michael cottrell on sat 20 nov 99
------------------
I am constructing a ball mill, but there are just a few things I need to =
know
before I get too far along, maybe someone can help
1: Is there an optimal speed for the rotation of the jar? if so what is it? =
and
how fast do the roller shafts need to rotate.
2:how fast do commercially constructed mills rotate, and are they on a =
rheostat?
3:how far apart should the roller shafts be?
luckily I know a guy who is a mechanical engineer to help me out, I will let=
you
know how it turns out, if its a smashing success, I will post plans on my
website
thanks-------Michael
Michael Cottrell
Black Diamond Pottery, Asheville North Carolina
http://www.crosswinds.net/=7Eblackdiamondpots
email: blackdiamondpots=40crosswinds.net
Gavin Stairs on sun 21 nov 99
At 05:06 PM 20/11/99 , you wrote:
>----------------------------Original message----------------------------
>------------------
>I am constructing a ball mill, but there are just a few things I need to know
>before I get too far along, maybe someone can help
>1: Is there an optimal speed for the rotation of the jar? if so what is
>it? and
>how fast do the roller shafts need to rotate.
Yes. Just a touch less than the speed at which the material begins to
free-fall. ( The critical speed can be calculated. If the pebbles free
fall, they tend to break, but that is when the greatest impact action takes
place. Just under the critical speed, the pebbles cascade down the slope,
and that is when the greatest wiping action occurs. If the speed is too
great, the pebbles all end up around the periphery, like on the spin cycle
of a washing machine, and there is no abrasive action at all. Below the
critical speed, the abrasion diminishes progressively as the rotation speed
lessens.) Depends on the diameter of the jar, the diameter of the
rollers. Your mechanical engineer friend should be able to calculate this.
>2:how fast do commercially constructed mills rotate, and are they on a
>rheostat?
They vary quite a lot, because the small ones are not optimally designed,
especially if they are meant to accept different sized barrels. Some have
speed control.
>3:how far apart should the roller shafts be?
Depends on the size of the barrel. The largest barrel should be stably
supported, and the smallest should not get jammed between. If you need
more range, let the idler roll be moveable.
>luckily I know a guy who is a mechanical engineer to help me out, I will
>let you
>know how it turns out, if its a smashing success, I will post plans on my
>website
>thanks-------Michael
>
>Michael Cottrell
>Black Diamond Pottery, Asheville North Carolina
>http://www.crosswinds.net/~blackdiamondpots
>email: blackdiamondpots@crosswinds.net
Gavin
Don Prey on sun 21 nov 99
In a message dated 11/20/99 10:07:39 PM, mcottrel@warren-wilson.edu writes:
<< I am constructing a ball mill, but there are just a few things I need to
know
before I get too far along, maybe someone can help
1: Is there an optimal speed for the rotation of the jar? if so what is it?
and
how fast do the roller shafts need to rotate.
2:how fast do commercially constructed mills rotate, and are they on a
rheostat?
3:how far apart should the roller shafts be?
luckily I know a guy who is a mechanical engineer to help me out, I will let
you
know how it turns out, if its a smashing success, I will post plans on my
website
thanks-------Michael
>>
Michael, when I constructed a small ball mill a few years ago I followed the
advice given in a book titled "Glazes from Natural Sources" by Brian
Sutherland. He gives the following jar rotation speed equation: revolutions
per minute equals 54.19 / sq. root of R times 0.64 and 0.87. R is the
internal radius of the jar in inches. The two factors 0.64 and 0.87
establish the lower and upper operating limits.......so there is some
tolerance for variation here. I used two quart jars with R equal to about 6
and things workd out ok. More good information in this book if you can find
a copy.... it was published in 1987 by B. T. Batsford Ltd, London, ISBN 0
7134 4204 2.
Don Prey in Oregon
P. S. as a check on the form of the equation: an R of 12 gives rpms of
roughly 35 and 48 (I'm reading from his graph)
Hank Murrow on sun 21 nov 99
>----------------------------Original message----------------------------
>------------------
>I am constructing a ball mill, but there are just a few things I need to know
>before I get too far along, maybe someone can help
>1: Is there an optimal speed for the rotation of the jar? if so what is
>it? and
>how fast do the roller shafts need to rotate.
>2:how fast do commercially constructed mills rotate, and are they on a
>rheostat?
