Re: lead screw or rack and pinion ?

Hi Craig,

I have been thinking about the drive more than anything else.

I am now comfortable about the table. leveling by shims. the gantry, two 3" sq steel box beams, horizontal. Router suspended
inbetween.

But it seems that gearing a stepper to gain power adds cost and complexity when a driven lead screw would keep things simple. My
first unit was a rotating nut and worked well. (probably because I'm a rotating nut)

Seems two large screws would be easy enough to spin and keep syncronized. That and the power is inherent in the system witout
adding additional gerboxes.

Dave

--- In CAD_CAM_EDM_DRO@y..., "cmedwards_lu" <cedwards@c...> wrote:
> 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:
>
> http://ourworld.compuserve.com/homepages/MJVanVoorhis/techdata.htm
>
> One thing that became very clear after playing around with the
> numbers is that aluminum extrusions are not very stiff, unless they
> get VERY large. I'm not really sure what should be considered
> an "acceptable" deflection as the gantry and head change position or
> as cutting forces on the tool change, but I decided rather
> arbitrarily that I would try to keep all deflections under .001 In.
> I doubt this would be good enough for a metal machining table, but
> since I'm mostly looking to work with wood and plastic, I figure it
> will be good enough for my very limited budget.
>
> If you want to get some idea of what a minimum solution might look
> like, check out http://www.shopbottools.com I'm sure you're planning
> on something better, but I do like the "horizontal" gantry approach.
> I'm still debating the pros/cons for the gantry layout, most
> commercial machines use a "vertical" approach of one rail over the
> other, but I like the wide tool bay offered by a twin beam horizotal
> gantry. Also depends how much Z travel you want. I'm planning on 12
> inches, evem though most of what I want to do will be flat panel
> work, because it really doesn't cost much extra at this stage.
>
> With regard to drive choice.... I still haven't decided yet. I've
> ruled out rotating lead or ballscrews because of the lenght of X axis
> and the corresponding size of a drive system needed, but I am
> considering a rotating nut design. Alternately, rack and pinion
> looks pretty good as well, especially if you ratio down the motor RPM
> using a gearhead or a toothed belt arrangement. I do want to use DC
> brushless servos , which develop more power at higher RPM, hence not
> a good idea to drive the pinion directly. But if you are using
> steppers... well the shopbot folks seem to have made it work.
>
> Just my 3 cents worth.... this is my first large format router as
> well... so I'm not nearly as experienced as others on this list.
> But I hope it's helpful all the same.
>
> -Craig Edwards
>
>
> --- In CAD_CAM_EDM_DRO@y..., "turbulatordude" <davemucha@j...> wrote:
> > Hi All,
> >
> > I am persuing the design of a 8ft x 4 ft router table, moving
> gantry
> > style and am looking at the best way to move the table.
> >
> > seems the consensus so far is to power both sides to prevent
> racking
> > of the moving gantry.
> >
> > the question is wheather it is better to use lead screws, driven by
> > timing belts, lead screws driven by gears and solid shafting, or
> rack
> > an pinion driven by timing belts or gears and solid shafting.
> >
> > of course alternate methods are welcomed. I'm in the design stages
> > and it looks like I am going with Hi Win linear rails and bearings.
> > (Mclem@ ??? on the list offers a good deal.
> >
> > I was thinking of using NEMA34 steppers to drive each axis, geared
> > down as speed is not too important (yet) and it looks like the
> table
> > is simple to build. I checked out welded tables and the twisting
> is
> > too much to handle. seems like you have to learn a multitude to
> > subjects to get a good result in this hobby. (Mariss makes driving
> > the steppers just too easy)
> >
> > Dave