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

Hi all,

I cnc'd the knee on my (then new) BP clone almost 2.5 yrs ago. I've
logged over 2000 hours of production time and over 4000 tool changes and
I can still hold .005" vertical any time I need to. The knee is just
fine to be cnc'd!

This link http://www.cncauto.com/knee11.htm helps explain the
benefits. I've also posted pics in the Photos section showing my gas
springs installed: NEMA 42 - Gas Springs

I've since converted to servos and can easily get 48 IPM rapids in Z
axis with my knee. I fitted two 175 lb gas springs (from McMaster Carr)
and these can hold the knee in position even with two 50 lb Kurt vices
on the table. I used a 5TPI rolled ballscrew from Rockford ballscrew.
I had to machine clearance inside the knee support post to clear the
ball return tube. The servo motor is equivalent to a Baldor MT-4090
(you can get the specs from Baldor's website) it's connected via a 2:1 belt.

Considering how much vertical clearance you need to get an R-8 tool out
of the spindle I can't imagine doing so many tool changes via the
quill's wimpy travel. Of course it depends on what kind of work you are
contemplating.

I did have some vibration with the knee gib at the beginning and it took
a bit of trial and error to get it set right (as tight as possible, but
no vibration during a rapid) but it hasn't missed a beat since. There
are no abnormal signs of wear - just nicely polished ways on X, Y & Z.
I keep my quill fully retracted and locked in position - and as rigid as
it can be.

Do the knee - you'll be glad you did!

Cheers, Peter


Jon Elson wrote:

Stephen Wille Padnos wrote:

>I'm not sure a counterweight system would work. You'd need a cable that
>goes over the top of the machine or something - this would probably get
>in the way of the machining envelope.
>
>