|Table tops are the platform
for conducting many types of measurements and processes. They can
serve as a mechanical reference between different components (such
as lasers, lenses, film plates, etc.) as well as simply providing
a quiet work surface. Tops typically use one of three constructions:
laminate, a solid material (granite) or
a lightweight honeycomb.
The choice of construction depends on the type and size of the application.
11 shows a typical laminated
construction. These are usually 2 to 4 in. thick and consist
of layers of steel and/or composite materials epoxy-bonded together
into a seamless stainless steel pan with rounded edges and corners.
A visco-elastic adhesive can be used between the plates to enhance
the damping provided by the composite layers. All bonding materials
are chosen to prevent delamination of the assembly due to heat,
humidity, or aging. The ferromagnetic stainless steel pan provides
a corrosion-resistant, durable surface which works well with magnetic
fixtures. “Standard” sizes for these tops range from
24 in. square to 6 x 12 ft, and can weigh anywhere from 100 - 5,000
lbs. This type of construction is not well suited to applications
which require large numbers of mounting holes (tapped or otherwise).
The ratio of steel to lightweight damping composite in the core
depends primarily on the desired mass for the top.
There are many applications
in which a heavy top is of benefit. It can lower the center-of-gravity
for systems in which gravitational stability is
an issue. If the payload is dynamically “active” (like
a microscope with a moving stage), then the increased mass will reduce
the reaction motions of the top. Lastly, steel is very strong, and
very high mass payloads may require this strength.
Granite and solid-composite
tops offer a relatively high mass and stiffness, provide moderate
levels of damping, and are cost effective in smaller sizes. Their non-magnetic
properties are desirable in many applications, and they can
be lapped to a precise surface. Mounting to granite surfaces
is difficult, however, and granite is
more expensive and less well damped than laminate tops
in larger sizes. The highest performing work surfaces are honeycomb
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table tops are very lightweight for their rigidity and are preferred
for applications requiring bolt-down mounting or larger working surfaces.
They can be made in any size from 1 ft on a side and a few in. thick,
to 5 x 16 ft and over 2 ft thick. Larger tops can also be “joined” to
make a surface which is almost unlimited in size or shape. The smaller
surfaces are often called “breadboards,” and
the larger sizes “optical
tops” or “optical
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||There are many other
benefits to using a honeycomb core. The open centers of the cells
allow an array of mounting holes to be placed on the table’s
surface. These holes may be capped to prevent liquid contaminants
from entering the core and “registered” with the core’s
cells. During the construction of TMC optical tops, the top skin
is placed face down against a reference surface (a lapped granite
block), and the epoxy, core, sidewalls, bottom skin, and damping
system built up on top of it. The whole assembly is clamped together
using up to 30 tons of force. This forces the top skin to take the
same shape (flatness) of the precision granite block. Once the epoxy
is cured, the table’s top skin keeps this precise flatness
(typically ±0.005 in.) over its entire surface.
TMC’s patented CleanTop® II design
allows the core to be directly bonded to the top and bottom skins
of the table. This improves the compressional stiffness of the core
and reduces the thermal relaxation time for the table. The epoxy
used in bonding the table is extremely rigid without being brittle
yet allows for thermal expansion and contraction of the table without
compromising the bond between the core and the skins.
Honeycomb core tables
can also be made out of a variety of materials, including nonmagnetic
stainless steel, aluminum for magnetically sensitive applications,
and super invar for applications demanding the highest grade of thermal
stability. Lastly, the individual cups sealing the holes in the top
skin (unique to TMC’s patented CleanTop® II design)
are made of stainless steel or nylon to resist a wide range of corrosive
The sidewalls of
the optical table can be made out of many materials as well. Some
of TMC’s competitors’ tops use a common “chipboard” sidewall
which, though well damped, is not very strong and can be easily
damaged in handling or by moisture. TMC tables use an all-steel
sidewall construction with constrained-layer damping to provide
equally high levels of damping with much greater mechanical strength.
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|The performance of an
optical table is characterized by its static and dynamic rigidity.
Both describe how the table flexes when subjected to an applied force.
The first is its response to a static load, while the second describes
the “free oscillations” of the table.
13 shows how the static rigidity of a table is measured. The
table is placed on a set of line contact supports. A force is applied
to the center of the table, and the table’s deflection ()
measured. This gives the static rigidity in terms of in/lbf
This rigidity is a function of the table’s dimensions and
the physical properties of the top and bottom skins, sidewalls,
core, and how they are assembled.
is a measure of the peak-to-peak motion of a table’s oscillations
when it is excited by an applied impulse force. When hit with a
hammer, several normal modes of
oscillation of the table are excited, and each “rings” with
its own frequency. Figure 14 shows
the four lowest frequency modes of a table. Dynamic compliance is
measured by striking the corner of a table with an impact testing
hammer (which measures the level of the impact’s force near
the corner of the table). The table’s response is measured
with an accelerometer fastened to the top as close to the location
of the impact as possible. The signals are fed to a spectrum analyzer
which produces a corner compliance
curve. This measures the deflection of the table in in/lbf
(or mm/N) for frequencies between 10 and 1,000 Hz.
|Each normal mode
resonance of the top appears as a peak in this curve at its resonant
frequency. The standard way to quote the dynamic compliance of a
top is to state the peak amplitude and frequency of the lowest frequency
peak (which normally dominates the response). Figure
15 shows the compliance curve for a table with low levels of
damping (to emphasize the resonant peaks). The peaks correspond
to the modes shown in Figure
14. The curve with a slope of 1/f 2is sometimes
referred to (erroneously) as the “mass line,” and it
represents the rigid-body motion of the table. “Mass line” is
misleading because the rigid-body response of the top involves rotational
as well as translational degrees of freedom, and, therefore, also
involves the two moments of inertia of the table in addition to
its mass. For this reason, this line may be 10 times or more above
the line one would calculate using the table’s mass alone.
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