Re: Do Stepper Motors need "Drag?"

--- In CAD_CAM_EDM_DRO@y..., tom.kay@n... wrote:

> Thanks Steve, great feedback. Oh, another quick thought; would
> leadscrews and plastic nuts be OK for the average hobbiest who is

not

> earning money with his benchtop mill? Or is the wear factor enough

of

> a pain/expense to go right to ballscrews, in your opinion?
>
> Cheers, Tom.
>
>
> --- In CAD_CAM_EDM_DRO@y..., stevesng@n... wrote:
> > --- In CAD_CAM_EDM_DRO@y..., tom.kay@n... wrote:
> > > I am converting a mill to cnc with Dan Mauch's 5 amp boards and
> his
> > > 521 oz-in stepper motors. I plan on using a 2:1 reduction from
> the
> > > motors to the screw.

Tom,

One man's trash is another man's treasure..... I have only seen
the plastic stuff used in lightly loaded mechanisms and formed
my opinion based on them. What I saw did not look like it was
up to milling machine usage, however we live in a world of
constantly evolving technology. There may be plastic systems
that can do the job. The "plastic" is actually going to be
some sort of low friction material, likely a composite. Good
old Teflon is very low friction, but flows under pressure.
There are many new composite systems that put graphite, moly,
or who knows what, into the plastic mixture. This could allow
a stiffer plastic to have low friction.

Keep in mind that anti-backlash setups work by pressing two
load bearing surfaces apart and against opposite flanks of the
screw thread, or some work by squeezing the V of the nut deeper
into the V of the thread. On high end ball screws, the balls
are precisely matched to the screw without using two nuts. This
is the same as pressing the nut deeper into the V of the thread.

Whichever is used, it must have enough force to resist being
pushed together, or out of the V as the load on the nut increases.
As with all systems, ability to resist load and rate of wear are
opposing requirements and must be traded off against one another.
Some of the other folks on
this list have done the math for how much force is needed to
turn the screw. This could be extended to figure the load on
the nut assembly and give you numbers to check against the
specs on the plastic nut systems. Whatever the case, in order
to carry the same load, a plastic nut is likely to be larger.

Until someone shows me a new "miracle" plastic nut system, I
will continue to recommend ball screws for anything the size of
a mill-drill.

Cheers,
Steve Stallings