The most common question asked when it
comes to vacuum hold-down is how much vacuum does my application
require? Vacuum systems should be evaluated based on the specific
applications. The smaller the part or thinner the material to be
held, the greater the volume of air flow required, thus increasing
the size of the vacuum pump. Many people state that you can’t have
enough vacuum. This is somewhat a true statement. However, it can be
a very costly fact. It is always easy to overbuy. Purchasing a
single 40HP or 50HP pump can cost in excess of $25,000, and the
purchase is only the initial cost. Depending on where your shop is
located, the utility rates can vary considerably. Keep in mind that
operating costs (utility) rarely go down, so these costs will
increase year to year.
Purchasing a number of smaller pumps can
help reduce the price of the pump as well as reduce the overhead to
run these pumps. If you are processing sheets of materials that have
large parts, then a single pump can be utilized. If the nest has
numerous smaller parts, then it is best to run all the pumps for
maximum air flow. There are other devices which can be purchased to
aid the vacuum hold-down system such as a spindle pressure foot, or
roller hold-down. These devices push down on the material while
being routed thus aiding the vacuum system.
Vacuum Table Basics
When selecting a vacuum table, the most
important thing to remember is how a vacuum table works. The
following six criteria should be carefully considered when
determining whether your CNC application could benefit from using
vacuum hold-down.
1.) Atmospheric pressure is
approximately 15 psi pounds/sq.in.). Each square inch of surface
area has a load of approximately 15 pounds on it. (This pressure is
more below sea level and less at the tops of mountains).
2.) When we have a box that has no air
in it; i.e., almost complete vacuum, the top and all other sides of
the box are being loaded with 15 pounds of pressure on each square
inch of surface. If the top of the box is 6”x6” then there is
6x6x15=540 pounds of load evenly distributed on the surface. The box
will collapse or bend in if it is not strong enough.
3.) A vacuum table; i.e., a box with
holes in it, has the “holes” pulling down on the object above it
with a pressure of 15 psi (assuming there is a complete vacuum).
Note that the pressure on the object is based on the surface area of
the hole in contact with the object being held, not the total
surface of the object being held.
4.) It is critical to realize that if
the object is being machined; i.e., undergoing a side load, the
actual force holding the object in place against the cutting force
is now the friction between the object and the vacuum table. The
magnitude of this relative friction force depends on the coefficient
of friction between the object and the table, in addition to the
actual vertical force on the object. Therefore, if the surface of
the vacuum table is very slippery; e.g., Teflon coated, the object
will tend to slip no matter how much downward force is being
applied. If the surface of the table is non-slippery; e.g., it is
rubber coated, then the object will tend to stay in place assuming
there is enough downward force being applied. Note that for small
pieces; e.g., small brass letters that have a surface area of less
than .5 square in., it is unlikely that they can be held reliably
with a vacuum table because the downward pressure on this object
would only be about 7.5 pounds. It is important to remember both
factors with respect to securing hold-down:
a.) contact surface area between the
actual vacuum holes and the object.
b.) relative coefficient of friction
between the object and the vacuum table surface
5.) Always consider the air permeability
of the object being held down. The permeability is the amount of air
that the object allows to pass through it. For example, a sponge is
very permeable while a piece of aluminum is usually not. The
permeability of the object determines whether a vacuum blower or a
vacuum pump should be selected. A vacuum pump usually generates a
very high vacuum; i.e., there is virtually no air in the vacuum
chamber, but vacuum pumps tend not to draw or pull a great volume of
air quickly. A vacuum blower tends to pull a great volume of air
very quickly, but it does not necessarily create a very “complete”
vacuum.
How big a vacuum pump or blower should I
select? Consider the following questions when deciding:
a.) How large a part is being held down?
b.) How large is the vacuum table and
its chambers?
c.) How much leakage will there be in
the system?
d.) How permeable is the material?
e.) How quickly will the material be
needed to clamp and unclamp?
Vacuum Pumps
Vacuum hold-down and which pump to
choose can be very complex subjects. There are several different
pumps available for use with a CNC machine and all have varying
specifications and price ranges. The proper selection of a vacuum
pump is imperative for optimal machine performance. To avoid paying
too much or not enough, vacuum systems need to be evaluated based on
the customer’s specific applications rather than cost alone.
There are two main items of concern when
looking at the specifications of a pump: vacuum level and flow.
Vacuum level is typically measured in terms of inches of mercury
(i.e. “Hg). This is the same term used when reading a barometer. So,
it’s no surprise that the vacuum utilized in CNC vacuum hold-down
works by atmospheric pressure applying 15 lbs/ sq.in. (psi at sea
level) in all directions. During a CNC routing operation, when you
use vacuum to remove air from one side of the material being cut
(the underside), then the atmospheric pressure on the other side
(topside) increases, in effect, pushing on the material. How much
hold-down force is applied to the material can be calculated by
reading the vacuum level gauge, multiplying that numeral by surface
area of the material (sq.in.) and multiplying that by 0.5. For
example, let’s look at a gauge that reads 22"Hg for a material
workpiece that is 24" square (576 sq.in.).
Clamping Force = Gauge Reading ("Hg) x
Material Surface Size (sq.in.) x .5 (22 x 576 x .5 = 6336 lbs)
Vacuum flow is the other important
specification to consider when opting for CNC vacuum hold-down. Flow
measures the volume of air pulled in by the pump. Measured in cubic
feet per minute, vacuum flow is referred to as either open flow (CFM)
or specific flow (SCFM). Open flow is the maximum flow without any
restriction on the air being drawn by the pump. Whereas, specific
flow refers to the level of air being drawn by the pump at a
specific level of vacuum, usually the optimum level in relation to
"Hg. Since SCFM is directly related to the vacuum’s level of maximum
clamping force (via the "Hg gauge reading), SCFM or specific flow is
the more relevant measure of vacuum flow.
