Re: Router plan made from steel- alum vs steel

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Dave Kowalczyk"
<dkowalcz@d...> wrote:

> Dave:
>
> Well, this is an interesting topic. In machine frames we don't
> stress materials anywhere near the yield point except for perhaps
> ball bearing contact surfaces.
>
> So the property that matters for structures is the stiffness, and
> to a lesser degree the vibration damping properties, dimensional
> stability, wear, etc.... Cast iron is king for this.
>
> Given the choice between aluminum and steel, go with the steel.
> It's stiffer, cheaper, expands less, and wears long.
>
> Although I can't recall the title, a book I've read on the

subject

> suggests that built up steel structures for big machine tools are
> almost as good as cast iron for damping properties owing to the
> joints.
>
> Your intuititon that the same design in steel will be stiffer is
> correct. The extra weight doesn't matter - a 2" square bar in
> aluminum and steel of equal lengths will sag about the same amount
> from their own weight.
>
> Dave Kowalczyk
> Mercer Island, WA
> Author of TurboCNC --> http://www.dakeng.com
>

There are merits to most everything. the neat thing about some
aluminum extrusions for frames is that one can often find some
extrusion that is not just a hollow tube, but an X interior with
slots on the outside to bolt things too. advantage aluminum.

steel is easier to find and easier to weld. advantage steel.

alum does not rust. -> alum
easier to drill -> alum

pound for pound, aluminum is stronger, but it is also lighter so it
means a thicker piece, often a lot thicker. advantage.... er.....
depends.

but, as the originator mentioned, aluminum is not universally
available so in some places steel is really the only choice.

also as since the steel is often much heavier, the gantry will
probably weigh more and therefor have some inertia problems for both
accel and decel.

but when at a steel yard it is odd to look at a 20 ft long x 12 dia
solid steel bar bend under it's own weight.


Dave

an one thing I did on my design was to make the table stand large
enough to accommodate full 4' x 8' ft sheets as a storage rack. I'm
not sure if the extra weight would make it more stable or the heavier
weight would make it harder to move. more tradeoffs.