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

Hi Hal

I won't quarrel with your figures...they seem about right.

The issue is the definition of specific stiffness...

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.

This is true for most alloys, hardened or unhardened.

One way to think of it is an identically dimensioned bar of different
materials.

The material with the highest specific stiffness will ring with the highest
note when struck.

FYI the highest specific stiffness of the common metals is beryllium.

OK? :)

Les

Leslie M.Watts
L M Watts Furniture
Tiger Georgia

Main page:
http://www.lmwatts.com
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http://www.lmwatts.com/shop.html
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-----Original Message-----
From: Hal Eckhart [mailto:hal@...]
Sent: Monday, October 25, 2004 8:19 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 6:23 PM, Leslie Watts wrote:

>Aluminum and steel are virtually identical in specific stiffness.

I don't think that's true. The modulus of elasticity for steel is
29,700,000 psi and it's 10,000,000 psi for aluminum (this will vary
based on type and temper, but it's the best info I can find). For
example, a 2" square steel tube with a 1/8" wall would deflect 0.11"
with 100# load in the middle of an 8' run. For aluminum, it would
deflect 0.33".

This info is from the cheesy calculator I hacked together at
<http://metalgeek.com/static/deflection.php>.

If you want to quarrel with my conclusions, I've posted my sources
there. I am NOT an expert on this, although I do bend a lot of metal.
And my limited testing seems to support the accuracy of the calculator.
But I'm certainly open to other ideas.

And one note about 8020. I've used it, although I wouldn't recommend it
for framing (for a hobbyist). Unless you're going to be reconfiguring
it later, it's awfully pricy. However, they do sell a slick little
sliding dohickey for it that seems like it would work fine for a plasma
Y axis.

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


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