Cheap leadscrew nut ( was Re: lead screw or rack and pinion ?

Hi Craig,

I spun the nut on my first machine.

I used a 1/2-13 rod to move the table.
Since The table moved at less than 3 pounds
and there was no pressure, it seemed acceptable.

anyway, I took a 3/4" pillow block bearing, and a 1" dia
piece of Delrin. turned the OD to 3/4 to a hard press
fit in the bearing, threaded 6" in the center for the
rod.

I left a 1/4" shoulder at 1" to act as a stop
on the bearing. and left enough room on the
other side of the stop for the timing gear.

I driled 3 holes in the side of the timing gear
and drilled and tapped cooresponding holes in the shoulder.

____|--|XXXX_____

----|__|XXXX-----

XXXX is where the bearing would sit.

on the protruding end, I put room to add
a thrust bearing to keep the mess in
the bearing. So far, the press fit has
been more than strong enough.

As for the mounting of a fixed screw, it was
just brackets and nuts. I tentioned the screw
on the brackets to prevent bending of the screw
as it was being compressed. One of the expers on here
let me know that under compression, that 1/2-13
would bend like a noodle at only 17 pounds.

I have some crude pictures of my parts.
If you would like to see them, send me
and e-mail.

Dave


Dave

--- In CAD_CAM_EDM_DRO@y..., "cmedwards_lu" <cedwards@c...> wrote:
> Conceptually I like the rotating nut approach also (maybe for the
> same reason... <g>), especially for low duty cycle "homebrew"
> machines. For me, getting into screws over 1" in dia. gets
> expensive, plus it seems like there is a lot of prep work / expense
> in the bearing setup. But no doubt it's a time proven method, and
if
> you the tools/skills to do it right...great! But I believe rack is
> cheaper... and would appear easier for a novice to work with, so I
> think I'll probobly end up trying that first. Unless you have a
good
> low cost design for a rotating nut design!
>
> The issue with the gearing is really more of a problem with servos
> than with steppers. Steppers are fine at lower RPM, but DC servos
> really want to typically run at higher RPM to develop more HP. But
> of course everything is in the details so it really depends on the
> specific motor, load inertia, accel desired, etc. There are some
> good motor sizing programs avail, check some of the recent posts on
> that subject. The thing thats kinda cool about timing belts for
> gearing is that if you don't get the design perfect you can "tweak"
> the ratios a bit....but you're right, no free lunch in terms of
> complexity!
>
> THANKS LES WATTS for the pointer to Grape.... it's a great free
> tool! Only problem is I'll probobly play with the design of the
> table for another month instead of cutting steel.... ah the plight
of
> an electrical engineer trying to do mechanical design.
>
> -Craig
>
> --- In CAD_CAM_EDM_DRO@y..., "turbulatordude" <davemucha@j...>
wrote:
> > 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
> >