>3:how far apart should the roller shafts be?
>luckily I know a guy who is a mechanical engineer to help me out, I will
>let you
>know how it turns out, if its a smashing success, I will post plans on my
>website
>thanks-------Michael
Dear Michael; There are some excellent remarks about ball mills in Appendix
7 of Cardew's "Pioneer Pottery". Here he discusses 'critical speed' among
other matters. I found that the centers of the roller shafts need to be
almost as wide as the diameter of the jar. It's a good idea to use a DC
motor and a speed controller to fine tune the jar speed. I assembled my
drive system from components bought at Burden's Surplus in Nebraska. Drives
a three gallon jar beautifully to grind my natural dug porcelain body. Got
my jar from Seattle Pottery Supply. Want to see yours, Hank in Eugene
Ray Aldridge on sun 21 nov 99
At 05:06 PM 11/20/99 EST, you wrote:
>----------------------------Original message----------------------------
>------------------
>I am constructing a ball mill, but there are just a few things I need to know
>before I get too far along, maybe someone can help
>1: Is there an optimal speed for the rotation of the jar?
Yes. The right speed is critical to the success of a ball mill. Too fast
and the charge just spins around the outside of the jar. Too slow and the
charge just slides. You want a cascade.
The right speed is a function of the jar's internal diameter. According to
Cardew, the right speed is 64 to 87% of critical speed, which can be found
by using this formula:
Critical speed in rpm=54.19 divided by the square root of R (when R is the
internal radius expressed in feet)
Hope this helps, and that I've transcribed Cardew accurately.
Ray
Aldridge Porcelain and Stoneware
http://www.goodpots.com
David Hendley on sun 21 nov 99
I've never constructed a ball mill, but I can tell you
that the rotation speed is Critical.
The speed (RPM) depends on the size of the jar.
I've never seen a ball mill that operates with a rheostat.
The correct speed is achieved through speed reduction
via a gearbox and/or pulleys.
How far apart the roller shafts are depends on the size
of the jar.
There are 2 excellent references for determining the critical
speed for a ball mill:
'Pioneer Pottery' by Michael Cardew
'The Potter's Alternative' by Harry Davis
Unfortunately, both books are out-of-print.
If you can't find them, I, or some other kind soul
from Clayart, would probably copy the relevant
pages for you.
--
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com/
----- Original Message -----
From: michael cottrell
To:
Sent: Saturday, November 20, 1999 4:06 PM
Subject: Ball mill questions
----------------------------Original message----------------------------
------------------
I am constructing a ball mill, but there are just a few things I need to
know
before I get too far along, maybe someone can help
1: Is there an optimal speed for the rotation of the jar? if so what is it?
and
how fast do the roller shafts need to rotate.
2:how fast do commercially constructed mills rotate, and are they on a
rheostat?
3:how far apart should the roller shafts be?
luckily I know a guy who is a mechanical engineer to help me out, I will let
you
know how it turns out, if its a smashing success, I will post plans on my
website
thanks-------Michael
Michael Cottrell
Black Diamond Pottery, Asheville North Carolina
http://www.crosswinds.net/~blackdiamondpots
email: blackdiamondpots@crosswinds.net
Dave Finkelnburg on sun 21 nov 99
Michael,
Can't answer your first question without knowing the inside diameter of
the jar. However, with the following you can answer the question yourself.
The aceptable speed (revolutions per minute) for any ball mill (or
pebble mill) depends on the diameter. As I'm sure you realize, you need to
keep the balls climbing up the wall of the mill, then tumbling down. If the
mill goes too fast, called "centrifuging," centrifugal force just holds the
balls against the outside of the mill and they don't tumble, hence don't do
any grinding or mixing.
Per Joseph Newton, in his, "Introduction to Metallurgy," the minimum
speed at which a mill charge will "centrifuge" can be calculated by the
formula, N = 54.19/square root of R, where N is the critical speed in
revolutions per minute, and R is the radius of the mill in feet. Newton
adds most commercial mills, "are operated at speeds ranging from 50 to 80
per cent of the theoretical critical speed."
My apologies for not taking the time to convert this formula to a more
convenient measure, like inches or centimeters, but I am sure you can do
that. I hope this is helpful. Look forward to seeing the plans for your
"smashing" success!
