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making grog--grinding devices

updated sat 4 sep 99

 

Dave Finkelnburg on tue 31 aug 99

Nikom,
You have met the enemy of hammer mills. They are simple and fast :-)
and make a lot of dust! :-(
First, you have to understand what happens when your material fractures.
Does it release so much fines on initially breaking, or only when fine
particles are ground even further? You could test this by simply welding
1/8-inch projections onto a steel plate, then rubbing it onto another steel
plate with some of your hammer-mill feed in between. If the product is
acceptable in size distribution, meaning the excessive fine fraction you are
getting now is formed by overgrinding, there are some things you might look
at.
There are two kinds of roller mills. In one kind the vertical rolls are
flung against the shell of the mill--these are most commonly used in
industry for fine grinding and are usually in a closed circuit with an air
classifier.
The other, which I suspect you are considering, consists of two
horizontal driven rolls turning towards each other. Feed falls onto the gap
between the two rolls. One roll is fixed, the other backed by powerful
springs against both bearings, so it can move away if it bites a piece which
is too hard to break. (Something has to give, better the spring than the
roller shaft!)
You can adjust the gap between two such rolls to prevent over-grinding.
This would work best with a product screen, where you would recycle oversize
back to the rolls and remove the undersize as product.
The diameter of the rolls determines the largest piece which can be fed
to the mill. The roll surface would benefit from being wear-resistant or it
will soon have an irregular gap. The rolls really need to be parallel at
the grinding surface to get uniform product size.
An alternative, which I don't recommend because it would probably be a
hassle, and low in capacity, would be to put rods in a ball mill (then it's
a rod mill) to get less fines. You have to control grind by controlling the
feed rate to the mill. The flood of material holds the rods apart and
reduces the formation of super-fine fraction, while breaking up the largest
pieces. This is usually preceded (in series) by a jaw crusher or other
device, which gets everything into a range so it can be crushed by the rods.
The initial crusher is sometimes in a screening circuit, with oversize
recycled back to the crushing device. You could also do wet grinding with a
rod mill, to control dust, and size with a spiral classifier, to regrind the
coarsest sand. As I said, probably too much hassle!
The crusher in front would be of benefit with your simple two-roller
mill also. It works best with feed sized somewhat.
Hope this is helpful!
Dave Finkelnburg
dfinkeln@cyberhighway.net


-----Original Message-----
From: Nikom Chimnok
To: CLAYART@LSV.UKY.EDU
Date: Thursday, August 26, 1999 10:00 AM
Subject: Making grog


----------------------------Original message----------------------------
Hello,
I am writing to pose the question, What is the superior machine for
grinding grog?

At present I am using a homemade hammermill, 5 HP, about 1800 RPM.
The screen has 1/8" holes, which makes a reasonable largest size, as these
are big rough pots. The problem is that 40-50% of the product is dust
smaller than 120 mesh, which I understand is the cutoff point.

I can get rid of the dust by screening, but this represents a
considerable waste of time and electricity, effectively doubling the price
of the grog and the time it takes to make it.

What machine will produce exactly the particle size I am
after--approximately an even mix in a range from 20 or 30 to 120 mesh? Would
a roller mill work better? I have Harry Davis' book, in which he describes
the construction, and it seems possible to make.

A ball mill doesn't seem like the answer. I have no experience with
jaw crushers, but do know they're formidably expensive. What machines do
grog-producing companies use? Are there any other alternatives?

Thanks in advance, if anyone can tell me anything.

Nikom in Thailand

Donn Buchfinck on wed 1 sep 99

ok this is how you can do it
get an old garbage disposal
hook it up
outside, you won't need water

get whatever you are going to make the grog out of it and do a very low bisque
020 or something, extrude hollow tubes of the clay that you can stack inside
an electric kiln.
the reason you grind the material when it is soft is that it breaks down much
easier.

when the tubs come out they will be fired but soft
run them through the garbage disposal, you will be amazes at how this works,
and how long the thing will last without water
then take the ground up material and put it into a bowl or some container, a
shallow dish and refire the stuff to a higher temperature.

