ivor & olive lewis on wed 16 sep 09
Twenty five years ago I was asked to run satellite classes for the regional
college of technical and further education. Clay work and pottery.
Knowing there was a pug mill available I asked if I could have the use of i=
t
for a term. and was told it did not really do anything.
When I got back to my classroom it set to work to pug some soft clay noting
that the lever of the pressing plate was bent.
The clay went in but nothing happened until I pressed the plate into the
hopper. Then it was a struggle to get a good output rate. Only when the
plate was pressed hard would clay emerge from the orifice.
Went home and thought about things.
Back in the 1960's I had worked with the maintenance Engineer of Associated
Chrome and Chemical Co (Yarm, N. Riding, Yorks.) on screw auger problems.
These were used to transport graded ore. Then, I knew the solution to the
non transporting of the clay through the pug.
The following day I stripped the machine down and reset the auger flights.
Assembled the machine. Never had to use the pressure plate again. The
adjusted flights sliced into the clay and drove it into the chamber as fast
as I could feed the stuff . This machine was an original Venco, made before
the introduction of vacuum pumps. Didn't need one, you could hear air
bubbles explode from the clay as pressure was exerted by the screw flights.
So, if any of you are having to use that handle to drive your clay through
your machine consider having the auger flights tuned.
Best regards,
Ivor Lewis,
Redhill,
South Australia
Vince Pitelka on wed 16 sep 09
Ivor Lewis wrote:
"So, if any of you are having to use that handle to drive your clay through
your machine consider having the auger flights tuned."
Ivor -
This conforms to the instructions provided by Harry Davis for building his
deairing pugmill design. When I built mine twenty five years ago, I paid
special care to the angle and tilt of the auger blades (what you refer to a=
s
the flights). In that case, the angle is slightly greater after the vacuum
zone, in order to move the clay slightly faster. That keeps the clay open
and loose in the vacuum zone after it passes through the shredder screens,
along the vacuum to affect all of the clay. Also, the blades are all
slightly tilted towards the extrusion end, which focuses a little more
effort on pushing the clay through the mill, and less on pushing the clay
against the outside walls of the barrel. The latter is very important as
well, because it is the friction with the barrel holding the clay back that
makes a pugmill work. That's why the blades must not come too close to the
inner surface of the barrel. If they skim the clay off the inside of the
barrel, the whole mass of the clay turns as one big cylinder and the pugmil=
l
does nothing. There must be some clearance to allow clay to stick to the
inside of the barrel, providing the friction and shearing required for
proper operation.
Harry Davis was adamant about how the machine would not work if the blade
clearance and angles were not exactly right. Over the years, I have
provided advice to a number of people building the Harry Davis deairing
pugmill. Some of those pugmills did not work properly initially, and the
problem was usually traced to the blade-mount angles. Once that was fixed,
the machines worked fine.
When I was hired at NDSU in Fargo, ND, the Walker pugmill we used for
processing our clay was very sluggish. I stripped it down and tweaked the
blades a bit to make the angle uniform throughout, and it doubled the
efficiency of the machine.
And you are right about the self-feeding. When I am pugging clay of an
appropriate consistency, I have only to toss chunks of clay into the hopper=
,
and the machine gobbles them up with no use of the plunger.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka
Larry Kruzan on wed 16 sep 09
Hi, many thanks to Ivor and Vince for at last explaining just why the littl=
e
Bailey I had was so much manual labor. I always felt that it was a pretty
good design but it just didn't work. I hope that the good folks there might
take a look at this since it would be such a simple fix to improve their
product.
Larry Kruzan
Lost Creek Pottery
www.lostcreekpottery.com
-----Original Message-----
From: Clayart [mailto:Clayart@LSV.CERAMICS.ORG] On Behalf Of Vince Pitelka
Sent: Wednesday, September 16, 2009 7:30 PM
To: Clayart@LSV.CERAMICS.ORG
Subject: Re: : Pugmills - Random thoughts about what is best
Ivor Lewis wrote:
"So, if any of you are having to use that handle to drive your clay through
your machine consider having the auger flights tuned."
Ivor -
This conforms to the instructions provided by Harry Davis for building his
deairing pugmill design. When I built mine twenty five years ago, I paid
special care to the angle and tilt of the auger blades (what you refer to a=
s
the flights). In that case, the angle is slightly greater after the vacuum
zone, in order to move the clay slightly faster. That keeps the clay open
and loose in the vacuum zone after it passes through the shredder screens,
along the vacuum to affect all of the clay. Also, the blades are all
slightly tilted towards the extrusion end, which focuses a little more
effort on pushing the clay through the mill, and less on pushing the clay
against the outside walls of the barrel. The latter is very important as
well, because it is the friction with the barrel holding the clay back that
makes a pugmill work. That's why the blades must not come too close to the
inner surface of the barrel. If they skim the clay off the inside of the
barrel, the whole mass of the clay turns as one big cylinder and the pugmil=
l
does nothing. There must be some clearance to allow clay to stick to the
inside of the barrel, providing the friction and shearing required for
proper operation.
Harry Davis was adamant about how the machine would not work if the blade
clearance and angles were not exactly right. Over the years, I have
provided advice to a number of people building the Harry Davis deairing
pugmill. Some of those pugmills did not work properly initially, and the
problem was usually traced to the blade-mount angles. Once that was fixed,
the machines worked fine.
When I was hired at NDSU in Fargo, ND, the Walker pugmill we used for
processing our clay was very sluggish. I stripped it down and tweaked the
blades a bit to make the angle uniform throughout, and it doubled the
efficiency of the machine.
And you are right about the self-feeding. When I am pugging clay of an
appropriate consistency, I have only to toss chunks of clay into the hopper=
,
and the machine gobbles them up with no use of the plunger.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka
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