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?

There's the other question, too, that the machine vibrates itself, but
someone running down the hall one floor above will vibrate the machine
as well; so I want lots of mass on the one hand, but can't have it TOO
rigidly bound to the floor. An engine-turner definitely told me that
he had his machine in the concrete-floored basement of his house, and
when somone walked too forcefully along the floor above, each step
would be an apparent defect. So I might want ot think more about the
complexities of dampening and insulation than merely about just
blithely laying on mass?




thanks for the advice!
-Bernard Arnest

In many (most) commercial designs, the mass is distributed where it is
needed. This produces both rigidity as well as the vibration dampening
you are talking about. If you try and build up smaller designs, you
usually find that the bearings aren't big enough, shafts aren't big
enough, etc to give you the rigidity you want.

Bernard Arnest wrote:
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?

There's the other question, too, that the machine vibrates itself, but
someone running down the hall one floor above will vibrate the machine
as well; so I want lots of mass on the one hand, but can't have it TOO
rigidly bound to the floor. An engine-turner definitely told me that
he had his machine in the concrete-floored basement of his house, and
when somone walked too forcefully along the floor above, each step
would be an apparent defect. So I might want ot think more about the
complexities of dampening and insulation than merely about just
blithely laying on mass?




thanks for the advice!
-Bernard Arnest

In general, wood frames on equipment damp out harmonic vibration much
better than metal, or concrete.
Bugs

Bernard Arnest wrote:
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?

Every component of every machine has one or more natural frequencies at
which it prefers to vibrate. Deform a component slightly and release it,
or strike it with a hammer, and it will vibrate at this frequency. The
problems come when a moving part of the machine tool starts shaking a
component at or near its natural frequency. This results in vibrations
of a large amplitude which make lots of noise and may reduce the
accuracy of your work. A common example of this would be a sheet metal
side panel vibrating in the cabinet of a table saw.

The usual way to combat vibration problems is to raise the natural
frequencies of all the components far above the frequency at which the
moving parts of the machine tool rotate. The basic equation for
determining a natural frequency is f = 1/(2*pi)*sqrt(k/m). f is the
natural frequency, k is the stiffness and m is the mass. This looks
complicated, and trying to apply it to any real component is even more
complicated, but it does give some useful insight. More stiffness
increases the natural frequency. More mass (weight) reduces it. But it's
difficult to make a component stiffer without also making it heavier. If
you take a simple steel plate and double its thickness, you make it
twice as heavy, but you make it eight times stiffer. So k/m increases by
a factor of four, and the natural frequency by a factor of two. In other
words, when you make components thicker, the stiffness wins and the
natural frequency rises. Another factor which affects vibration is
damping. Imagine striking a metal component with a hammer and listening
to it ring. If the ring persists for a long time, there's little
damping, but if it dies down quickly there's a lot. High damping is
usually regarded as a good thing in machine design; it draws energy out
of the vibrating system and reduces the amplitude of the vibration.
Unless it's really high damping doesn't affect the natural frequency much.

Weight can be useful in making a machine more stable - i.e., harder to
knock over, and less inclined to shake annoyingly on your wooden floor
or bench - but from the point of view of vibration of individual machine
components it's a bad thing because it lowers the natural frequency.
However, it goes hand-in-hand with stiffness and rugged construction, so
people generally view it as a good thing. I think that most times people
talk about heavy machines being advantageous they are actually talking
about stiffness as opposed to weight. Cast iron is a good compromise
material to use for building machine tools because although it's heavy,
it's extremely stiff, has high internal damping and is resistant to
wear. It's main disadvantage is that it's a fairly brittle metal.

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?

If you have problems with stand vibrating, or the saw bouncing up and
down on a wooden floor, it will probably improve the situation. It won't
run as smoothly as a true one ton saw because you're adding weight and
stiffness in a rather crude way. The one ton saw will have the stiffness
in more useful places: it'll likely have a stiffer table, stiffer legs,
a larger diameter shaft for the blade, etc. By filling the base of a
lighter saw with concrete you basically have a light saw sitting on a
block of concrete. The weight is in the concrete, not in the components
of the machine which take the stress.

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 should work, but I'd personally prefer to use a heavy fabricated or
cast iron stand. Containers filled with concrete can give you problems
like corrosion between the concrete and the metal, and they aren't so
neat. If you just want the weight, and aren't worried about making the
stand stiffer, you could add the concrete in the form of blocks which
can be removed to make transport easier.

