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A Vacuum System for Holding Work
on the Lathe Part I
by Hal Mahon©
hal.mahon@umb.edu
See Part 2 here
Vacuum chucking has many advantages for holding work
on the lathe. Neither mortise nor tenon is required and there are no
screws to leave marks on your work. Any turner on most lathes may use
the system described here. The only restriction is that your work is not
so porous or has so many worm holes as to preclude forming a vacuum, and
even then there are tricks that may enable you to overcome some of these
difficulties. Of course the surface of your work must be sufficiently
smooth to allow the gasket to seal to the vacuum chuck.
Vacuum chucking is for any wood turner who wants to improve the quality
of their work by improving access to the inside and outside of their
turning. It is frequently used near the end of a project in which the
work has been held with a conventional 4 jaw chuck or faceplate and the
next step is to reverse your work for access to its bottom to remove the
mortise or tenon, or blemishes from screws. The vacuum chuck is a
delightful tool for this part of your work.
Mickey Goodman has written about
vacuum chucking. See his extensive and
helpful article at
http://cnew.org/library/article_vac_chuck/vacuum_chucking.htm. Google can
bring up more information. Our purpose here is to provide practical
information aimed at turners who want to make some part, or all, of a
vacuum chucking system, including making your own vacuum pump. Off the
shelf commercial systems may be purchased at 4 figure prices. The
information here enables you to make a fine system that may exceed the
performance of the most expensive systems because it can be tailor
made to your needs and to your lathe. Depending on your choices you may
build a fine system at a cost two orders of magnitude less than that of
commercial systems.
Holding a bowl will be explained in more detail later. Basically a
gasket makes an airtight seal between the bowl’s bottom (or side) and
the vacuum chuck to allow work inside the bowl. An airtight seal between
the inside of the bowl and the chuck allows you to work on the outside
and bottom of the bowl. By evacuating air inside this seal the greater
atmospheric pressure outside forces the bowl against the chuck. This
force can be considerable and can resist significant force from a gouge
or sanding. Admitting air into the sealed space releases the bowl from
the vacuum chuck. Mounting and unmounting the bowl can be done quickly
and conveniently so your vacuum chuck system should be designed with
efficacy and convenience in mind as well as economy.
A vacuum chucking system has 4 components:
- Vacuum pump
- Control manifold connecting pump to the lathe
spindle adapter
- Spindle adapter allowing lathe to spin while
manifold connection remains stationary
- The vacuum chuck itself
The first part of this series is about the vacuum pump
and the second part deals with the rest of the system.
Vacuum Pumps—where can you find a pump?
Very good pumps are made by
Gast and
Rietschle &
Thomas are included in high end systems. Check eBay for vacuum pumps
(and Google for additional information and addresses not included in
this article).
Clinton Electric Motor Service (978-365-7652) had 4 pumps ranging in
price from $85 to $350. You would not be unhappy with the service and
guarantee offered by CEMS. (In addition CEMS can replace worn lathe
bearings and rewind lathe motors. It is also a source for DC motors and
helpful information to convert your belt driven lathe or drill press to
variable speed.)
Sources of used vacuum pumps may be sought where they have been used
i.e. by dairies, dentists, hospitals, industry, veneer presses, boat
building, type setting, and in HVAC control systems. Vacuum pumps may be
hiding in barns, and salvage areas of medical and industrial facilities.
Low priced pumps or even freebies may be lurking close by as was the
Gast vacuum pump that I acquired a few years before I started wood
turning.
Of course vacuum is available at the intake opening of a regular air
compressor. The air intakes of two smaller air pumps are shown in
Figures 1 and 2.
 FIGURE
1
The pump in Figure 1 is a Gast air compressor.
FIGURE 2
The pump in Figure 2 is a 12 volt automotive unit from
a variable height suspension system. The white felt intake filter has
been removed and is shown positioned to the right of the intake (vacuum)
port. To connect either of these pumps to your vacuum system a fixture
from e.g. maple with a pipe fitting would need to be configured for the
opening and epoxied in place.
 FIGURE
3
Figure 3 shows pipes fitted to the intake ports of a
larger air compressor. If a vacuum reservoir, such as a ten gallon size
portable air tank, is included the pump can operate intermittently. For
intermittent operation a constant vacuum regulator should be provided,
see
http://www.joewoodworker.com/veneering/welcome.htm. This site is a
source of such a regulator and also a source for most of the parts
discussed in this article if they cannot be obtained locally.
