on inside an optimal milling jar, and notes a few proven and very inexpensive designs for same.

 

Anyone too poor to buy the book can read all the info they need from Lloyd's posts on rec.pyrotechnics as well.

[techtwit]

OK, OK,OK I'm getting it — the book too. Until it comes in and I've read it I'll shuddup. Thanks for the needed extra little shove!