All pump manufacturers have performance
curves that show the specific vacuum level vs. flow rate. Take for
example, a 10HP pump that is rated for 11"Hg @ 105 cfm and has an
open flow rating of 280 cfm. When the open flow reaches 280 cfm, the
vacuum level is at 0"Hg, which using the clamping force equation
above translates into zero hold-down. But, at 105 cfm the pump will
not drop below 11"Hg until it exceeds that flow rate. At 106 cfm and
above, the vacuum level will drop, effectively decreasing your
vacuum hold-down.
Once the vacuum begins to decrease, the
part or parts you are machining can shift and move due to the loss
of clamping force. This is primarily a concern in nested-based
manufacturing where flow-through methods are used. Flow- through is
the method of using an mdf scavenger board or sacrificial board to
protect the CNC machine’s surface. A sacrificial board is made out
of porous material so that air can be vacuumed through the board,
helping secure the workpiece to the table. A sacrificial board (i.e.
a sheet of MDF) is put between the CNC machine’s table surface and
underneath the material being routed. The scavenger board allows the
CNC Router tool bit to penetrate the workpiece while the board
protects the surface and the vacuum table’s grid structure.
The pump previously mentioned was used
to illustrate the value of specific flow rate based on the usage of
a 10HP regenerative vacuum blower. This example was the easiest
method in which to illustrate how vacuum pumps and specific flow
rates operate in the real world. There are many other pumps to
choose from such as rotary vanes, positive displacement blowers, and
rotary screw pumps. Prices from one system to another can fluctuate
a great deal, but these few are the most commonly used units in the
CNC Router industry. The differences in a cost-to-performance ratio
from one system to another are best described by comparing the pros
and cons of each system. For each system, a 10HP example will be
used in relation to "Hg at scfm, decibel levels, approximate cost,
typical applications and maintenance requirements.
In comparison to the other vacuum
systems, a 10HP regenerative blower is the lowest in cost, generally
retailing for around $5,000. Regenerative vacuum blowers, consist of
an electric motor coupled via belt or direct drive to the vacuum
pump impeller. The motor rotates the impeller, drawing air in the
inlet and discharging the air through the exhaust, creating vacuum.
Because the inexact dimensional tolerances from the impeller to the
housing, air is allowed to escape, resulting in lower vacuum
pressure. This is known as slippage. Regenerative vacuum blowers
typically generate low vacuum pressure, but generate greater volume
of air as described in the earlier example of 11"Hg at 105 scfm. The
noise consideration is a very real concern for these products. A
regenerative vacuum blower runs in a decibel range of 85 to 95. This
type of vacuum is ideal for holding less dense materials such as
foam and other porous materials. The required maintenance is minimal
with regenerative blowers, usually limited to replacing air filters.
Rotary vane vacuum pumps also consist of
an electric motor coupled via belt or direct drive to the vacuum
pump impeller made up of self-lubricating carbon vanes. The vanes
rotate in the pump housing drawing air in the inlet and discharging
the air through the exhaust, creating vacuum.
Because of the close tolerances
generated by the carbon vanes, greater vacuum pressure results. Up
to 25"Hg at 173 scfm can be achieved for an $8,300 10HP unit. The
increased vacuum results in superior hold- down capability, and is
ideal for wood, plastic, and other nonporous sheet-like materials.
In addition, these shop friendly pumps run quieter with a decibel
range of 80 to 85. The downside to using these pumps is that the
carbon vanes need to be replaced after 6000 to 8000 hours depending
on conditions. Increased heat in the pump reduces vane life; dirty
and/or blocked air filters and relief valves will cause the vacuum
pump to heat and decrease vane life.
The electric motor for a positive
displacement rotary blower is also coupled via belt or direct drive,
but instead of carbon vanes it connects to two rotors that rotate in
the opposite direction.
When the rotors pass the blower inlet,
it traps a quantity of air through the blower housing and discharges
this compressed air out the exhaust. Because the pump compresses the
air, when it is exhausted it is very loud due to the release of this
pressure. Typical decibel ranges for a $8,500 10 HP unit run from 95
to 100. Positive displacement rotary blowers should be enclosed
providing some sound protection. A max vacuum of 15"Hg @ 250 scfm
makes these units ideal for wood, plastic, and other nonporous
sheet-like materials. Regular maintenance such as oil changes are
required between 5000-6000 hours of operation. Proper disposal of
these oils are required.
Lastly, rotary screw vacuum pumps, which
are the most costly pump at about $15,000 for a 10 HP unit, are
ideal for wood, plastic, and other nonporous sheet-like materials.
They have their electric motor coupled
via belt or direct drive to counter-rotating twin screws. These
pumps are the most complex of all, but also yield the greatest
vacuum at 29"Hg at 150 scfm. They require cooling systems with
complex electrical system controls to operate the unit.
The twin screws are machined to
precision clearances in relation to the compression housing. The
counter rotating screws cause a low pressure area in the suction
port. Air is drawn in and trapped by the rotating screws and
transported to the discharge end. This does result in a high decibel
range of 95 to 100, and maintenance of these units includes regular
oil changes (for oil pumps) and expensive repairs on switches and
shut down devices. All the pumps mentioned have their place in the
CNC market. The level of performance varies from one model to
another and also from manufacturer to manufacturer. Regardless,
maximum vacuum and efficiency is the goal. There are simple ways to
increase efficient vacuum such as routing gasket around the
perimeter of the vacuum table (if applicable), or seal the edges of
the scavenger board. The yardstick for proper vacuum table hold-down
is a system that performs at 11"Hg or higher (a specific scfm).
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