Some turners have suggested using the handy Shop Vac
that most of us have for cleaning up. Thus the availability is an
advantage. However, with a clean filter a good shop vacuum can only pull
a vacuum of up to 6 inches of mercury (see below for definition),
significantly less than the pumps discussed below. Some vacuums use air
through the motor for cooling. The low airflow when used for vacuum
chucking endangers overheating the motor. Finally, a major reason for my
disfavor is that they tend to be quite noisy.
 FIGURE
4
A venturi type vacuum source is shown in Figure 4
together with a collection of fittings the purpose of which will be
explained later. The venturi produces a vacuum from the side port by
forcing 4 to 6 cubic feet per minute of compressed air through from the
larger end. A silencer should be attached to the output. This can make a
reliable and simple vacuum pump that will be noisy in operation and may
be expensive to operate continuously, depending on the air compressor.
Grizzly (800 523 4777) sells complete venturi systems for which they
recommend a 2 to 4 hp compressor delivering air at 85 psi. My preference
is for a quieter system and one with which I would have less concern
about the cost of operation.
How to make your own vacuum pump
Although I already have a good Gast vacuum pump and do not need another,
my purpose here is to show by my example how to make a fine, reliable
vacuum pump that can be essentially free except for the cost of a few
fittings. A pump you can make will have multiple uses such as pumping
automotive tires, vacuum veneering and for a vacuum hold down table for
routing and sanding, in addition to use with your lathe. The several
recycling centers I visit have refrigerators, freezers, dehumidifiers
and air conditioners that have been disposed, usually because they have
lost their refrigerant.
 FIGURE
5
Figure 5 shows the compressor I removed from a Maytag
refrigerator. (Most refrigeration motors are much more compact than this
unit.)When removing the refrigeration pump a pair of large diagonal
cutters may be used to cut and crimp the pipes. Or use a tubing cutter
and put tape over the ends to keep out dirt. Do not use a hacksaw to
avoid metal particles getting into the pump. Leave 8 to 18 inches of
pipe connected to the pump, longer is better. The pipe may be cut to a
convenient length later with tubing cutters. Be careful bending pipe to
avoid kinking and avoid breakage at welded joints. Use care when
removing the pump as it may be filled with oil. In operation the oil may
mist from the output of the pump and a longer length of tubing coiled
vertically will lead it to drain back into the pump. A length of spring
with an ID matching the OD of the pipe is useful for bending without
kinking. One woodturner has reported more than ten years of trouble free
operation with a pump from an apartment size, below counter
refrigerator.
A common unit to describe the strength of vacuum is inches of mercury.
At sea level a perfect vacuum can support a column of mercury 30 inches
high, which corresponds to 14.7 pounds per square inch. This is
atmospheric pressure at sea level. The force on a bowl with an area of
50 sq-in (about 8” in diameter) under a vacuum of 25 inches of mercury
(abbreviated 25 ” Hg) would be 613 pounds (50 x 14.7 x 25/30 = 612.5).
This is a considerable force and helps explain the effectiveness of the
vacuum chuck.
The graph above shows how
force varies with inches of vacuum and chuck diameter. For example at 20
inches vacuum and with an 8 inch diameter chuck the graph shows that the
force holding the bowl to the chuck is 500 pounds, or 275 pounds force
for a 6 inch diameter chuck. With 15 inches vacuum a 6 inch diameter
chuck would provide the holding force of a good size person standing on
your work pushing with their weight down against the chuck. As shown in
this graph the larger the diameter of the chuck the greater is the
holding force for a given vacuum. However with out care too much force
could be applied to a bowl. Damage could result if the wall of the bowl
is too thin or the vacuum applied is too great.
The valve shown in the
Vacuum System Schematic below can be adjusted to limit the vacuum and
hence the force applied to an acceptable level yet sufficient to provide
adequate holding force.
This refrigerator pump shown in Figure 5 was free, runs quietly and
pulled a vacuum of 28 inches of mercury on my lathe, which is close to
the maximum possible of 30 inches . This was higher than obtained from
my Gast vacuum pump. (The vacuum measured is also highly dependant on
the quality of the seal to the bowl on the lathe.) I returned this pump
to recycling after testing because it was bulkier than my other options.
Electrical connections are shown in the left side of the Schematic,
including the ground wire connected to the metal case of the vacuum
pump. There is an on/off switch. Some induction motors will have one or
two starting capacitors. The refrigeration unit in figure 5 did not have
an external starting capacitor. The manifold is on the right side of the
Schematic. There is a trap to capture dust before it enters the
mechanical part of the pump. The vacuum line leads from the pump through
the dust trap to the valve. This control valve is for adjusting the
level of vacuum applied to the chuck. The gauge is located where I can
conveniently watch it while I am turning. The gauge gives a measure of
the strength of the vacuum, and with reference to the graph above, the
force applied. I keep an eye on the gauge while turning to assure that
the gasket remains tight. A quick release valve saves time in quickly
reducing the vacuum to remove the work held in the chuck.
