Re: [CAD_CAM_EDM_DRO] Re: Router plan made from steel- alum vs steel

Hi Everyone;


I'm begining to believe that there are to many guys named Dave
responding to this thread :)

Dave; you do hilight some important points. I look at it this way
Steel can be a fine base for a machine, but it will take more effort to
get the stiffness required based on weight. There is not a huge
difference in cost either once one considers the effort required to keep
the steel frame from flexing. One could always throw a lot of metal
into a steel structure and hope for the best but then costs will sky rocket.

When it comes to off the cuff engineering with steel, with respect to
something like a router table, you best friend will be lots of
triangles. I'm sure this is how many a home grown router is
implemented. For me though I would have more confidence in a
combination of aluminum extrusions and aluminum plate given equal lack
of real mechanical engineering and a fixed budget.

Maybe its me, but you are talking a great deal of effort and resources
required to weld up a steel frame and end up with it striaght, square
and level. Most of the robotic frames, fabricated from welded steel,
that I've seen are usually milled or planed after fabrication. Not
many of us have the resources to do that on a router platform. I could
see where a well designed frame welded up out of steel would be nice,
but I doubt that there is a well designed frame floating about in the
public domain. That is a frame that has been verified through
computer simulation. If there is I'd be the first to give it a serious
look.

The alternative is a design that has been independantly implemented by a
number of people and has field proven itself. This is most likely
what the original poster is looking for.

Thanks
Dave


turbulatordude wrote:

>--- 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.
>
>
>
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