From what I have been able to find, a vacuum of 10 inches should be
sufficient for a vacuum chuck.
What I can't seem to find is the minimum cfm needed.

Used pumps are fairly easy to find, but I'm not sure what is the minimum
size I need.

Dave in Cherrylog, GA

"Dave W" wrote: (clip)What I can't seem to find is the minimum cfm
needed.(clip)
^^^^^^^^^^^^^^
The capacity of the pump affects two things:
1.) How fast does the system reach a usable vacuum?
b.) How much leakage will the system tolerate, and still work?

If you use a pump with more than enough capacity, you will be able to turn
wood without spending lots of time sealing leaks. You will have a bleed
valve, which will usually be partly open, so you don't get TOO MUCH vacuum.

If you go for a pump with very little volume flow, the slightest leak will
get you in trouble. I say there is no lower limit, but I advise you to get
the biggest pump you can find and afford. Mine is a 3/4 HP Gast, and I am
very happy. (It's a bit noisy, though.)

Dave I built Vacuum System for my Lathe and the details are on my Web
Site
where there is a two part article in the Hints and Tips Section.

http://www.laymar-crafts.co.uk

The cfm of the Vacuum pump in reality only needs to overcome the amount
of leakage you may get on the system, but this is difficult to predict
as seapage through certain woods can occur.

Best advise is to make the bit you make as Air Tight as possible, I
used Vacuum Grease on all Joints and this works well providing joints
are not to Loose/Wide.

The other area where cfm has an influence is in the length of time it
will take to Evaacuate the Volume between the Pump and the Base of the
Piece you are holding.

If you assume the system has no leaks then a 4 cfm Pump would take
approximately 1 minute to empty 4cu ft of System which is an enormous
Volume, my Pump is rated at 2.4cfm and takes less than 1 second to pull
a 26 inch Vacumm.

This is for the System + any Leaks.

Hope this Helps

Richard

Dave W wrote:
From what I have been able to find, a vacuum of 10 inches should be
sufficient for a vacuum chuck.
What I can't seem to find is the minimum cfm needed.

Used pumps are fairly easy to find, but I'm not sure what is the
minimum
size I need.

Dave in Cherrylog, GA

dave - 2 to 3 cfm is about right. More is better. See my article on vacuum
chucks/pumps either on my web (www.wbnoble.com) or in the tips section of
www.woodturners.org




"Dave W" wrote in message
...
From what I have been able to find, a vacuum of 10 inches should be
sufficient for a vacuum chuck.
What I can't seem to find is the minimum cfm needed.

Used pumps are fairly easy to find, but I'm not sure what is the minimum
size I need.

Dave in Cherrylog, GA

"Dave W" wrote in message
...
From what I have been able to find, a vacuum of 10 inches should be
sufficient for a vacuum chuck.
What I can't seem to find is the minimum cfm needed.

Used pumps are fairly easy to find, but I'm not sure what is the minimum
size I need.

Dave in Cherrylog, GA
==================

If you're a fairly handy "do it yourself" type, you can build a good system
for little cash outlay. Find a wrecking yard that has some 60's and 70's
cars left. Find one that has a 2 cyclinder air conditioner "V" style
compressor (York). These are a preferred style since they can be easily set
up vertically and be stable. Take the clutch assy apart, and use epoxy to
fill it, or braze it to make it a solid assembly. Use a metal bracket to
mount it, using a 1/4 HP or larger electric motor to turn it. Unless you
will be turning something very porous, you will need a bleeder valve to
reduce pressure. Too much vacuum will implode a platter or large bowl.
Rotary vacuum couplers are available thru the usual wood turning suppliers.
Cover the faceplate with a closed cell foam rubber (like a wet suit). A
vacuum gauge placed between compressor and lathe is a "nice to have "
accessory that allows you to monitor your system. An in-line air filter will
assure a clean running compressor.

