Re: Knee for z-axis control?

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

>
>
> doug98105 <DougR@a...> wrote:
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "pondindustrial"
>
> wrote:
> > Hi all,
> >
> > I will be retrofitting my Bridgeport mill soon and know that

some

> > have used the knee for z-axis control. Does anyone here have any
> > experience with this method? If so, what should I take into
> > consideration if I decide to go this route? The mill has a 9" X
> 48"
> > bed.
> >
> > Is a counterweight to offset some of the lifting load preferable?
> > Did you use the original acme screw/nut, or retro with a

ballscrew?

> > What would be a reasonably sufficient drive in terms of motor
> torque
> > and gear reduction?
> > What are the pros & cons of using the knee opposed to the quill?
> >
> >
> > Thanks, Eric
>
> To my knowledge there are no commercial CNC knee mills (B'port

clone

> types) with the Z on the knee. Ever wonder why? You would think
> that at least a few builders of CNC machines might use this method
> if it had some merit.
>
> In spite of the apparent success of some conversions mentioned

here,

> to me it seems like total nonsense to have that much mass jumping

up

> and down at traverse rates we normally expect of a CNC mill.
>
> Just my $.02,
>
> Doug
>
> Mazak and others traverse the whole powerhead on dovetail ways for

Z with .0005 reliable repeatability globally within the envelope.
They have no quill at all. Okuma and Howa refridgerate the quill so
it will remain the right size and tight (fighting the deflection of
an extended quill) . I wonder why they use such unorthodox methods?
Maybe someone should call and tell them its nonsense....

>
> Ron
>

Ron,

I'm well aware of what Mazak and others do. I see very little
similarity between those type machines and Z-ing the knee of a
B'port type mill where a good percentage of the machine's mass is in
the knee assembly.

I'll still pose the question, if Z-ing the knee is better than the
quill, why isn't it done on factory made commercial CNC knee mills?

The knee is easier to do for a conversion of a manual mill, are we
talking easier or better? Bridgeport, themselves, went to a lot of
trouble to build a quill Z retrofit, it's surprising they didn't
take the easy way and Z the knee. Maybe they didn't think it was
such a great idea?

Again, it's just opinion based on my experience with CNC mills,

Doug