Re: [CAD_CAM_EDM_DRO] Re: lead screw or rack and pinion ?

Craig,
I think you are on the right track with structural analysis. Let me
give you some links to some free programs I use and I think you
will like.

The first is Grape. It is a very easy to use Finite element program with
nice things like animated deflection simulations. You can pop a simple
3-d structure in and run it in a matter of minutes.
It is at:
http://www.grapesoftware.mb.ca/




The second is BEAM 2D. This is great for very quick what if calculations
of beam deflections. Just click on a cross section and material and
type in loads and supports and hit run.
It is at:
http://www.orandsystems.com/

For most gantry or moving table designs I use 0.001" deflection/100lb load.
This will make a very stiff structure good enough for metal working
machines.
A woodworking machine can be much less stiff.

I use A-36 low carbon structural steel. What really is critical is stiffness
rather than strength. Aluminum has a specific stiffness of 102 in;
steel is 97. So they give virtually the same stiffness per
unit weight... but steel is much much cheaper!!

And of course big EI makes for good stiffness... in general I goes up as the
square of a cross section dimension. I use square tubing in space
frames.

I use a scraping method I have mentioned before to get long accurate
surfaces
to mount linear rails on. Lately I have had good results with filled epoxy
and bondo.
Apply a thin layer to the steel and "scrape" (actually sand) it. After a
rail is
bolted on you have the added benefit of a constrained layer damper.

Les

Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.rabun.net/~leswatts/wattsfurniturewp.html
engineering page:
http://www.rabun.net/~leswatts/shop.html

----- Original Message -----
From: "cmedwards_lu" <cedwards@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Sunday, June 30, 2002 8:27 PM
Subject: [CAD_CAM_EDM_DRO] Re: lead screw or rack and pinion ?


> Dave,
>
> I too have been designing a table for a 4 x 8 router (orginally
> moving table.... but have since changed to moving gantry due to floor
> space considerations). After considering many options I'm going to
> go with a welded steel frame, using various structural steel shapes
> mostly C channel and square tubing. Of course, that will still leave
> a lot of work to prepare the mounting surfaces for the 25mm linear
> rails, but I plan to grind/scrape using a mechanist level (I'll
> probobly cheat and end using some shim material as well <g>). The
> main thing I'm analysing as I'm designing the gantry and side beams
> is the "stiffness" vs. weight of the steel members. Too heavy a
> gantry and it greatly adds cost to the servo/drive system, but of
> course the more rigid the better. It's no doubt better to analysis
> the whole frame using finite analysis tools, but I'm lacking those so
> I'm just calculating deflections for various beam / force combos
> using a simple spread sheet. which I'll upload. Really more of
> a "what if" game than a real engineering analysis. You can get the
> moment of inertia figures for common shapes from here: