You're right, of course. A cast ribbed table can be made quite flat.
Unfortunately it isn't very stiff, and will deform under its own weight.
One step up from simple ribs is a cored casting, with minimum open space on
the back. Better than that is the honeycomb, if you can make it.

The optical tables are made with honeycomb to optimize stiffness to weight,
and more important for their application, the damping. These were developed
to make holograms on. Vibrations of more than .1 microns make holography
impossible. Vibrations damp out much more quickly on the honeycomb table
than they do in solid or cast tables. The Newport website I referred to
easily has a lot of documention on this point. They plot dynamic stiffness
as a function of drive frequency.

When I speculated that a servo to kill vibration might be a good thing, I
was referring to not only table vibration, but ways bouncing around, columns
flexing, spindles and bearings flopping around, screws winding up and
unwinding, and so on.

Elliot Burke

>Message: 25
> Date: Thu, 18 Jan 2001 11:18:45 -0700
> From: "Smoke" <gordonr@...>
>Subject: Re: flat, light surfaces

>Personally, if I wanted a flat surface, i'd go with a cast ribbed table

that

>has been stress reliefed and then blanchard ground on both sides.

>Smoke