Re: [CAD_CAM_EDM_DRO] Re: How bout this idea??

Hi, somehow we forget about climb milling! if there is backlash no matter
where the encoder is you wont climb mill or if you do the machine will not
last long. It will pound it's self to death. cnc goes both ways
milling,especially when you go around a corner or do you lift the tool and
finnish from the other end?!! cul brian f.

----- Original Message -----
From: "deanc500" <deanc@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Wednesday, March 13, 2002 11:04 PM
Subject: [CAD_CAM_EDM_DRO] Re: How bout this idea??


> --- In CAD_CAM_EDM_DRO@y..., Doug Fortune <pentam@c...> wrote:
> > sspeed33317 wrote:
> >
> > > I came up with this idea , using regular DC motors , not servomotors
> > > mounting linear encoders right to all the axes
> > >
> > > would eliminate the need for Servo motors , would eliminate
> > > the need for Zero lash ball screws ,would eliminate the need
> > > for expensive servo motors , wouldn't have to compensate for any
> > > backlash because it would be reading actual table movement .
> > >
> > > And the benefit of the ACME screws friction to help hold
> > > the table in position .
> > >
> > > And you could tie in for DRO use ,or tie into an existing DRO
> > > for CNC use if you already have one.
> > >
> > > Good idea??? Any body have input??? .
> > > Already in service??
> >
> > Yup, great idea ... been using it for a few months now
> > (see my webpage where I sell this type of acme leadscrew
> > with an encoder nib on the end for mounting the encoder).
> >
> > The only minor catch is that you still have to make sure
> > your motor-coupler-screw (or motor-gearing-screw) interface
> > has very little backlash (compared to the resolution of the
> > encoder mounted on the screw), otherwise a nasty oscillation
> > can be set up (I'm guessing that a few encoder counts of
> > backlash is OK).
> >
> > Now your table will still appear to have backlash if your
> > table nut has backlash (this is a candidate for compensation
> > in software). Luckily I have found using long length nuts
> > (greater than 3:1 length:dia ratio) have under 1 thou backlash
> > (when new & unworn). I use a 6:1 ratio and there is no measurable
> > backlash.
> >
> > Note that (expensive but worth it in some applications) ballscrews
> > have other desirable features beyond just the low/no backlash
> > ballnuts, so just because you can eliminate backlash doesn't mean
> > that ballscrews aren't the perfect answer (for some applications).
> >
> >
> > Doug Fortune
> > http://www.cncKITS.com
>
> Hi Tim and Doug,
>
> Doug, know from our experiments how and encoder on a lead screw can
> cause trouble. There has to be next to no backlash in the system. So
> again, your long acme nuts or spring loaded backlash nuts would have
> to be used.
> I also have an idea that may be crazy, but here it is anyway. There
> have recently been a couple of threads discussing the virtues of
> ballscrews vs acme (and all its grades) vs regular threaded rod. All
> of these methods are bad for reasons of cost or poor accuracy. How
> about if you removed the encoder from the screw altogether. I saw a
> photo somewhere recently of a benchtop mill that had a dro where the
> encoders were driven by a cable. The fellow had even used a pulley
> system to increase the resolution. So what would happen if you had
> inexpensive screws and anti backlash nuts of some kind, and cable
> driven encoders. Seems to me that your accuracy would not be affected
> by the screw at all. Just a thought.
>
> I also like the idea of using just any dc motor and I have tried
> several with good results. They are a dime a dozen, and are good
> enough for hobby use. I just look for motors that have a relitivly low
> rotor inertia. I can usually tell how a motor will act by spinning it
> by hand. A good one is usually long and skinny, and wont spin by hand
> very easily. Hook it up to 1 or 2 volts and see if it cogs. If it
> doesnt, its a keeper.
> Dean
>
>
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