there you have it grog
or go an buy some

Donn Buchfinck
San Francisco

Nikom Chimnok on wed 1 sep 99

Dave,
I have some evidence that the second case you mention is the true
one--the fine particles are getting overground. I tested this by replacing
my 1/8 inch screen with a 3/16. Undersized particles (<120 mesh) were
reduced from around 50% to around 25%. Big change! If I went to a 1/4 inch
screen, and the progression is linear, I shouldn't get any undersized
particles at all. I would, of course, have a lot of oversized, but these can
be reground.
I see now I should have measured the comparative flow rates, but I
can only imagine that the material will flow much faster through big holes
than it will through little ones. Right?
But some other factor is at work. David Hendley and I have been
corresponding off list because we thought no one else was interested, and he
has exactly the opposite problem with his International Harvester corn
grinder. His particles come out too big, and he has to throw them back in
for a second pass.
So before looking for alternatives grinders, I must say I am sure
there is something about hammermill design I do not understand. What is it?
I like our hammermill--it's totally homemade, uses no exotic alloys (we do
use Japanese bearings, since the cheap Chinese ones are such junk) and is
totally homemade, thus totally repairable by guys with 6th grade educations.

So if you can answer my last question, What don't I understand, I'm
sure you know I'll appreciate it.

Thanks,
Nikom
******************************************************************
At 11:54 31/8/99 EDT, you wrote:
>----------------------------Original message----------------------------
>Nikom,
> You have met the enemy of hammer mills. They are simple and fast :-)
>and make a lot of dust! :-(
> First, you have to understand what happens when your material fractures.
>Does it release so much fines on initially breaking, or only when fine
>particles are ground even further? You could test this by simply welding
>1/8-inch projections onto a steel plate, then rubbing it onto another steel
>plate with some of your hammer-mill feed in between. If the product is
>acceptable in size distribution, meaning the excessive fine fraction you are
>getting now is formed by overgrinding, there are some things you might look
>at.

Nikom Chimnok on fri 3 sep 99

Howdy Donn,

I want you to understand first that I appreciate your response and
the effort you have made in addressing my problem. But perhaps it will
tickle you to know how funny it sounds from where I am sitting.

It is not that we do not have garbage disposal units here: we just
call them by different names, such as dogs, cats, chickens, rats, and ants.

Second, the only electric kiln I own is 9X9X4 inches.

Third, I need to make a ton and a half of grog in one day.

Fourth, I have about fifteen tons of broken bisque lying around,
with which I can either surface a road or make grog.

And finally, they don't sell grog in this country.

So, while impressed by your innovativeness, I think the method you
advise is probably more appropriate for an artist who makes small numbers of
expensive pieces, than a factory that ships containerloads cheap. There's a
big difference between here and there. Old Mel (whose pots I admire
tremendously, by the way) recently made a comment about leaving ten $60 pots
out of the back of his kiln because they don't come out perfect there. If I
could make exact copies of those pots (not that I could, you understand)
they'd be worth about $8 each. If I want $60 for a pot, it has to be so big
that only 2 of them would fit in Mel's train kiln. Money changes everything
(didn't somebody famous say that?)

Best wishes,
Nikom in Thailand, waiting anxiously to see if 4 huge pots, made of 500
kilos (dry ingredient weight)of meticulously prepared clay are gonna crack
or not.
********************************************************************

At 14:11 1/9/99 EDT, you wrote:
>----------------------------Original message----------------------------
>ok this is how you can do it
>get an old garbage disposal
>hook it up
>outside, you won't need water
>
>get whatever you are going to make the grog out of it and do a very low bisque
>020 or something, extrude hollow tubes of the clay that you can stack inside
>an electric kiln.
>the reason you grind the material when it is soft is that it breaks down much
>easier.
>
>when the tubs come out they will be fired but soft
>run them through the garbage disposal, you will be amazes at how this works,
> and how long the thing will last without water
>then take the ground up material and put it into a bowl or some container, a
>shallow dish and refire the stuff to a higher temperature.
>
>there you have it grog
>or go an buy some
>
>Donn Buchfinck
>San Francisco
>
>