RE: [CAD_CAM_EDM_DRO] Re: sourcing motion control parts - steel vs aluminum

Hal,
Good questions and very appropriate for this group I think.

You sure are correct in that stiffness is super important in machine tools.
You apply a force and a deflection occurs in the direction of that force.

Strange things start to happen if the force is applied in rapid short
oscillations
though.

Take a rubber band and hook it to a weight like a pencil or something. Hold
the
free end of the band and slowly move your hand up and down. When you hand
moves up,
the pencil dangling from the other end moves up as well. Now move your hand
faster
up and down. At some point you will observe that when your hand is moving up
the pencil
is moving down! We call this a complex conjugate pole pair, but the
gibberish doesn't matter.
The important thing is that you are commanding an up movement and the pencil
goes down.

If this is a cnc machine tool the fact that the motion is opposite of that
desired
creates massive instability in the control system. It's a real bad thing.

To make the frequency that this happens as high as possible and thus less a
problem
is very desirable. This is done by maximizing SPECIFIC STIFFNESS among other
things.

Like I said, steel and aluminum are about the same in this parameter. I do
use aluminum
a lot for some things because it's lesser elongation often makes it easier
to machine.

But I use steel for machine structures... It's cheaper, more thermally
stable, and as
you said three times the elastic modulus of aluminum.

Les


Leslie M.Watts
L M Watts Furniture
Tiger Georgia

Main page:
http://www.lmwatts.com
Engineering:
http://www.lmwatts.com/shop.html
Cnc surplus for sale:
http://www.lmwatts.com/forsale.html
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http://www.lmwatts.com/signwp.html

-----Original Message-----
From: Hal Eckhart [mailto:hal@...]
Sent: Monday, October 25, 2004 8:51 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: RE: [CAD_CAM_EDM_DRO] Re: sourcing motion control parts - steel
vs aluminum

On 10/25/04 8:28 PM, Leslie Watts wrote:

>Steel has about three times the modulus of aluminum, but it is also
>three times the density!
>
>Specific stiffness is the ratio of those two parameters, so they are
>about the same.

Okay, I'll bite. What exactly does "specific stiffness" have to do with
the rigidity of a frame? (no sarcasm intended; I'm just curious)

Hal Eckhart - Casa Forge - Minneapolis MN - <http://www.casaforge.com>