Ken Moon
Webberville, TX

if you do as keith suggests, be sure to run some oil through the pump every
few days, AC pumps depend on oil in the working fluid, and there is none in
the air. Also, be sure to add a filter
"Ken Moon" wrote in message
ink.net...

"Dave W" wrote in message
...
From what I have been able to find, a vacuum of 10 inches should be
sufficient for a vacuum chuck.
What I can't seem to find is the minimum cfm needed.

Used pumps are fairly easy to find, but I'm not sure what is the minimum
size I need.

Dave in Cherrylog, GA
==================

If you're a fairly handy "do it yourself" type, you can build a good
system
for little cash outlay. Find a wrecking yard that has some 60's and 70's
cars left. Find one that has a 2 cyclinder air conditioner "V" style
compressor (York). These are a preferred style since they can be easily
set
up vertically and be stable. Take the clutch assy apart, and use epoxy to
fill it, or braze it to make it a solid assembly. Use a metal bracket to
mount it, using a 1/4 HP or larger electric motor to turn it. Unless you
will be turning something very porous, you will need a bleeder valve to
reduce pressure. Too much vacuum will implode a platter or large bowl.
Rotary vacuum couplers are available thru the usual wood turning
suppliers.
Cover the faceplate with a closed cell foam rubber (like a wet suit). A
vacuum gauge placed between compressor and lathe is a "nice to have "
accessory that allows you to monitor your system. An in-line air filter
will
assure a clean running compressor.

Ken Moon
Webberville, TX

Dave W wrote:
From what I have been able to find, a vacuum of 10 inches should be
sufficient for a vacuum chuck.
What I can't seem to find is the minimum cfm needed.

Used pumps are fairly easy to find, but I'm not sure what is the minimum
size I need.

Dave in Cherrylog, GA



A smaller passage between the chuck and pump is better than a larger
one, since there's less air to evacuate and you don't have to take into
consideration air flow as with a compressor. With this in mind, cfm may
not be as important as you think--an efficient low cfm pump can be
better than an inefficient hi cfm one.
My 1/4 hp Gast pump has fairly low cfm rating, but it's only a matter of
waiting a few seconds more to reach max vacuum than say, a 3/4 hp pump.

BTW, a good source for closed-cell foam rubber seal material: 2 mm thick
craft foam in 9 x 12 sheets, with adhesive backing. Available at craft
stores and Walfart for about 60 cents each, many choices of colors. Also
great for jam chucks. Moldable around compound curves (doesn't buckle).


Ken Grunke
rural La Farge, WI
http://www.token.crwoodturner.com/

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

allow me to most humbly disagree. a small passage means high reynolds #
loss and inefficiency. A small pump, however "efficient" (whatever that
means) has some maximum flow rate. if you turn a thin walled vessel, or
have any irregularities, warpage, etc, you will have leakage. The small
pump will at some level of leakage, have inadequate flow and will be unable
to obtain sufficient vacuum to reliably hold the piece. Having a small
orofice like you suggest will aggrevate the problem.

Your 1/4 hp gast pump actually has a pretty good flow rating, refer to
http://www.gastmfg.com/pdf/rotvane/r...ne_catalog.pdf
to see the specifications for your pump - you didn't specify the model. The
0323 series, for example, has a zero pressure flow of around 3 CFM. I use a
3/4 hp pump that has a 0 pressure flow of about 10 cfm - I find it is far
easier to hold objects that have leakage from cracks, worm holes, or just
the pores of the wood. 3 CFM is adequate, but more is better. If you will
only turn large items (a foot in diameter or more) a shop vac is adequate
and has lots and lots of flow.

