RE: RE:rE:re:Re: not stiff enough! (machine design considerations )

Ballendo,

With regard to gantry and bridge style machines, I am a little confused. In
a "normal" mill (like a Bridgeport or whatever), the operator is in front of
the machine and the X-axis is right-left. The Y-axis is in-out.

Your description below seems to indicate that the X and Y of the
gantry/bridge machines is opposite to that of the conventional machines,
with the in-out motion being the X-axis. Is this true?

If so, I could see a real problem arise if you used the same g-code program
to make parts on a MaxNC and, say, a Minirobo. They'd be flipped around!

Any thoughts?

Gary

> Date: Tue, 23 Jan 2001 21:41:26 -0000
> From: ballendo@...
> Subject: RE:rE:re:Re: not stiff enough! (machine design
> considerations)
>
> Richard,
>
> Good to know our suggestions are helping!
>
> Some thoughts on machine design:
> One of the MAJOR disadvantages of a 'gantry' style machine (like you
> have) is "axis error buildup". What this means is that ANY 'flex-
> ability' in X WILL BE ADDED to Y, and whatever 'flex-ability' is in Y
> (which now includes the X problems) WILL BE ADDED to Z! Finally,
> whatever Z has on its' own will be added...
>
> A bridge design "solves" this problem by NOT connecting Y support to
> the X axis. But at the cost of larger footprint, longer slide
> travels, etc.
>
> The reason I'm saying all this to you is your statement re: changing
> the Z axis shafts... This is like putting a solid performer (think
> circus acrobats) on the shoulders of two wobbly, 'supporters'!
>
> In a gantry design, the X axis (being 'lowest' in the stack) needs to
> be STIFF!!!
>
> Keep in mind that any mass you add at
> "the top" of the stack is like putting the fat man on the shoulders
> of the child... (circus again)
>
> It may even be worth making the X travel shorter, and "picking it up"
> by making the Y travel greater... When using unsupported shafts this
> will definitely increase overall machine stiffness (all else being
> equal).
>
> Hope this helps.
>
> Ballendo
>
> >Thanks all those who responded to my stiffness quiestions.
> >
> >Taking cuts no more than 1/2" the bit diameter, and running the
> >routine twice has solved 90% of the problems.
> >Still get some flex when the 1/4" router bit plunges in. I'm putting
> >the 3/4" rails on Z, and trying different bits.
> >Richard S