Re: [CAD_CAM_EDM_DRO] Re: Sherline Z-axis ballscrew feed.

At 04:26 15/09/2003 +0000, you wrote:

>Tim:
> Wouldn't two nuts on the leadscrew result in too much drag for the
>stepper motor for the Z-axis?

That is certainly a potential problem. Any method that removes all the
backlash will increase the friction. However, unlike the pinched, slotted
nut method, which doesn't cope at all well with leadscrews that are worn in
the middle but not at the ends, using 2 nuts spring loaded against each
other will remove backlash and also adjust to any differential wear on the
screw.

>I thought that one of the attractions
>of using ballscrews was the low friction and less wear than normal
>thread-type leadscrews?

That is certainly one of the attractions, especially with mills like the
Taig and the Sherline, where the motors are required to lift the motor
assembly as well as the spindle. Result is hopefully that you need smaller
motors to get the same performance. The downside is that the pitch of a
ballscrew will certainly be coarser than the 20 TPI you see on the Taig &
Sherline mills by probably a factor of 2 or so, so it may be necessary to
use a reduction drive to keep the same resolution (if this is important to
you).

>I think a ballscrew for the z-axis is a
>fine idea. Now, if someone can just do it and work the bugs out,
>we can just retrofit our mills. ; ) -Jim

Easier said than done unfortunately. There is very little room in the Z
ways of the Taig mill to fit a ballscrew - the nut would need to have a max
diameter of around 16-18mm if I remember rightly, and on the Sherline there
is even less room. Similar problems with the X and Y if you need to go
there. Ballnuts are big relative to the diameter of the screw, and so far,
I haven't come across a screw/nut combination that will give both the right
length of travel and the right diameter of nut - as the diameter of
screw/nut gets smaller, you start to run up against the max screw length
that they recommend for that diameter. Might be possible to achieve with
the nut mounted externally, but this would not be a neat solution.

Yes, you could certainly re-design the whole of the Z axis, but at that
point, you might as well consider going the whole way & designing a mill
around one of the inexpensive cast iron X/Y tables of Eastern origin; the
result would probably be cheaper than buying a new Sherline, and would
definitely be stiffer.


Regards,
Tony