"but it's only a matter of waiting a few seconds more to reach max vacuum
than say, a 3/4 hp pump" - this is just plain wrong under many
circumstances. It is true if there is zero leakage. It is probably false
if the total leakage approaches 1 or 2 CFM or so -- referring to the curves
in the manual whose link I provided above, you can see that at a flow of 2
CFM your pump can develop a max of 10 inches of vac, whereas at a flow of 2
cfm leakage, the 1023 series can develop 20 inches or more. You could amuse
yourself by writing out the differential equations for this if you like,
from the provided curves, they are linear in the operational region of
interest, but I don't think it's worth the effort to do that.

I would say, based on experience and tests using a variety of pumps, that a
open or zero pressure flow of less than 2.5 to 3 CFM means that the pump
will be marginal as a vacuum chuck - it may be totally fine for many other
purposes, for example evacuating molds, AC systems, etc, but not for wood
turning. A flow of much over 10 CFM is also probably not needed, if you
have that much leakage, the losses elsewhere in the system will prevent it
from working.

"Ken Grunke" wrote in message
...
Dave W wrote:
From what I have been able to find, a vacuum of 10 inches should be
sufficient for a vacuum chuck.
What I can't seem to find is the minimum cfm needed.

Used pumps are fairly easy to find, but I'm not sure what is the minimum
size I need.

Dave in Cherrylog, GA



A smaller passage between the chuck and pump is better than a larger
one, since there's less air to evacuate and you don't have to take into
consideration air flow as with a compressor. With this in mind, cfm may
not be as important as you think--an efficient low cfm pump can be
better than an inefficient hi cfm one.
My 1/4 hp Gast pump has fairly low cfm rating, but it's only a matter of
waiting a few seconds more to reach max vacuum than say, a 3/4 hp pump.

BTW, a good source for closed-cell foam rubber seal material: 2 mm thick
craft foam in 9 x 12 sheets, with adhesive backing. Available at craft
stores and Walfart for about 60 cents each, many choices of colors. Also
great for jam chucks. Moldable around compound curves (doesn't buckle).


Ken Grunke
rural La Farge, WI
http://www.token.crwoodturner.com/

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet
News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+
Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption
=----

Among other comments, william_b_noble wrote:

If you will
only turn large items (a foot in diameter or more) a shop vac is adequate
and has lots and lots of flow.

I'm trying to figure out why only large items. Because it's a weak
vacuum, but the large area of the piece produces enough total pressure
to ensure adequate gripping power?

How large does your work have to be in order to use a shop vac?

Thanks.

Owen Davies

"william_b_noble" wrote in message
news:1115448791.a702db2e45a5b5805b9a5f6457eee204@t eranews...
allow me to most humbly disagree. a small passage means high reynolds #
loss and inefficiency. A small pump, however "efficient" (whatever that
means) has some maximum flow rate. if you turn a thin walled vessel, or
have any irregularities, warpage, etc, you will have leakage. The small
pump will at some level of leakage, have inadequate flow and will be
unable
to obtain sufficient vacuum to reliably hold the piece. Having a small
orofice like you suggest will aggrevate the problem.



I would say, based on experience and tests using a variety of pumps, that
a
open or zero pressure flow of less than 2.5 to 3 CFM means that the pump
will be marginal as a vacuum chuck - it may be totally fine for many other
purposes, for example evacuating molds, AC systems, etc, but not for wood
turning. A flow of much over 10 CFM is also probably not needed, if you
have that much leakage, the losses elsewhere in the system will prevent it
from working.

It's a bit like deciphering a dust collector rated in HP (or SearsPower)
rather than flow rate @ some pressure, or just flow rate. You've got to be
consistent in your units first, then figure out which are important for what
you do. Have to be aware of some of the tradeoffs as well. Bill's note on
passage size is spot on.

"Excess" capacity on a nice piece of hard maple can quickly translate to a
flying piece of red oak if you're not careful.

Vacuum chucking is something I'll try once the last is out of college - on
my new BIG lathe.

allow me to most humbly disagree. a small passage means high reynolds #
loss and inefficiency.