There's the other question, too, that the machine vibrates itself, but
someone running down the hall one floor above will vibrate the machine
as well; so I want lots of mass on the one hand, but can't have it TOO
rigidly bound to the floor. An engine-turner definitely told me that
he had his machine in the concrete-floored basement of his house, and
when somone walked too forcefully along the floor above, each step
would be an apparent defect. So I might want ot think more about the
complexities of dampening and insulation than merely about just
blithely laying on mass?

I'm surprised at the engine turner's tale, but perhaps someone here can
confirm it? You certainly do get vibration isolation systems in
laboratories which consist of a large concrete block sitting on flexible
rubber mountings. Again, someone here can probably give you more
information.

I hope I haven't muddied the waters here. As you can see, vibration is a
very complex subject. In summary, to answer your first question, no it
isn't just the weight: it's weight in the right places!

Best wishes,

Chris Tidy

Another thing I should have mentioned is that stiffness plays an
important role in resisting constant cutting forces, as well as the
cyclic forces which cause vibration. For example, if the table of your
mill distorts due to cutting forces, you get an inaccurate component.

If vibration is a big issue for you, I'd suggest looking out for a
proper 1 ton saw. Ultimately this is a much better solution to the problem.

Best wishes,

Chris

"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.

I think weight is a by product as opposed to a design constraint. Vibration
damping and stiffness is the design objective. Construction material is the
key. Cast structures are more dead than formed structures and iron is more
dead and stiffer than aluminum. Adding concrete to a formed construction
changes its resonance frequency, but doesn't add much stiffness.
Steve

"Bernard Arnest" wrote in message
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?

There's the other question, too, that the machine vibrates itself, but
someone running down the hall one floor above will vibrate the machine
as well; so I want lots of mass on the one hand, but can't have it TOO
rigidly bound to the floor. An engine-turner definitely told me that
he had his machine in the concrete-floored basement of his house, and
when somone walked too forcefully along the floor above, each step
would be an apparent defect. So I might want ot think more about the
complexities of dampening and insulation than merely about just
blithely laying on mass?




thanks for the advice!
-Bernard Arnest

"Bernard Arnest" wrote in
message
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?

There's the other question, too, that the
machine vibrates itself, but
someone running down the hall one floor above
will vibrate the machine
as well; so I want lots of mass on the one hand,
but can't have it TOO
rigidly bound to the floor. An engine-turner
definitely told me that
he had his machine in the concrete-floored
basement of his house, and
when somone walked too forcefully along the
floor above, each step
would be an apparent defect. So I might want ot
think more about the
complexities of dampening and insulation than
merely about just
blithely laying on mass?




thanks for the advice!
-Bernard Arnest


I was told by an old mechanical engineer that
machine tools should be compared by the tonne.
There is no substitute for copious amounts of cast
iron!

does cast iron have different damping properties than, say, steel
plates that have been welded or bolted together?

Christopher Tidy wrote:

Another thing I should have mentioned is that stiffness plays an
important role in resisting constant cutting forces, as well as the
cyclic forces which cause vibration. For example, if the table of your
mill distorts due to cutting forces, you get an inaccurate component.

If vibration is a big issue for you, I'd suggest looking out for a
proper 1 ton saw. Ultimately this is a much better solution to the problem.

Re the defects OP (Bernard Arnest) mentioned, where he wrote
An engine-turner definitely told me that
he had his machine in the concrete-floored basement of his house, and
when somone walked too forcefully along the floor above, each step
would be an apparent defect. So I might want to think more about the
complexities of dampening and insulation than merely about just
blithely laying on mass?
and you wrote
I'm surprised at the engine turner's tale, but perhaps someone here can
confirm it?

Although his damping example referred to a table saw,
the machine he plans to build is a rose engine, as in (eg)
http://www.pledge.co.uk/ref/rosengin.htm or
http://www.rgmwatches.com/engine.html and in a pattern from
such an engine, just as for a moire pattern, I think the
tiniest variation can make an easily visible difference.
-jiw

Bernard Arnest wrote:
does cast iron have different damping properties than, say, steel
plates that have been welded or bolted together?

Yes. Cast iron has higher internal damping than mild steel.

Chris