Re: [CAD_CAM_EDM_DRO] cncKITS.com announcing pre-machined acme leadscrew assemblies...

> So the motors don't have encoders on them yet,

the screws are stand-alone, and so one can use any
handy motor (the appropriate sized motor coupler is
is supplied with the screw). You could even put a
pully on there, in place of the motor.

If your motor already has an encoder, then don't worry
about it. But many motors (stepper or DC, and especially
DC permanent magnet motors that were not originally
intended for servo operation do not have dual shafts.

This innovation allows the incorporation of an encoder
while using those motors (otherwise you're expensive
alternative is to replace your existing single shaft
motors with dual shaft motors - and still have to add
the encoder).

Another scenario: a guy has put together a working
open loop stepper motor system (the stepper motor has
a single shaft and no encoder). At any time in the
future, to make a closed-loop feedback system he merely
needs to add an encoder on the end of the shaft to
complete the mechanical feedback generation aspect of the
upgrade.

> I was wondering if there was any bearing for the
> far end of the screw (the one that doesn't have the
> roller-bearings)? Is this the end that attaches
> to the motor?

No bearing there at the motor end. This would
over-constrain the design and make it suseptable to
binding due to misalignment or thermal causes. The
motors bearings support up to a few feet of screw weight,
but are otherwise isolated to just supplying the torque.

> I thought that one end of a screw was supposed to be fixed,

Yes exactly, thats the end with the taper roller bearings.

> .. the other to float, so that temperature changes wouldn't
> tend to bow it.

yes exactly, that is the motor coupler end. The rubber
spider separates the motor coupler from the leadscrew coupler
and allows for minor misalignment and thermal growth of the
screw.

> What about backlash error- is there an anti-backlash
> nut that fits these screws?

I am using 3" long acme coupling nuts, which pretty much
eliminate the backlash. As they wear, backlash will no
doubt reappear at some point. I am also developing a 6"
square combo threadblock/UHMW slider using 1.5" dia stainless
steel tubes. Building them is 'a lot' easier said than done
but when they are made to spec they slide very nicely and
there is no measurable backlack from the acme thread. These
will form the 'linear movement' components of my future
router kits.

I haven't tried any antibacklash nuts, but I am sure they
would work fine (even though they might add some drag and
cause some additional wear). They'd need to be for screws
1" dia x 5 threads per inch, normal right-hand (tightening)
thread. However, I think their expense is not justified
compared to the above two options (3" coupling nut & UHMW nut).

Doug Fortune
http://www.cncKITS.com