Quite right. Good vacuum system design is to use large diameter tubing. In
a practical sense, just make sure your pump has a flow rate much higher than
your system's leakage rate. It is easier to use an oversize pump that will
keep ahead of the leaks than it is to use a miniscule flow pump and track
down leaks. A 1/3HP, 5cfm, pump developing up to 28.5" Hg should be
sufficient for anyone's work clamping setup and not be too hard on the
pocket book. HVAC repairers use such a pump and they aren't too expensive.
Dan

I'm not sure that the Renolds # is applicable to a Gas [Air] only
applies to Fluids for determining whether you have Laminar or Turbulant
Flow for Thermodynamic Calculations.

Surely once you have a Vacuum the Pipe/Tube is empty so you are Sucking
Nothing other than any Air that Leaks in? Therefore Pipe Size/System
Volume is probably best kept to a minimum?

"Dave W" wrote in message
...
From what I have been able to find, a vacuum of 10 inches should be
sufficient for a vacuum chuck.
What I can't seem to find is the minimum cfm needed.

Used pumps are fairly easy to find, but I'm not sure what is the minimum
size I need.

Dave in Cherrylog, GA

===== This has been the most erudite discussion regarding vacuum pumps and
I have gained the following knowledge on how to select one: Get the biggest
pump you can, but not too big, or not too small; use the largest hose you
can, but not too large or not too small. I'm sure glad that I hadn't read
that before I had Bill Noble sell me one of his pumps, as it was it was just
the right size and I followed the instructions and used just the right hose,
it holds the bowl or blank like it was glued on. Thanks, Bill!!!

Leif

wrote: I'm not sure that the Renolds #
is applicable to a Gas [Air] only applies to Fluids for determining whether
you have Laminar or Turbulant Flow for Thermodynamic Calculations. Surely
once you have a Vacuum the Pipe/Tube is empty so you are Sucking Nothing
other than any Air that Leaks in? Therefore Pipe Size/System
Volume is probably best kept to a minimum?
^^^^^^^^^^^^^^^^
I usually edit a post down to the "bare bones" that I want to comment on.
However, I have to quarrel with most of what you say. Reynolds Number
applies to "fluids." Air is a fluid. It has to do with calculating the
pressure drop in fluid flow, in both laminar and turbulent flow, and in
estimating when the transition from laminar to turbulent takes place.

In a practical vacuum system the pipe is NOT empty. There is a pressure
gradient in the pipe from the chuck all the way to the pump suction. This
gradient is what causes the air to flow toward the pump. Making the pipe
smaller increases the flow resistance, and reduces the capacity of the pump
to cope with leaks. In any practical system, the volume in the pipe will be
quite small compared to the capacity of the pump. The extra volume you add
by making the pipe large will delay the pull-down by a second or two,
perhaps. But, making the pipe too small could result in so much pressure
gradient that the pressure inside of the bowl will rise and interfere with
the holding power of the chuck.

Incidentally, though it has not been mentioned, for the same reason that the
pipe should be large, it should also be short.

only large items because most shop vacs pull somewhere between 3 and 5
inches of mercury - that won't hold a small item. There is a handy dandy
table of forces (diameter, vac) in my article on vac chucks that may be
helpful. Figure that a shop vac will get you to 3 or so PSI of holding
force - so with a 12 inch diameter item, that's about 36 square inches, so
about 100 pounds of force - enough to hold a bowl while you lightly clean up
the foot.
With a 3 inch diameter object, that's 9 square inches, and you get around 30
pounds of force - not enough to counter even sanding pressure.

bill
"Owen Davies" wrote in message
...
Among other comments, william_b_noble wrote:

If you will
only turn large items (a foot in diameter or more) a shop vac is
adequate
and has lots and lots of flow.

I'm trying to figure out why only large items. Because it's a weak
vacuum, but the large area of the piece produces enough total pressure
to ensure adequate gripping power?

How large does your work have to be in order to use a shop vac?

Thanks.

Owen Davies