"Bernard Arnest" wrote in
oups.com:

Hi,

This is a rather general question, but here goes:

I hung around the woodworking newsgroups for a long while, and there
were regular discussions of the advantages of weight. You want as
much mass as possible to absorb vibration. Heavy machinery was
quality machinery. Etc.

Now, is it just weight? That is, if I take my table saw at home,
which is one of those hybrids that's like a cabinet saw but with an
open base; it's fairly stable as-is. If I box in the legs with sheet
metal and fill it up with concrete or lead or whatever, is it now the
same as a one-ton oliver table saw in terms of running smoothly?

So if I've made designs to build a rose engine over the summer (I've
decided to go through with it): it is normally a half-ton machine,
much of that in the cast iron base, I would wager. All that is in
steel and brass in the original will be replicated in steel and brass.
But if I weld up some 1/4"-3/4" sheet metal to form the stand, can I
just fill it up with concrete? Will that make it run smoother and
give me better work?

This might be a more elegant solution for a home built machine:

http://pergatory.mit.edu/perg/awards/shear_tube.html

It will allow you to keep the weight way down and will be stiffer than
cast iron and will absorb vibration as well as or better than a polymer
concrete base.

You can use all steel welded construction which eliminates the problem
of pouring a one off casting. It also makes machining the mounting
surfaces a relative breeze.

Filling the base with concrete will add weight to be sure. But it won't
always dampen vibration. If you look at a machine that uses a polymer
concrete base, like a new Hardinge, you'll see that the entire base is
made from polymer concrete. It's not just a steel or cast iron base
filled with concrete. The polymer concrete is poured around the mounting
structures, rather than the mounting structures being filled. This way
vibraton damping is certain.

Stiffness is just as desireable as vibration damping in a precision
machine tool. The problem is a welded steel base while its very stiff,
rings like a bell. Machine tool design is all about trade offs. Heavy
slides damp vibration very well but they are difficult to position
accurately. Cast iron dampens vibration better than steel but it sags
and flexes. Just ask anyone who has set up an engine lathe. The bed
twists and sags easily. If the engine lathe bed was steel, it would stay
straight. It would also vibrate terribly making the machine nearly
useless.

Large diameter spindle bearings offer more support and can bear more
load than a smaller diameter set. But at a given RPM the larger bearings
will run hotter which isn't very desirable.

The key I guess is in finding the right balance for the application.