Re: Driving a Bridgeport Knee

--- In CAD_CAM_EDM_DRO@yahoogroups.com, R Rogers <rogersmach@y...>
wrote:

> The knee due to its mass and precision of the acme screw is far

more accurate than the X and Y will ever be. And there is zero
backlash. The Z mechanism is twice the resolution of X and Y i.e. 1
rev equals .100 opposed to the .200 per rev of X and Y. A ballscrew
is not needed. Backlash in the bevel gears can be removed by shimming
or compensate for it in the software. The major drawback to driving
the knee is slower acceleration due to the weight and the need for a
larger servo to power it. The quill wouldnt be any more accurate.
Probably not as accurate.

>
> Ron

The best acme screw spec I can find is .003" per foot and that is
today's technology. Couple that with bevel gear error and what do
you have???

On the other hand, most of us have not had a problem with this error.

On the third hand, what if we want to try new things on our CNC like
making molds, now how accurate do we have to be. If we leave the
acme screw in and bevel gear, will we limit ourselves later?

On the 4th hand, are we kidding ourselves that we can get ½ thou
accuracy from our Bridgeport. Probably. My guess, working on a metal
parts for a couple hours, best ballscrews, good controls, .003" error
min. But hey,I am an optimist!!!

And round and round it goes. I have been hashing this over in my
mind for months.


Wally

>
>
> Polaraligned <polaraligned@o...> wrote:
>
>
> Well, I don't think they get it very much below .001". You will
> have backlash to contend with. But the bigger question is how
> accurate are those gears anyway? People get ground screws over
> rolled ones- yet settle for sloppy bevel gears? I just like the
> idea of driving the leadscrew directly and saving the bevel gears
> for manual operation of the machine. I may even get a ballscrew
> for the Z axis, but I have not really decided if this is just a

waste.

> As it is all commercial conversions (that I have seen) drive the

quill

> because it is more accurate. To each his own...
>
> Scott
>
>
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "cnc_4_me" wrote:
> >
> > Well for what it is worth, the people i talked to about driving

with

> > bevel gears said to shim them and take out the backlash.
> >
> > Wally
> >
> >
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Polaraligned"
> >
> wrote:
> > >
> > > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "cnc_4_me"
> > wrote:
> > > >
> > > > I was wondering if this was feasable also, if my machine

wasn't

> > > > buried under a ton of stuff i would investigate further.

While i

> > do
> > > > not have any answers i do like hearing your ideas or anyone

elses

> > on
> > > > the subject. One thing i am affraid of with usaual methode of
> > > > driving the knee, is breaking the bevel gear with to much
> > > > horsepower...
> > > >
> > > > Wally
> > > >
> > >
> > >
> > > I really am resistant to driving through the bevel gears as they
> > > are very sloppy. I also like to be able to move all axis by hand
> > > should the need arise or for the "quickie" machine job. My

machine

> > > is currently completely apart for a rebuild, and I can see no

reason

> > > why this won't work. Just can't bring myself to drive through
> > > the damn bevel gears....
> > >
> > > Scott
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > > >
> > > > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Polaraligned"
> > > >
> wrote:
> > > > >
> > > > > I am thinking about driving the knee with a pulley just

below

> > the
> > > > > bearing on the Z-axis leadscrew. I would machine the

leadscrew

> > down
> > > > > to accept the pulley. The motor would be mounted on the

side

> > of the
> > > > > knee, and to get the timing belt out I would cut a

rectangular

> > slot
> > > > in
> > > > > the side of the knee.
> > > > >
> > > > > Advantages:
> > > > > Motor is out of the way.
> > > > > I can still use the handle to move the knee via the bevel

gears.

> > > > > (can't leave the handle permanantly attached!)
> > > > >
> > > > > Disadvantages:
> > > > > Lose about 2 inches off the bottom of the knee travel
> > > > > Changing belts would be a real pain in the you know what.
> > > > > Permanantly alter the knee with hole (which does not bother

me)

> > > > >
> > > > >
> > > > > I think the motor will have no problem with clearances.
> > > > > Does this sound good? or am I missing something.
> > > > >
> > > > > Scott
>
>
>
>
>
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