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

One correction - that's .0005" - not .005" - I work in mm and screwed
up the conversion to your antiquated units of measure... I should also
point out that while my ballscrews are rolled, they are only accurate to
.003" per foot worst case. But they're very repeatable - I may have to
adjust my zero point to compensate, but only every few hours when things
warm up a bit. Once adjusted, I can hold a very tight tolerance when
needed.

Peter Renolds wrote:

>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.
>>
>>
>>
>>
>
>
>
>
>
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