Dave Finkelnburg
Idaho Fire Pottery
dfinkeln@cyberhighway.net
-----Original Message-----
From: michael cottrell
To: CLAYART@LSV.UKY.EDU
Date: Saturday, November 20, 1999 3:08 PM
Subject: Ball mill questions
----------------------------Original message----------------------------
------------------
I am constructing a ball mill, but there are just a few things I need to
know
before I get too far along, maybe someone can help
1: Is there an optimal speed for the rotation of the jar? if so what is it?
and
how fast do the roller shafts need to rotate.
2:how fast do commercially constructed mills rotate, and are they on a
rheostat?
3:how far apart should the roller shafts be?
luckily I know a guy who is a mechanical engineer to help me out, I will let
you
know how it turns out, if its a smashing success, I will post plans on my
website
thanks-------Michael
Michael Cottrell
Black Diamond Pottery, Asheville North Carolina
http://www.crosswinds.net/~blackdiamondpots
email: blackdiamondpots@crosswinds.net
Paul Stubbs on sun 21 nov 99
In message <011801bf32f7$68230660$62a156c6@michael.warren-wilson.edu>,
michael cottrell writes
>----------------------------Original message----------------------------
>------------------
>I am constructing a ball mill, but there are just a few things I need to know
>before I get too far along, maybe someone can help
>1: Is there an optimal speed for the rotation of the jar? if so what is it? and
>how fast do the roller shafts need to rotate.
>2:how fast do commercially constructed mills rotate, and are they on a
>rheostat?
>3:how far apart should the roller shafts be?
>luckily I know a guy who is a mechanical engineer to help me out, I will let
>you
>know how it turns out, if its a smashing success, I will post plans on my
>website
>thanks-------Michael
>
>Michael Cottrell
>Black Diamond Pottery, Asheville North Carolina
>http://www.crosswinds.net/~blackdiamondpots
>email: blackdiamondpots@crosswinds.net
The best advice I can give is to refer you to Michael Cardews "A Pioneer
Potter" And " The Potters Alternative" Harry Davis " Both essential
reading for the serious equipment-builder.John Harlow in the UK has a
good simple design it utilises a polythene drum that drops into a
cradle. If you want to email him for full details let me know.
I built a conventional style BM 20 odd years ago and used rubber
rollers from an old washing mangle running in plain plumber block
bearings and driven by a chain drive from a small lawn mower, noisy! The
jar is made from a strong stoneware body and the neck is sealed with a
child's plastic ball. Its still going strong.
A cautionary note, if you do get someone to help fabricate some parts
for you stay in charge of the project, some engineers have this notion
that they always know best.
--
Paul Stubbs
Ray Aldridge on mon 22 nov 99
At 04:32 PM 11/21/99 EST, you wrote:
>
>Michael, when I constructed a small ball mill a few years ago I followed the
>advice given in a book titled "Glazes from Natural Sources" by Brian
>Sutherland. He gives the following jar rotation speed equation: revolutions
>per minute equals 54.19 / sq. root of R times 0.64 and 0.87. R is the
>internal radius of the jar in inches. The two factors 0.64 and 0.87
>establish the lower and upper operating limits.......so there is some
>tolerance for variation here. I used two quart jars with R equal to about 6
>and things workd out ok. More good information in this book if you can find
>a copy.... it was published in 1987 by B. T. Batsford Ltd, London, ISBN 0
>7134 4204 2.
>Don Prey in Oregon
>P. S. as a check on the form of the equation: an R of 12 gives rpms of
>roughly 35 and 48 (I'm reading from his graph)
>
It's always good to do the arithmetic and see if it makes sense. In this
case it doesn't, because R is actually supposed to be the internal radius
expressed in *feet.* Using inches, the maximum speed would be about 14 rpm.
Ray
Aldridge Porcelain and Stoneware
http://www.goodpots.com
Don Prey on tue 23 nov 99
In a message dated 11/22/99 3:48:34 PM, pbwriter@fwb.gulf.net writes:
<< It's always good to do the arithmetic and see if it makes sense. In this
case it doesn't, because R is actually supposed to be the internal radius
expressed in *feet.* Using inches, the maximum speed would be about 14 rpm.
Ray >>
Ray is right, of course. My confusion arose because the graph I was looking
at was rpm vs. radius in inches. The graphed data is correct, but as Ray
says, the dimension of R in the equation is "feet". Sorry for any confusion.
Don Prey in Oregon
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