Hi all, 
Since I’m cheap I’d prototype with a fiberglass tube and fill with urethane 
foam. How much does the modulus change between a rectangular tube and 
elliptical geometry?
Box with corner braces that are viscous damped. It all comes down to load and 
frequencies. 
I’m just the aging dummy in the corner. ;-)

> On May 13, 2024, at 2:36 PM, Chris Albertson <albertson.ch...@gmail.com> 
> wrote:
> 
> 
> 
>> On May 13, 2024, at 1:45 PM, Todd Zuercher via Emc-users 
>> <emc-users@lists.sourceforge.net <mailto:emc-users@lists.sourceforge.net>> 
>> wrote:
>> 
>> Anyone have any brilliant ideas to stiffen a woefully inadequate cross beam 
>> on a gantry router without adding too much mass?  What is there now is a 4" 
>> x 8" rectangular 3/8" walled extrusion that is 145" long.
>> 
>> Under normal jogging commands the two servos control the ends of this gantry 
>> reasonably well, but while the axis is homing the thing shakes and wobbles 
>> terribly bad.  Also If I put a dial indicator in the center of the bridge 
>> and hit the bridge forward or backward it will flex and wobble enough to 
>> displace the dial indicator +/-0.03 and it takes nearly a dozen wobbles to 
>> dampen it.  But on the ends the servo's only have a few thousandths of give.
>> 
>> I'm less concerned about the actual stiffness and more worried about 
>> dampening the wobble.
> 
> 
> What is the extrusion made of, I assume it is some kind of aluminum alloy.    
> The simplest but expensive option is to replace it with a stronger/stiffer 
> material with the same dimensions.   Of course Titanium comes to mind but 
> that is maybe not in the budget.    Carbon fiber could work and it is 
> possible to DIY carbon fiber beams with just hand tools.   I have made 4 
> meter long racing kayaks with carbon, using just a paint brush and scissors 
> in one weekend.   
> 
> The first class way is to make a female mold and polish it well so the part 
> looks nice.   The cheap way is to make one like they make surfboards.  You 
> start with a foam block, shape it then wrap it in fiber and resin.
> 
> The neat thing about carbon composite is that you are not limited to the 
> extrusion shape.   I would make the entire beam a compound curve with no flat 
> or straight or cylindrical sections,  Maybe like a very elongated American 
> football but with ovil cross section.    
> 
> I like to use the car hood story.  A flat sheet of sheet steel is bendable by 
> hand.  But after they stamp it into the shape of a car hood it becomes rigid. 
>   So rather then a square tube, way not oval but with a larger diameter in 
> the center where all the bending force is?
> 
> The way you make it is to first make a full-size model out of wood and bondo. 
> Do a test-fit and give it an automotice grade paint finsh and then paste wax. 
>  Make a fiber glass mold, then from that your part.   Yes that is a lot of 
> work.  This is why you have an aluminum extrusion there now, because that was 
> easy and cheap.
> 
> 
> 
> 
> 
> 
>> 
>> Todd Zuercher
>> P. Graham Dunn Inc.<http://www.pgrahamdunn.com/index.php>
>> 630 Henry Street
>> Dalton, Ohio 44618
>> Phone:  (330)828-2105ext. 2031
>> 
>> 
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