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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work.

vacuum base for bell jar



 
 
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  #1  
Old May 16th 05, 10:40 PM
Chuck Sherwood
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Default vacuum base for bell jar

OK, I got a vacuum pump and I found a 1/2 gallon bell jaw at Fisher
for $42. I didn't see a base so I am wondering about making one.

I figure an aluminum plate would work fine. How thick of a plate
do I need for a 1/2 gallon bell jar? Is 3/8 enough or do I need
to go to 1/2 ?

thanks
chuck
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  #3  
Old May 17th 05, 01:41 AM
Leo Lichtman
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"Chuck Sherwood" wrote: (clip) How thick of a plate do I need for a 1/2
gallon bell jar? (clip)
^^^^^^^^^^^^^
Depends. If the bell jar is an inch in diameter and several feet tall, the
aluminum plate would have to support only 11.5 lb, so it could be pretty
thin G.


  #4  
Old May 17th 05, 02:17 AM
Tom Gardner
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If the 3/8 plate sucks up into the bell jar.....go to 1/2"

I should think the .375 would do the job.


Thus, the birth of vacuum forming!


  #5  
Old May 17th 05, 02:29 AM
jim rozen
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In article , Leo
Lichtman says...

... If the bell jar is an inch in diameter and several feet tall, the
aluminum plate would have to support only 11.5 lb, so it could be pretty
thin G.


But if it were a foot in diameter, then the force at the
center of the plate would be about 1600 pounds or so. The
height of the jar does not matter.

FWIW a commercial system might have a one foot dia bell jar
and a half inch thick stainless baseplate. The baseplate
has to be rigid enough to not oilcan excessively, because
the innards of the thing being pumped down are bolted to
it.

Jim


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  #6  
Old May 17th 05, 06:11 AM
Leo Lichtman
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"jim rozen" wrote: But if it were a foot in diameter, then the force at
the center of the plate would be about 1600 pounds or so. The height of the
jar does not matter. (clip)
^^^^^^^^^^^^
We're just kidding around, aren't we? So. I will point out that the OP
specified that the bell jar is about 1/2 gallon. So, if it is a foot in
diameter, it will be about 2" tall. Your force calculation gets the correct
number, but the force will not be at the *center* of the base--it will be
uniformly distributed over the entire area.

The only point I was trying to make in my facetious posting was that the
required thickness of the base cannot be specified unless the diameter is
known.

As a practical matter, a 1/2 gallon bell jar is probably about 6" in
diameter, so it will have a force on it of about 400 lb. I would make the
base out of whatever is available in the range of 1/4" to 3/4", just based
on two things: 1.) You don't want it to "oilcan". 2.) You don't want it
to be too heavy to handle easily.


  #8  
Old May 17th 05, 07:56 AM
Cydrome Leader
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Chuck Sherwood wrote:
OK, I got a vacuum pump and I found a 1/2 gallon bell jaw at Fisher
for $42. I didn't see a base so I am wondering about making one.

I figure an aluminum plate would work fine. How thick of a plate
do I need for a 1/2 gallon bell jar? Is 3/8 enough or do I need
to go to 1/2 ?


I'd go with steel of 1/2" or thicker simply because I've never seen the base of a bell jar made out of anythign that wasn't thick steel. Al might flex and break your seal or chip the bell jar.
  #9  
Old May 17th 05, 01:04 PM
jim rozen
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In article , Leo
Lichtman says...

Your force calculation gets the correct
number, but the force will not be at the *center* of the base--it will be
uniformly distributed over the entire area.


Quite right, of course. I quite the entire force number often
because it impresses folks, and makes them respect bell jars
a bit more. The idea that it's a glass jar with two tons of
force involved somewhere in there makes it easier to convince
them that it has to be covered in wire mesh.

The only point I was trying to make in my facetious posting was that the

required thickness of the base cannot be specified unless the diameter is
known.


Ah, got it!

As a practical matter, a 1/2 gallon bell jar is probably about 6" in
diameter, so it will have a force on it of about 400 lb. I would make the
base out of whatever is available in the range of 1/4" to 3/4", just based
on two things: 1.) You don't want it to "oilcan". 2.) You don't want it
to be too heavy to handle easily.


Sounds good to me.

Jim


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==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================
  #10  
Old May 17th 05, 03:13 PM
Chuck Sherwood
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Default

OK, I got a vacuum pump and I found a 1/2 gallon bell jaw at Fisher
for $42. I didn't see a base so I am wondering about making one.


FYI, the bell jar is 6-7 inchs in diameter.
thanks for the input. I will find some think plate.
 




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