There were 5 derelict air conditioners at another recycling center. I
removed two air conditioners with capacities of 5500 BTUh and 11,600
BTUh . Both tested well. My favorite of the two was the slightly quieter
11,600 BTUh pump, shown in Figure 6 with its cabinet removed. The vacuum
intake pipe is the larger of the two pipes to the pump.
 FIGURE
6
The starting capacitor is shown as the cylinder in the
upper right hand corner of Figure 6. The starting capacitor (and its
twin if it is in a separate cylinder) must be removed and saved. There
are often two capacitors as shown in the Vacuum System Schematic above.
In this instance both capacitors were in the same plastic cylinder.
These are usually connected using clips so removal requires some pulling
on the clips, but a pair of pliers eases removal. Make a diagram noting
wire colors and how the wires run between essential parts to aid in
rewiring later. Your wiring diagram should have some similarity to the
electrical wiring in the Vacuum System Schematic. This shows how a wire
from one of the incoming 115V power wires goes to the capacitor(s) and
via a fourth wire, to the pump. In some AC units you may be able to
identify only one capacitor.
FIGURE 7
Figure 7 shows the 11,600 BTUh removed from its case
and mounted in a rack with handle for convenience moving the unit. The
plastic cylinder with two capacitors is mounted below the On/Off switch.
The transparent bowl is the dust trap with its cover removed and resting
below.
Although this 11,600 BTUh unit is rated at 1184 Watts and 10.7 amps, in
operation on my lathe it varied between 310 W and 320 W as compared to
my Gast vacuum pump, which measured 325 W with the same bowl to chuck
seal. The 320 W measured versus its rating of 1184 W is explained by the
much lower gas through put as used in my vacuum chucking system,
compared to its operation as an air conditioner.
Initially when I first set up my vacuum system I included a large, 10
gallon size air reservoir. However in tests with and without the
reservoir I could detect absolutely no advantage of the reservoir with
either vacuum pump. I tried the reservoir with different bowls with
different qualities of seals. An air reservoir would be an extra expense
you do not need.
Testing and Trouble Shooting
In testing with the intake vacuum line shut off by its quick disconnect,
the pump would run for less than a minute and then shut down. This did
not happen with any of the other pumps I tried. The cause for automatic
shutdown is not firmly explained. As soon as I connected the pump to the
Vacuum System Manifold the problem went away. Apparently the extremely
high vacuum with no external connection (and presumably no incoming air
from a minute leak) caused an overload of the 11,600 BTUh pump. In
actual operation with a vacuum chuck on my lathe , over time and under a
variety of conditions, there has never been a subsequent shut down.
There is a difference between the two pumps when power is shut off. The
Gast pump does not hold its vacuum with its power shut off. The vacuum
inside the vacuum chuck and bowl returns to zero within a few moments.
The AC pump holds its vacuum when power is shut off. Because I included
a quick disconnect valve in my vacuum manifold design I can admit air
into the vacuum chuck and quickly remove my bowl. Thus this difference
between these two pumps is insignificant for my use. Others might find
this difference important.
I compared the time to evacuate the ten gallon portable air tank to 25
inches of mercury for the Gast and the AC pump mounted as shown in
Figure 7. My Gast vacuum pump pulled the air down to 25 “ Hg in about 3
seconds, but could go to no higher vacuum. The AC pump in Figure 7
required 10 seconds to evacuate the ten gallon tank to 25 “ Hg and then
continued to a vacuum of 28.5 “ Hg.
Conclusion
There is little difference in performance between the two pumps. The
Gast vacuum pump and AC pump both work well. The Gast is faster in pump
down, but the difference in time is not important at all. My biggest
disappointment with the Gast pump is that it is not tightly enclosed
against dust. My AC vacuum pump is totally sealed, and pulls a better
vacuum, a slightly important factor when using a 2” diameter chuck for
small work. My time, the cost of a copper pipe fitting and an On/Off
switch with enclosure consisted of my costs over my Gast pump. (The
parts shown in the manifold section of the Schematic are needed for both
pumps.) I cannot say how long will either pump last. Having already done
it once, I can swap in a free new AC pump from the recycling center in
an hour should I suffer a failure. In conclusion my preference favors
the 11,600 BTUh air conditioner vacuum pump over my commercial Gast pump
primarily because of its lack of total enclosure against dust.
The final part of this series on A Vacuum System for Holding Work on the
Lathe follows next month.
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Posted Feb 2007 |
