Re: [CAD_CAM_EDM_DRO] Knee for z-axis control?

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?
>
>

The quill on a J head is limited at about 5" of travel. But, it is light,
tight, and forever concentric with the spindle rotation.

The knee can give 12"+ of travel, but it is the heaviest moving part of
the mill,
and may not always move parallel to the spindle, even IF the machine was
carefully trammed. The way the knee is built, it puts a huge side load
on the
angled part of the dovetail evey time the knee is lowered. Eventually the
vertical ways on the base as well as the mating ways on the knee become
worn.

Due to the weight, it is almost guaranteed that the knee screw on any used
Bridgeport is no longer accurate, due to differential wear.

For these reasons, I chose to automate the quill. I made a whole
contraption
to attach to the front of the head and mount a ballscre as close to the
front
of the main housing as possible. It drives the quill through the slot
for the
stop ring.

Jon